[PATCH 04/16] drbd: dirty bitmap

[Philipp Reisner]

DRBD maintains a dirty bitmap in case it has to run without peer node or
without local disk. Writes to the on disk dirty bitmap are minimized by the
activity log (=AL). Each time an extent is evicted from the AL the part of
the bitmap no longer covered by the AL is written to disk.

Signed-off-by: Philipp Reisner <philipp.reisner@xxxxxxxxxx>
Signed-off-by: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>

---
diff --git a/drivers/block/drbd/drbd_bitmap.c b/drivers/block/drbd/drbd_bitmap.c
new file mode 100644
index 0000000..2858ef5
--- /dev/null
+++ b/drivers/block/drbd/drbd_bitmap.c
@@ -0,0 +1,1273 @@
+/*
+   drbd_bitmap.c
+
+   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.
+
+   Copyright (C) 2004-2008, LINBIT Information Technologies GmbH.
+   Copyright (C) 2004-2008, Philipp Reisner <philipp.reisner@xxxxxxxxxx>.
+   Copyright (C) 2004-2008, Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>.
+
+   drbd is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   drbd is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with drbd; see the file COPYING.  If not, write to
+   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <linux/bitops.h>
+#include <linux/vmalloc.h>
+#include <linux/string.h>
+#include <linux/drbd.h>
+#include <asm/kmap_types.h>
+#include "drbd_int.h"
+
+/* OPAQUE outside this file!
+ * interface defined in drbd_int.h
+
+ * convetion:
+ * function name drbd_bm_... => used elsewhere, "public".
+ * function name      bm_... => internal to implementation, "private".
+
+ * Note that since find_first_bit returns int, at the current granularity of
+ * the bitmap (4KB per byte), this implementation "only" supports up to
+ * 1<<(32+12) == 16 TB...
+ */
+
+/*
+ * NOTE
+ *  Access to the *bm_pages is protected by bm_lock.
+ *  It is safe to read the other members within the lock.
+ *
+ *  drbd_bm_set_bits is called from bio_endio callbacks,
+ *  We may be called with irq already disabled,
+ *  so we need spin_lock_irqsave().
+ *  And we need the kmap_atomic.
+ */
+struct drbd_bitmap {
+	struct page **bm_pages;
+	spinlock_t bm_lock;
+	/* WARNING unsigned long bm_*:
+	 * 32bit number of bit offset is just enough for 512 MB bitmap.
+	 * it will blow up if we make the bitmap bigger...
+	 * not that it makes much sense to have a bitmap that large,
+	 * rather change the granularity to 16k or 64k or something.
+	 * (that implies other problems, however...)
+	 */
+	unsigned long bm_set;       /* nr of set bits; THINK maybe atomic_t? */
+	unsigned long bm_bits;
+	size_t   bm_words;
+	size_t   bm_number_of_pages;
+	sector_t bm_dev_capacity;
+	struct semaphore bm_change; /* serializes resize operations */
+
+	atomic_t bm_async_io;
+	wait_queue_head_t bm_io_wait;
+
+	unsigned long  bm_flags;
+
+	/* debugging aid, in case we are still racy somewhere */
+	char          *bm_why;
+	struct task_struct *bm_task;
+};
+
+/* definition of bits in bm_flags */
+#define BM_LOCKED       0
+#define BM_MD_IO_ERROR  1
+#define BM_P_VMALLOCED  2
+
+static int bm_is_locked(struct drbd_bitmap *b)
+{
+	return test_bit(BM_LOCKED, &b->bm_flags);
+}
+
+#define bm_print_lock_info(m) __bm_print_lock_info(m, __func__)
+static void __bm_print_lock_info(struct drbd_conf *mdev, const char *func)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	if (!__ratelimit(&drbd_ratelimit_state))
+		return;
+	dev_err(DEV, "FIXME %s in %s, bitmap locked for '%s' by %s\n",
+	    current == mdev->receiver.task ? "receiver" :
+	    current == mdev->asender.task  ? "asender"  :
+	    current == mdev->worker.task   ? "worker"   : current->comm,
+	    func, b->bm_why ?: "?",
+	    b->bm_task == mdev->receiver.task ? "receiver" :
+	    b->bm_task == mdev->asender.task  ? "asender"  :
+	    b->bm_task == mdev->worker.task   ? "worker"   : "?");
+}
+
+void drbd_bm_lock(struct drbd_conf *mdev, char *why)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	int trylock_failed;
+
+	if (!b) {
+		dev_err(DEV, "FIXME no bitmap in drbd_bm_lock!?\n");
+		return;
+	}
+
+	trylock_failed = down_trylock(&b->bm_change);
+
+	if (trylock_failed) {
+		dev_warn(DEV, "%s going to '%s' but bitmap already locked for '%s' by %s\n",
+		    current == mdev->receiver.task ? "receiver" :
+		    current == mdev->asender.task  ? "asender"  :
+		    current == mdev->worker.task   ? "worker"   : current->comm,
+		    why, b->bm_why ?: "?",
+		    b->bm_task == mdev->receiver.task ? "receiver" :
+		    b->bm_task == mdev->asender.task  ? "asender"  :
+		    b->bm_task == mdev->worker.task   ? "worker"   : "?");
+		down(&b->bm_change);
+	}
+	if (__test_and_set_bit(BM_LOCKED, &b->bm_flags))
+		dev_err(DEV, "FIXME bitmap already locked in bm_lock\n");
+
+	b->bm_why  = why;
+	b->bm_task = current;
+}
+
+void drbd_bm_unlock(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	if (!b) {
+		dev_err(DEV, "FIXME no bitmap in drbd_bm_unlock!?\n");
+		return;
+	}
+
+	if (!__test_and_clear_bit(BM_LOCKED, &mdev->bitmap->bm_flags))
+		dev_err(DEV, "FIXME bitmap not locked in bm_unlock\n");
+
+	b->bm_why  = NULL;
+	b->bm_task = NULL;
+	up(&b->bm_change);
+}
+
+/* word offset to long pointer */
+static unsigned long *__bm_map_paddr(struct drbd_bitmap *b, unsigned long offset, const enum km_type km)
+{
+	struct page *page;
+	unsigned long page_nr;
+
+	/* page_nr = (word*sizeof(long)) >> PAGE_SHIFT; */
+	page_nr = offset >> (PAGE_SHIFT - LN2_BPL + 3);
+	BUG_ON(page_nr >= b->bm_number_of_pages);
+	page = b->bm_pages[page_nr];
+
+	return (unsigned long *) kmap_atomic(page, km);
+}
+
+static unsigned long * bm_map_paddr(struct drbd_bitmap *b, unsigned long offset)
+{
+	return __bm_map_paddr(b, offset, KM_IRQ1);
+}
+
+static void __bm_unmap(unsigned long *p_addr, const enum km_type km)
+{
+	kunmap_atomic(p_addr, km);
+};
+
+static void bm_unmap(unsigned long *p_addr)
+{
+	return __bm_unmap(p_addr, KM_IRQ1);
+}
+
+/* long word offset of _bitmap_ sector */
+#define S2W(s)	((s)<<(BM_EXT_SHIFT-BM_BLOCK_SHIFT-LN2_BPL))
+/* word offset from start of bitmap to word number _in_page_
+ * modulo longs per page
+#define MLPP(X) ((X) % (PAGE_SIZE/sizeof(long))
+ hm, well, Philipp thinks gcc might not optimze the % into & (... - 1)
+ so do it explicitly:
+ */
+#define MLPP(X) ((X) & ((PAGE_SIZE/sizeof(long))-1))
+
+/* Long words per page */
+#define LWPP (PAGE_SIZE/sizeof(long))
+
+/*
+ * actually most functions herein should take a struct drbd_bitmap*, not a
+ * struct drbd_conf*, but for the debug macros I like to have the mdev around
+ * to be able to report device specific.
+ */
+
+static void bm_free_pages(struct page **pages, unsigned long number)
+{
+	unsigned long i;
+	if (!pages)
+		return;
+
+	for (i = 0; i < number; i++) {
+		if (!pages[i]) {
+			printk(KERN_ALERT "drbd: bm_free_pages tried to free "
+					  "a NULL pointer; i=%lu n=%lu\n",
+					  i, number);
+			continue;
+		}
+		__free_page(pages[i]);
+		pages[i] = NULL;
+	}
+}
+
+static void bm_vk_free(void *ptr, int v)
+{
+	if (v)
+		vfree(ptr);
+	else
+		kfree(ptr);
+}
+
+/*
+ * "have" and "want" are NUMBER OF PAGES.
+ */
+static struct page **bm_realloc_pages(struct drbd_bitmap *b, unsigned long want)
+{
+	struct page **old_pages = b->bm_pages;
+	struct page **new_pages, *page;
+	unsigned int i, bytes, vmalloced = 0;
+	unsigned long have = b->bm_number_of_pages;
+
+	BUG_ON(have == 0 && old_pages != NULL);
+	BUG_ON(have != 0 && old_pages == NULL);
+
+	if (have == want)
+		return old_pages;
+
+	/* Trying kmalloc first, falling back to vmalloc.
+	 * GFP_KERNEL is ok, as this is done when a lower level disk is
+	 * "attached" to the drbd.  Context is receiver thread or cqueue
+	 * thread.  As we have no disk yet, we are not in the IO path,
+	 * not even the IO path of the peer. */
+	bytes = sizeof(struct page *)*want;
+	new_pages = kmalloc(bytes, GFP_KERNEL);
+	if (!new_pages) {
+		new_pages = vmalloc(bytes);
+		if (!new_pages)
+			return NULL;
+		vmalloced = 1;
+	}
+
+	memset(new_pages, 0, bytes);
+	if (want >= have) {
+		for (i = 0; i < have; i++)
+			new_pages[i] = old_pages[i];
+		for (; i < want; i++) {
+			page = alloc_page(GFP_HIGHUSER);
+			if (!page) {
+				bm_free_pages(new_pages + have, i - have);
+				bm_vk_free(new_pages, vmalloced);
+				return NULL;
+			}
+			new_pages[i] = page;
+		}
+	} else {
+		for (i = 0; i < want; i++)
+			new_pages[i] = old_pages[i];
+		/* NOT HERE, we are outside the spinlock!
+		bm_free_pages(old_pages + want, have - want);
+		*/
+	}
+
+	if (vmalloced)
+		set_bit(BM_P_VMALLOCED, &b->bm_flags);
+	else
+		clear_bit(BM_P_VMALLOCED, &b->bm_flags);
+
+	return new_pages;
+}
+
+/*
+ * called on driver init only. TODO call when a device is created.
+ * allocates the drbd_bitmap, and stores it in mdev->bitmap.
+ */
+int drbd_bm_init(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	WARN_ON(b != NULL);
+	b = kzalloc(sizeof(struct drbd_bitmap), GFP_KERNEL);
+	if (!b)
+		return -ENOMEM;
+	spin_lock_init(&b->bm_lock);
+	init_MUTEX(&b->bm_change);
+	init_waitqueue_head(&b->bm_io_wait);
+
+	mdev->bitmap = b;
+
+	return 0;
+}
+
+sector_t drbd_bm_capacity(struct drbd_conf *mdev)
+{
+	ERR_IF(!mdev->bitmap) return 0;
+	return mdev->bitmap->bm_dev_capacity;
+}
+
+/* called on driver unload. TODO: call when a device is destroyed.
+ */
+void drbd_bm_cleanup(struct drbd_conf *mdev)
+{
+	ERR_IF (!mdev->bitmap) return;
+	bm_free_pages(mdev->bitmap->bm_pages, mdev->bitmap->bm_number_of_pages);
+	bm_vk_free(mdev->bitmap->bm_pages, test_bit(BM_P_VMALLOCED, &mdev->bitmap->bm_flags));
+	kfree(mdev->bitmap);
+	mdev->bitmap = NULL;
+}
+
+/*
+ * since (b->bm_bits % BITS_PER_LONG) != 0,
+ * this masks out the remaining bits.
+ * Rerturns the number of bits cleared.
+ */
+static int bm_clear_surplus(struct drbd_bitmap *b)
+{
+	const unsigned long mask = (1UL << (b->bm_bits & (BITS_PER_LONG-1))) - 1;
+	size_t w = b->bm_bits >> LN2_BPL;
+	int cleared = 0;
+	unsigned long *p_addr, *bm;
+
+	p_addr = bm_map_paddr(b, w);
+	bm = p_addr + MLPP(w);
+	if (w < b->bm_words) {
+		cleared = hweight_long(*bm & ~mask);
+		*bm &= mask;
+		w++; bm++;
+	}
+
+	if (w < b->bm_words) {
+		cleared += hweight_long(*bm);
+		*bm = 0;
+	}
+	bm_unmap(p_addr);
+	return cleared;
+}
+
+static void bm_set_surplus(struct drbd_bitmap *b)
+{
+	const unsigned long mask = (1UL << (b->bm_bits & (BITS_PER_LONG-1))) - 1;
+	size_t w = b->bm_bits >> LN2_BPL;
+	unsigned long *p_addr, *bm;
+
+	p_addr = bm_map_paddr(b, w);
+	bm = p_addr + MLPP(w);
+	if (w < b->bm_words) {
+		*bm |= ~mask;
+		bm++; w++;
+	}
+
+	if (w < b->bm_words) {
+		*bm = ~(0UL);
+	}
+	bm_unmap(p_addr);
+}
+
+static unsigned long __bm_count_bits(struct drbd_bitmap *b, const int swap_endian, const enum km_type km)
+{
+	unsigned long *p_addr, *bm, offset = 0;
+	unsigned long bits = 0;
+	unsigned long i, do_now;
+
+	while (offset < b->bm_words) {
+		i = do_now = min_t(size_t, b->bm_words-offset, LWPP);
+		p_addr = __bm_map_paddr(b, offset, km);
+		bm = p_addr + MLPP(offset);
+		while (i--) {
+#ifndef __LITTLE_ENDIAN
+			if (swap_endian)
+				*bm = lel_to_cpu(*bm);
+#endif
+			bits += hweight_long(*bm++);
+		}
+		__bm_unmap(p_addr, km);
+		offset += do_now;
+	}
+
+	return bits;
+}
+
+static unsigned long bm_count_bits(struct drbd_bitmap *b)
+{
+	return __bm_count_bits(b, 0, KM_IRQ1);
+}
+
+static unsigned long bm_count_bits_swap_endian(struct drbd_bitmap *b)
+{
+	return __bm_count_bits(b, 1, KM_USER0);
+}
+
+void _drbd_bm_recount_bits(struct drbd_conf *mdev, char *file, int line)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long flags, bits;
+
+	ERR_IF(!b) return;
+
+	/* IMO this should be inside drbd_bm_lock/unlock.
+	 * Unfortunately it is used outside of the locks.
+	 * And I'm not yet sure where we need to place the
+	 * lock/unlock correctly.
+	 */
+
+	spin_lock_irqsave(&b->bm_lock, flags);
+	bits = bm_count_bits(b);
+	if (bits != b->bm_set) {
+		dev_err(DEV, "bm_set was %lu, corrected to %lu. %s:%d\n",
+		    b->bm_set, bits, file, line);
+		b->bm_set = bits;
+	}
+	spin_unlock_irqrestore(&b->bm_lock, flags);
+}
+
+/* offset and len in long words.*/
+static void bm_memset(struct drbd_bitmap *b, size_t offset, int c, size_t len)
+{
+	unsigned long *p_addr, *bm;
+	size_t do_now, end;
+
+#define BM_SECTORS_PER_BIT (BM_BLOCK_SIZE/512)
+
+	end = offset + len;
+
+	if (end > b->bm_words) {
+		printk(KERN_ALERT "drbd: bm_memset end > bm_words\n");
+		return;
+	}
+
+	while (offset < end) {
+		do_now = min_t(size_t, ALIGN(offset + 1, LWPP), end) - offset;
+		p_addr = bm_map_paddr(b, offset);
+		bm = p_addr + MLPP(offset);
+		if (bm+do_now > p_addr + LWPP) {
+			printk(KERN_ALERT "drbd: BUG BUG BUG! p_addr:%p bm:%p do_now:%d\n",
+			       p_addr, bm, (int)do_now);
+			break; /* breaks to after catch_oob_access_end() only! */
+		}
+		memset(bm, c, do_now * sizeof(long));
+		bm_unmap(p_addr);
+		offset += do_now;
+	}
+}
+
+/*
+ * make sure the bitmap has enough room for the attached storage,
+ * if neccessary, resize.
+ * called whenever we may have changed the device size.
+ * returns -ENOMEM if we could not allocate enough memory, 0 on success.
+ * In case this is actually a resize, we copy the old bitmap into the new one.
+ * Otherwise, the bitmap is initiallized to all bits set.
+ */
+int drbd_bm_resize(struct drbd_conf *mdev, sector_t capacity)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long bits, words, owords, obits, *p_addr, *bm;
+	unsigned long want, have, onpages; /* number of pages */
+	struct page **npages, **opages = NULL;
+	int err = 0, growing;
+	int opages_vmalloced;
+
+	ERR_IF(!b) return -ENOMEM;
+
+	drbd_bm_lock(mdev, "resize");
+
+	dev_info(DEV, "drbd_bm_resize called with capacity == %llu\n",
+			(unsigned long long)capacity);
+
+	if (capacity == b->bm_dev_capacity)
+		goto out;
+
+	opages_vmalloced = test_bit(BM_P_VMALLOCED, &b->bm_flags);
+
+	if (capacity == 0) {
+		spin_lock_irq(&b->bm_lock);
+		opages = b->bm_pages;
+		onpages = b->bm_number_of_pages;
+		owords = b->bm_words;
+		b->bm_pages = NULL;
+		b->bm_number_of_pages =
+		b->bm_set   =
+		b->bm_bits  =
+		b->bm_words =
+		b->bm_dev_capacity = 0;
+		spin_unlock_irq(&b->bm_lock);
+		bm_free_pages(opages, onpages);
+		bm_vk_free(opages, opages_vmalloced);
+		goto out;
+	}
+	bits  = BM_SECT_TO_BIT(ALIGN(capacity, BM_SECT_PER_BIT));
+
+	/* if we would use
+	   words = ALIGN(bits,BITS_PER_LONG) >> LN2_BPL;
+	   a 32bit host could present the wrong number of words
+	   to a 64bit host.
+	*/
+	words = ALIGN(bits, 64) >> LN2_BPL;
+
+	if (get_ldev(mdev)) {
+		D_ASSERT((u64)bits <= (((u64)mdev->ldev->md.md_size_sect-MD_BM_OFFSET) << 12));
+		put_ldev(mdev);
+	}
+
+	/* one extra long to catch off by one errors */
+	want = ALIGN((words+1)*sizeof(long), PAGE_SIZE) >> PAGE_SHIFT;
+	have = b->bm_number_of_pages;
+	if (want == have) {
+		D_ASSERT(b->bm_pages != NULL);
+		npages = b->bm_pages;
+	} else {
+		if (FAULT_ACTIVE(mdev, DRBD_FAULT_BM_ALLOC))
+			npages = NULL;
+		else
+			npages = bm_realloc_pages(b, want);
+	}
+
+	if (!npages) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	spin_lock_irq(&b->bm_lock);
+	opages = b->bm_pages;
+	owords = b->bm_words;
+	obits  = b->bm_bits;
+
+	growing = bits > obits;
+	if (opages)
+		bm_set_surplus(b);
+
+	b->bm_pages = npages;
+	b->bm_number_of_pages = want;
+	b->bm_bits  = bits;
+	b->bm_words = words;
+	b->bm_dev_capacity = capacity;
+
+	if (growing) {
+		bm_memset(b, owords, 0xff, words-owords);
+		b->bm_set += bits - obits;
+	}
+
+	if (want < have) {
+		/* implicit: (opages != NULL) && (opages != npages) */
+		bm_free_pages(opages + want, have - want);
+	}
+
+	p_addr = bm_map_paddr(b, words);
+	bm = p_addr + MLPP(words);
+	*bm = DRBD_MAGIC;
+	bm_unmap(p_addr);
+
+	(void)bm_clear_surplus(b);
+	if (!growing)
+		b->bm_set = bm_count_bits(b);
+
+	spin_unlock_irq(&b->bm_lock);
+	if (opages != npages)
+		bm_vk_free(opages, opages_vmalloced);
+	dev_info(DEV, "resync bitmap: bits=%lu words=%lu\n", bits, words);
+
+ out:
+	drbd_bm_unlock(mdev);
+	return err;
+}
+
+/* inherently racy:
+ * if not protected by other means, return value may be out of date when
+ * leaving this function...
+ * we still need to lock it, since it is important that this returns
+ * bm_set == 0 precisely.
+ *
+ * maybe bm_set should be atomic_t ?
+ */
+unsigned long drbd_bm_total_weight(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long s;
+	unsigned long flags;
+
+	/* if I don't have a disk, I don't know about out-of-sync status */
+	if (!get_ldev_if_state(mdev, D_NEGOTIATING))
+		return 0;
+
+	ERR_IF(!b) return 0;
+	ERR_IF(!b->bm_pages) return 0;
+
+	spin_lock_irqsave(&b->bm_lock, flags);
+	s = b->bm_set;
+	spin_unlock_irqrestore(&b->bm_lock, flags);
+
+	put_ldev(mdev);
+
+	return s;
+}
+
+size_t drbd_bm_words(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	ERR_IF(!b) return 0;
+	ERR_IF(!b->bm_pages) return 0;
+
+	return b->bm_words;
+}
+
+unsigned long drbd_bm_bits(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	ERR_IF(!b) return 0;
+
+	return b->bm_bits;
+}
+
+/* merge number words from buffer into the bitmap starting at offset.
+ * buffer[i] is expected to be little endian unsigned long.
+ * bitmap must be locked by drbd_bm_lock.
+ * currently only used from receive_bitmap.
+ */
+void drbd_bm_merge_lel(struct drbd_conf *mdev, size_t offset, size_t number,
+			unsigned long *buffer)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long *p_addr, *bm;
+	unsigned long word, bits;
+	size_t end, do_now;
+
+	end = offset + number;
+
+	ERR_IF(!b) return;
+	ERR_IF(!b->bm_pages) return;
+	if (number == 0)
+		return;
+	WARN_ON(offset >= b->bm_words);
+	WARN_ON(end    >  b->bm_words);
+
+	spin_lock_irq(&b->bm_lock);
+	while (offset < end) {
+		do_now = min_t(size_t, ALIGN(offset+1, LWPP), end) - offset;
+		p_addr = bm_map_paddr(b, offset);
+		bm = p_addr + MLPP(offset);
+		offset += do_now;
+		while (do_now--) {
+			bits = hweight_long(*bm);
+			word = *bm | lel_to_cpu(*buffer++);
+			*bm++ = word;
+			b->bm_set += hweight_long(word) - bits;
+		}
+		bm_unmap(p_addr);
+	}
+	/* with 32bit <-> 64bit cross-platform connect
+	 * this is only correct for current usage,
+	 * where we _know_ that we are 64 bit aligned,
+	 * and know that this function is used in this way, too...
+	 */
+	if (end == b->bm_words)
+		b->bm_set -= bm_clear_surplus(b);
+
+	spin_unlock_irq(&b->bm_lock);
+}
+
+/* copy number words from the bitmap starting at offset into the buffer.
+ * buffer[i] will be little endian unsigned long.
+ */
+void drbd_bm_get_lel(struct drbd_conf *mdev, size_t offset, size_t number,
+		     unsigned long *buffer)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long *p_addr, *bm;
+	size_t end, do_now;
+
+	end = offset + number;
+
+	ERR_IF(!b) return;
+	ERR_IF(!b->bm_pages) return;
+
+	spin_lock_irq(&b->bm_lock);
+	if ((offset >= b->bm_words) ||
+	    (end    >  b->bm_words) ||
+	    (number <= 0))
+		dev_err(DEV, "offset=%lu number=%lu bm_words=%lu\n",
+			(unsigned long)	offset,
+			(unsigned long)	number,
+			(unsigned long) b->bm_words);
+	else {
+		while (offset < end) {
+			do_now = min_t(size_t, ALIGN(offset+1, LWPP), end) - offset;
+			p_addr = bm_map_paddr(b, offset);
+			bm = p_addr + MLPP(offset);
+			offset += do_now;
+			while (do_now--)
+				*buffer++ = cpu_to_lel(*bm++);
+			bm_unmap(p_addr);
+		}
+	}
+	spin_unlock_irq(&b->bm_lock);
+}
+
+/* set all bits in the bitmap */
+void drbd_bm_set_all(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	ERR_IF(!b) return;
+	ERR_IF(!b->bm_pages) return;
+
+	spin_lock_irq(&b->bm_lock);
+	bm_memset(b, 0, 0xff, b->bm_words);
+	(void)bm_clear_surplus(b);
+	b->bm_set = b->bm_bits;
+	spin_unlock_irq(&b->bm_lock);
+}
+
+/* clear all bits in the bitmap */
+void drbd_bm_clear_all(struct drbd_conf *mdev)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	ERR_IF(!b) return;
+	ERR_IF(!b->bm_pages) return;
+
+	spin_lock_irq(&b->bm_lock);
+	bm_memset(b, 0, 0, b->bm_words);
+	b->bm_set = 0;
+	spin_unlock_irq(&b->bm_lock);
+}
+
+static void bm_async_io_complete(struct bio *bio, int error)
+{
+	struct drbd_bitmap *b = bio->bi_private;
+	int uptodate = bio_flagged(bio, BIO_UPTODATE);
+
+
+	/* strange behaviour of some lower level drivers...
+	 * fail the request by clearing the uptodate flag,
+	 * but do not return any error?!
+	 * do we want to WARN() on this? */
+	if (!error && !uptodate)
+		error = -EIO;
+
+	if (error) {
+		/* doh. what now?
+		 * for now, set all bits, and flag MD_IO_ERROR */
+		__set_bit(BM_MD_IO_ERROR, &b->bm_flags);
+	}
+	if (atomic_dec_and_test(&b->bm_async_io))
+		wake_up(&b->bm_io_wait);
+
+	bio_put(bio);
+}
+
+static void bm_page_io_async(struct drbd_conf *mdev, struct drbd_bitmap *b, int page_nr, int rw) __must_hold(local)
+{
+	/* we are process context. we always get a bio */
+	struct bio *bio = bio_alloc(GFP_KERNEL, 1);
+	unsigned int len;
+	sector_t on_disk_sector =
+		mdev->ldev->md.md_offset + mdev->ldev->md.bm_offset;
+	on_disk_sector += ((sector_t)page_nr) << (PAGE_SHIFT-9);
+
+	/* this might happen with very small
+	 * flexible external meta data device */
+	len = min_t(unsigned int, PAGE_SIZE,
+		(drbd_md_last_sector(mdev->ldev) - on_disk_sector + 1)<<9);
+
+	bio->bi_bdev = mdev->ldev->md_bdev;
+	bio->bi_sector = on_disk_sector;
+	bio_add_page(bio, b->bm_pages[page_nr], len, 0);
+	bio->bi_private = b;
+	bio->bi_end_io = bm_async_io_complete;
+
+	if (FAULT_ACTIVE(mdev, (rw & WRITE) ? DRBD_FAULT_MD_WR : DRBD_FAULT_MD_RD)) {
+		bio->bi_rw |= rw;
+		bio_endio(bio, -EIO);
+	} else {
+		submit_bio(rw, bio);
+	}
+}
+
+# if defined(__LITTLE_ENDIAN)
+	/* nothing to do, on disk == in memory */
+# define bm_cpu_to_lel(x) ((void)0)
+# else
+void bm_cpu_to_lel(struct drbd_bitmap *b)
+{
+	/* need to cpu_to_lel all the pages ...
+	 * this may be optimized by using
+	 * cpu_to_lel(-1) == -1 and cpu_to_lel(0) == 0;
+	 * the following is still not optimal, but better than nothing */
+	unsigned int i;
+	unsigned long *p_addr, *bm;
+	if (b->bm_set == 0) {
+		/* no page at all; avoid swap if all is 0 */
+		i = b->bm_number_of_pages;
+	} else if (b->bm_set == b->bm_bits) {
+		/* only the last page */
+		i = b->bm_number_of_pages - 1;
+	} else {
+		/* all pages */
+		i = 0;
+	}
+	for (; i < b->bm_number_of_pages; i++) {
+		p_addr = kmap_atomic(b->bm_pages[i], KM_USER0);
+		for (bm = p_addr; bm < p_addr + PAGE_SIZE/sizeof(long); bm++)
+			*bm = cpu_to_lel(*bm);
+		kunmap_atomic(p_addr, KM_USER0);
+	}
+}
+# endif
+/* lel_to_cpu == cpu_to_lel */
+# define bm_lel_to_cpu(x) bm_cpu_to_lel(x)
+
+/*
+ * bm_rw: read/write the whole bitmap from/to its on disk location.
+ */
+static int bm_rw(struct drbd_conf *mdev, int rw) __must_hold(local)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	/* sector_t sector; */
+	int bm_words, num_pages, i;
+	unsigned long now;
+	char ppb[10];
+	int err = 0;
+
+	WARN_ON(!bm_is_locked(b));
+
+	/* no spinlock here, the drbd_bm_lock should be enough! */
+
+	bm_words  = drbd_bm_words(mdev);
+	num_pages = (bm_words*sizeof(long) + PAGE_SIZE-1) >> PAGE_SHIFT;
+
+	/* on disk bitmap is little endian */
+	if (rw == WRITE)
+		bm_cpu_to_lel(b);
+
+	now = jiffies;
+	atomic_set(&b->bm_async_io, num_pages);
+	__clear_bit(BM_MD_IO_ERROR, &b->bm_flags);
+
+	/* let the layers below us try to merge these bios... */
+	for (i = 0; i < num_pages; i++)
+		bm_page_io_async(mdev, b, i, rw);
+
+	drbd_blk_run_queue(bdev_get_queue(mdev->ldev->md_bdev));
+	wait_event(b->bm_io_wait, atomic_read(&b->bm_async_io) == 0);
+
+	if (test_bit(BM_MD_IO_ERROR, &b->bm_flags)) {
+		dev_alert(DEV, "we had at least one MD IO ERROR during bitmap IO\n");
+		drbd_chk_io_error(mdev, 1, TRUE);
+		drbd_io_error(mdev, TRUE);
+		err = -EIO;
+	}
+
+	now = jiffies;
+	if (rw == WRITE) {
+		/* swap back endianness */
+		bm_lel_to_cpu(b);
+		/* flush bitmap to stable storage */
+		drbd_md_flush(mdev);
+	} else /* rw == READ */ {
+		/* just read, if neccessary adjust endianness */
+		b->bm_set = bm_count_bits_swap_endian(b);
+		dev_info(DEV, "recounting of set bits took additional %lu jiffies\n",
+		     jiffies - now);
+	}
+	now = b->bm_set;
+
+	dev_info(DEV, "%s (%lu bits) marked out-of-sync by on disk bit-map.\n",
+	     ppsize(ppb, now << (BM_BLOCK_SHIFT-10)), now);
+
+	return err;
+}
+
+/**
+ * drbd_bm_read() - Read the whole bitmap from its on disk location.
+ * @mdev:	DRBD device.
+ */
+int drbd_bm_read(struct drbd_conf *mdev) __must_hold(local)
+{
+	return bm_rw(mdev, READ);
+}
+
+/**
+ * drbd_bm_write() - Write the whole bitmap to its on disk location.
+ * @mdev:	DRBD device.
+ */
+int drbd_bm_write(struct drbd_conf *mdev) __must_hold(local)
+{
+	return bm_rw(mdev, WRITE);
+}
+
+/**
+ * drbd_bm_write_sect: Writes a 512 (MD_SECTOR_SIZE) byte piece of the bitmap
+ * @mdev:	DRBD device.
+ * @enr:	Extent number in the resync lru (happens to be sector offset)
+ *
+ * The BM_EXT_SIZE is on purpose exactle the amount of the bitmap covered
+ * by a single sector write. Therefore enr == sector offset from the
+ * start of the bitmap.
+ */
+int drbd_bm_write_sect(struct drbd_conf *mdev, unsigned long enr) __must_hold(local)
+{
+	sector_t on_disk_sector = enr + mdev->ldev->md.md_offset
+				      + mdev->ldev->md.bm_offset;
+	int bm_words, num_words, offset;
+	int err = 0;
+
+	mutex_lock(&mdev->md_io_mutex);
+	bm_words  = drbd_bm_words(mdev);
+	offset    = S2W(enr);	/* word offset into bitmap */
+	num_words = min(S2W(1), bm_words - offset);
+	if (num_words < S2W(1))
+		memset(page_address(mdev->md_io_page), 0, MD_SECTOR_SIZE);
+	drbd_bm_get_lel(mdev, offset, num_words,
+			page_address(mdev->md_io_page));
+	if (!drbd_md_sync_page_io(mdev, mdev->ldev, on_disk_sector, WRITE)) {
+		int i;
+		err = -EIO;
+		dev_err(DEV, "IO ERROR writing bitmap sector %lu "
+		    "(meta-disk sector %llus)\n",
+		    enr, (unsigned long long)on_disk_sector);
+		drbd_chk_io_error(mdev, 1, TRUE);
+		drbd_io_error(mdev, TRUE);
+		for (i = 0; i < AL_EXT_PER_BM_SECT; i++)
+			drbd_bm_ALe_set_all(mdev, enr*AL_EXT_PER_BM_SECT+i);
+	}
+	mdev->bm_writ_cnt++;
+	mutex_unlock(&mdev->md_io_mutex);
+	return err;
+}
+
+/* NOTE
+ * find_first_bit returns int, we return unsigned long.
+ * should not make much difference anyways, but ...
+ *
+ * this returns a bit number, NOT a sector!
+ */
+#define BPP_MASK ((1UL << (PAGE_SHIFT+3)) - 1)
+static unsigned long __bm_find_next(struct drbd_conf *mdev, unsigned long bm_fo,
+	const int find_zero_bit, const enum km_type km)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long i = -1UL;
+	unsigned long *p_addr;
+	unsigned long bit_offset; /* bit offset of the mapped page. */
+
+	if (bm_fo > b->bm_bits) {
+		dev_err(DEV, "bm_fo=%lu bm_bits=%lu\n", bm_fo, b->bm_bits);
+	} else {
+		while (bm_fo < b->bm_bits) {
+			unsigned long offset;
+			bit_offset = bm_fo & ~BPP_MASK; /* bit offset of the page */
+			offset = bit_offset >> LN2_BPL;    /* word offset of the page */
+			p_addr = __bm_map_paddr(b, offset, km);
+
+			if (find_zero_bit)
+				i = find_next_zero_bit(p_addr, PAGE_SIZE*8, bm_fo & BPP_MASK);
+			else
+				i = find_next_bit(p_addr, PAGE_SIZE*8, bm_fo & BPP_MASK);
+
+			__bm_unmap(p_addr, km);
+			if (i < PAGE_SIZE*8) {
+				i = bit_offset + i;
+				if (i >= b->bm_bits)
+					break;
+				goto found;
+			}
+			bm_fo = bit_offset + PAGE_SIZE*8;
+		}
+		i = -1UL;
+	}
+ found:
+	return i;
+}
+
+static unsigned long bm_find_next(struct drbd_conf *mdev,
+	unsigned long bm_fo, const int find_zero_bit)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long i = -1UL;
+
+	ERR_IF(!b) return i;
+	ERR_IF(!b->bm_pages) return i;
+
+	spin_lock_irq(&b->bm_lock);
+	if (bm_is_locked(b))
+		bm_print_lock_info(mdev);
+
+	i = __bm_find_next(mdev, bm_fo, find_zero_bit, KM_IRQ1);
+
+	spin_unlock_irq(&b->bm_lock);
+	return i;
+}
+
+unsigned long drbd_bm_find_next(struct drbd_conf *mdev, unsigned long bm_fo)
+{
+	return bm_find_next(mdev, bm_fo, 0);
+}
+
+#if 0
+/* not yet needed for anything. */
+unsigned long drbd_bm_find_next_zero(struct drbd_conf *mdev, unsigned long bm_fo)
+{
+	return bm_find_next(mdev, bm_fo, 1);
+}
+#endif
+
+/* does not spin_lock_irqsave.
+ * you must take drbd_bm_lock() first */
+unsigned long _drbd_bm_find_next(struct drbd_conf *mdev, unsigned long bm_fo)
+{
+	/* WARN_ON(!bm_is_locked(mdev)); */
+	return __bm_find_next(mdev, bm_fo, 0, KM_USER1);
+}
+
+unsigned long _drbd_bm_find_next_zero(struct drbd_conf *mdev, unsigned long bm_fo)
+{
+	/* WARN_ON(!bm_is_locked(mdev)); */
+	return __bm_find_next(mdev, bm_fo, 1, KM_USER1);
+}
+
+/* returns number of bits actually changed.
+ * for val != 0, we change 0 -> 1, return code positiv
+ * for val == 0, we change 1 -> 0, return code negative
+ * wants bitnr, not sector.
+ * Must hold bitmap lock already. */
+
+int __bm_change_bits_to(struct drbd_conf *mdev, const unsigned long s,
+	const unsigned long e, int val, const enum km_type km)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long *p_addr = NULL;
+	unsigned long bitnr;
+	unsigned long last_page_nr = -1UL;
+	int c = 0;
+
+	for (bitnr = s; bitnr <= e; bitnr++) {
+		ERR_IF (bitnr >= b->bm_bits) {
+			dev_err(DEV, "bitnr=%lu bm_bits=%lu\n", bitnr, b->bm_bits);
+		} else {
+			unsigned long offset = bitnr>>LN2_BPL;
+			unsigned long page_nr = offset >> (PAGE_SHIFT - LN2_BPL + 3);
+			if (page_nr != last_page_nr) {
+				if (p_addr)
+					__bm_unmap(p_addr, km);
+				p_addr = __bm_map_paddr(b, offset, km);
+				last_page_nr = page_nr;
+			}
+			if (val)
+				c += (0 == __test_and_set_bit(bitnr & BPP_MASK, p_addr));
+			else
+				c -= (0 != __test_and_clear_bit(bitnr & BPP_MASK, p_addr));
+		}
+	}
+	if (p_addr)
+		__bm_unmap(p_addr, km);
+	b->bm_set += c;
+	return c;
+}
+
+/* returns number of bits actually changed.
+ * for val != 0, we change 0 -> 1, return code positiv
+ * for val == 0, we change 1 -> 0, return code negative
+ * wants bitnr, not sector */
+int bm_change_bits_to(struct drbd_conf *mdev, const unsigned long s,
+	const unsigned long e, int val)
+{
+	unsigned long flags;
+	struct drbd_bitmap *b = mdev->bitmap;
+	int c = 0;
+
+	ERR_IF(!b) return 1;
+	ERR_IF(!b->bm_pages) return 0;
+
+	spin_lock_irqsave(&b->bm_lock, flags);
+	if (bm_is_locked(b))
+		bm_print_lock_info(mdev);
+
+	c = __bm_change_bits_to(mdev, s, e, val, KM_IRQ1);
+
+	spin_unlock_irqrestore(&b->bm_lock, flags);
+	return c;
+}
+
+/* returns number of bits changed 0 -> 1 */
+int drbd_bm_set_bits(struct drbd_conf *mdev, const unsigned long s, const unsigned long e)
+{
+	return bm_change_bits_to(mdev, s, e, 1);
+}
+
+/* returns number of bits changed 1 -> 0 */
+int drbd_bm_clear_bits(struct drbd_conf *mdev, const unsigned long s, const unsigned long e)
+{
+	return -bm_change_bits_to(mdev, s, e, 0);
+}
+
+/* the same thing, but without taking the spin_lock_irqsave.
+ * you must first drbd_bm_lock(). */
+int _drbd_bm_set_bits(struct drbd_conf *mdev, const unsigned long s, const unsigned long e)
+{
+       /* WARN_ON(!bm_is_locked(b)); */
+       return __bm_change_bits_to(mdev, s, e, 1, KM_USER0);
+}
+
+/* returns bit state
+ * wants bitnr, NOT sector.
+ * inherently racy... area needs to be locked by means of {al,rs}_lru
+ *  1 ... bit set
+ *  0 ... bit not set
+ * -1 ... first out of bounds access, stop testing for bits!
+ */
+int drbd_bm_test_bit(struct drbd_conf *mdev, const unsigned long bitnr)
+{
+	unsigned long flags;
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long *p_addr;
+	int i;
+
+	ERR_IF(!b) return 0;
+	ERR_IF(!b->bm_pages) return 0;
+
+	spin_lock_irqsave(&b->bm_lock, flags);
+	if (bm_is_locked(b))
+		bm_print_lock_info(mdev);
+	if (bitnr < b->bm_bits) {
+		unsigned long offset = bitnr>>LN2_BPL;
+		p_addr = bm_map_paddr(b, offset);
+		i = test_bit(bitnr & BPP_MASK, p_addr) ? 1 : 0;
+		bm_unmap(p_addr);
+	} else if (bitnr == b->bm_bits) {
+		i = -1;
+	} else { /* (bitnr > b->bm_bits) */
+		dev_err(DEV, "bitnr=%lu > bm_bits=%lu\n", bitnr, b->bm_bits);
+		i = 0;
+	}
+
+	spin_unlock_irqrestore(&b->bm_lock, flags);
+	return i;
+}
+
+/* returns number of bits set */
+int drbd_bm_count_bits(struct drbd_conf *mdev, const unsigned long s, const unsigned long e)
+{
+	unsigned long flags;
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long *p_addr = NULL, page_nr = -1;
+	unsigned long bitnr;
+	int c = 0;
+	size_t w;
+
+	/* If this is called without a bitmap, that is a bug.  But just to be
+	 * robust in case we screwed up elsewhere, in that case pretend there
+	 * was one dirty bit in the requested area, so we won't try to do a
+	 * local read there (no bitmap probably implies no disk) */
+	ERR_IF(!b) return 1;
+	ERR_IF(!b->bm_pages) return 1;
+
+	spin_lock_irqsave(&b->bm_lock, flags);
+	for (bitnr = s; bitnr <= e; bitnr++) {
+		w = bitnr >> LN2_BPL;
+		if (page_nr != w >> (PAGE_SHIFT - LN2_BPL + 3)) {
+			page_nr = w >> (PAGE_SHIFT - LN2_BPL + 3);
+			if (p_addr)
+				bm_unmap(p_addr);
+			p_addr = bm_map_paddr(b, w);
+		}
+		ERR_IF (bitnr >= b->bm_bits) {
+			dev_err(DEV, "bitnr=%lu bm_bits=%lu\n", bitnr, b->bm_bits);
+		} else {
+			c += (0 != test_bit(bitnr - (page_nr << (PAGE_SHIFT+3)), p_addr));
+		}
+	}
+	if (p_addr)
+		bm_unmap(p_addr);
+	spin_unlock_irqrestore(&b->bm_lock, flags);
+	return c;
+}
+
+
+/* inherently racy...
+ * return value may be already out-of-date when this function returns.
+ * but the general usage is that this is only use during a cstate when bits are
+ * only cleared, not set, and typically only care for the case when the return
+ * value is zero, or we already "locked" this "bitmap extent" by other means.
+ *
+ * enr is bm-extent number, since we chose to name one sector (512 bytes)
+ * worth of the bitmap a "bitmap extent".
+ *
+ * TODO
+ * I think since we use it like a reference count, we should use the real
+ * reference count of some bitmap extent element from some lru instead...
+ *
+ */
+int drbd_bm_e_weight(struct drbd_conf *mdev, unsigned long enr)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	int count, s, e;
+	unsigned long flags;
+	unsigned long *p_addr, *bm;
+
+	ERR_IF(!b) return 0;
+	ERR_IF(!b->bm_pages) return 0;
+
+	spin_lock_irqsave(&b->bm_lock, flags);
+	if (bm_is_locked(b))
+		bm_print_lock_info(mdev);
+
+	s = S2W(enr);
+	e = min((size_t)S2W(enr+1), b->bm_words);
+	count = 0;
+	if (s < b->bm_words) {
+		int n = e-s;
+		p_addr = bm_map_paddr(b, s);
+		bm = p_addr + MLPP(s);
+		while (n--)
+			count += hweight_long(*bm++);
+		bm_unmap(p_addr);
+	} else {
+		dev_err(DEV, "start offset (%d) too large in drbd_bm_e_weight\n", s);
+	}
+	spin_unlock_irqrestore(&b->bm_lock, flags);
+	return count;
+}
+
+/* set all bits covered by the AL-extent al_enr */
+unsigned long drbd_bm_ALe_set_all(struct drbd_conf *mdev, unsigned long al_enr)
+{
+	struct drbd_bitmap *b = mdev->bitmap;
+	unsigned long *p_addr, *bm;
+	unsigned long weight;
+	int count, s, e, i, do_now;
+	ERR_IF(!b) return 0;
+	ERR_IF(!b->bm_pages) return 0;
+
+	spin_lock_irq(&b->bm_lock);
+	if (bm_is_locked(b))
+		bm_print_lock_info(mdev);
+	weight = b->bm_set;
+
+	s = al_enr * BM_WORDS_PER_AL_EXT;
+	e = min_t(size_t, s + BM_WORDS_PER_AL_EXT, b->bm_words);
+	/* assert that s and e are on the same page */
+	D_ASSERT((e-1) >> (PAGE_SHIFT - LN2_BPL + 3)
+	      ==  s    >> (PAGE_SHIFT - LN2_BPL + 3));
+	count = 0;
+	if (s < b->bm_words) {
+		i = do_now = e-s;
+		p_addr = bm_map_paddr(b, s);
+		bm = p_addr + MLPP(s);
+		while (i--) {
+			count += hweight_long(*bm);
+			*bm = -1UL;
+			bm++;
+		}
+		bm_unmap(p_addr);
+		b->bm_set += do_now*BITS_PER_LONG - count;
+		if (e == b->bm_words)
+			b->bm_set -= bm_clear_surplus(b);
+	} else {
+		dev_err(DEV, "start offset (%d) too large in drbd_bm_ALe_set_all\n", s);
+	}
+	weight = b->bm_set - weight;
+	spin_unlock_irq(&b->bm_lock);
+	return weight;
+}
--
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