[PATCH] mm: remove global locks from mm/highmem.c

From: Peter Zijlstra
Date: Sun Jan 28 2007 - 09:14:00 EST

Eradicate global locks.

- kmap_lock is removed by extensive use of atomic_t, a new flush
scheme and modifying set_page_address to only allow NULL<->virt
transitions.

A count of 0 is an exclusive state acting as an entry lock. This is done
using inc_not_zero and cmpxchg. The restriction on changing the virtual
address closes the gap with concurrent additions of the same entry.

- pool_lock is removed by using the pkmap index for the
page_address_maps.

By using the pkmap index for the hash entries it is no longer needed to
keep a free list.

This patch has been in -rt for a while but should also help regular
highmem machines with multiple cores/cpus.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
include/linux/mm.h | 32 ++-
mm/highmem.c | 433 ++++++++++++++++++++++++++++++-----------------------
2 files changed, 276 insertions(+), 189 deletions(-)

Index: linux/include/linux/mm.h
===================================================================
--- linux.orig/include/linux/mm.h
+++ linux/include/linux/mm.h
@@ -543,23 +543,39 @@ static __always_inline void *lowmem_page
#endif

#if defined(WANT_PAGE_VIRTUAL)
-#define page_address(page) ((page)->virtual)
-#define set_page_address(page, address) \
- do { \
- (page)->virtual = (address); \
- } while(0)
-#define page_address_init() do { } while(0)
+/*
+ * wrap page->virtual so it is safe to set/read locklessly
+ */
+#define page_address(page) \
+ ({ typeof((page)->virtual) v = (page)->virtual; \
+ smp_read_barrier_depends(); \
+ v; })
+
+static inline int set_page_address(struct page *page, void *address)
+{
+ if (address)
+ return cmpxchg(&page->virtual, NULL, address) == NULL;
+ else {
+ /*
+ * cmpxchg is a bit abused because it is not guaranteed
+ * safe wrt direct assignment on all platforms.
+ */
+ void *virt = page->virtual;
+ return cmpxchg(&page->vitrual, virt, NULL) == virt;
+ }
+}
+void page_address_init(void);
#endif

#if defined(HASHED_PAGE_VIRTUAL)
void *page_address(struct page *page);
-void set_page_address(struct page *page, void *virtual);
+int set_page_address(struct page *page, void *virtual);
void page_address_init(void);
#endif

#if !defined(HASHED_PAGE_VIRTUAL) && !defined(WANT_PAGE_VIRTUAL)
#define page_address(page) lowmem_page_address(page)
-#define set_page_address(page, address) do { } while(0)
+#define set_page_address(page, address) (0)
#define page_address_init() do { } while(0)
#endif

Index: linux/mm/highmem.c
===================================================================
--- linux.orig/mm/highmem.c
+++ linux/mm/highmem.c
@@ -14,6 +14,11 @@
* based on Linus' idea.
*
* Copyright (C) 1999 Ingo Molnar <mingo@xxxxxxxxxx>
+ *
+ * Largely rewritten to get rid of all global locks
+ *
+ * Copyright (C) 2006 Red Hat, Inc., Peter Zijlstra <pzijlstr@xxxxxxxxxx>
+ *
*/

#include <linux/mm.h>
@@ -27,18 +32,14 @@
#include <linux/hash.h>
#include <linux/highmem.h>
#include <linux/blktrace_api.h>
+
#include <asm/tlbflush.h>
+#include <asm/pgtable.h>

-/*
- * Virtual_count is not a pure "count".
- * 0 means that it is not mapped, and has not been mapped
- * since a TLB flush - it is usable.
- * 1 means that there are no users, but it has been mapped
- * since the last TLB flush - so we can't use it.
- * n means that there are (n-1) current users of it.
- */
#ifdef CONFIG_HIGHMEM

+static int __set_page_address(struct page *page, void *virtual, int pos);
+
unsigned long totalhigh_pages __read_mostly;

unsigned int nr_free_highpages (void)
@@ -52,164 +53,208 @@ unsigned int nr_free_highpages (void)
return pages;
}

-static int pkmap_count[LAST_PKMAP];
-static unsigned int last_pkmap_nr;
-static __cacheline_aligned_in_smp DEFINE_SPINLOCK(kmap_lock);
+/*
+ * count is not a pure "count".
+ * 0 means its owned exclusively by someone
+ * 1 means its free for use - either mapped or not.
+ * n means that there are (n-1) current users of it.
+ */
+static atomic_t pkmap_count[LAST_PKMAP];
+static atomic_t pkmap_hand;

pte_t * pkmap_page_table;

static DECLARE_WAIT_QUEUE_HEAD(pkmap_map_wait);

-static void flush_all_zero_pkmaps(void)
+/*
+ * Try to free a given kmap slot.
+ *
+ * Returns:
+ * -1 - in use
+ * 0 - free, no TLB flush needed
+ * 1 - free, needs TLB flush
+ */
+static int pkmap_try_free(int pos)
{
- int i;
-
- flush_cache_kmaps();
+ if (atomic_cmpxchg(&pkmap_count[pos], 1, 0) != 1)
+ return -1;

- for (i = 0; i < LAST_PKMAP; i++) {
- struct page *page;
+ /*
+ * TODO: add a young bit to make it CLOCK
+ */
+ if (!pte_none(pkmap_page_table[pos])) {
+ struct page *page = pte_page(pkmap_page_table[pos]);
+ unsigned long addr = PKMAP_ADDR(pos);
+ pte_t *ptep = &pkmap_page_table[pos];
+
+ VM_BUG_ON(addr != (unsigned long)page_address(page));
+
+ if (!__set_page_address(page, NULL, pos))
+ BUG();
+ flush_kernel_dcache_page(page);
+ pte_clear(&init_mm, addr, ptep);

- /*
- * zero means we don't have anything to do,
- * >1 means that it is still in use. Only
- * a count of 1 means that it is free but
- * needs to be unmapped
- */
- if (pkmap_count[i] != 1)
- continue;
- pkmap_count[i] = 0;
+ return 1;
+ }

- /* sanity check */
- BUG_ON(pte_none(pkmap_page_table[i]));
+ return 0;
+}

- /*
- * Don't need an atomic fetch-and-clear op here;
- * no-one has the page mapped, and cannot get at
- * its virtual address (and hence PTE) without first
- * getting the kmap_lock (which is held here).
- * So no dangers, even with speculative execution.
- */
- page = pte_page(pkmap_page_table[i]);
- pte_clear(&init_mm, (unsigned long)page_address(page),
- &pkmap_page_table[i]);
+static inline void pkmap_put(atomic_t *counter)
+{
+ switch (atomic_dec_return(counter)) {
+ case 0:
+ BUG();

- set_page_address(page, NULL);
+ case 1:
+ wake_up(&pkmap_map_wait);
}
- flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
}

-static inline unsigned long map_new_virtual(struct page *page)
+#define TLB_BATCH 32
+
+static int pkmap_get_free(void)
{
- unsigned long vaddr;
- int count;
+ int i, pos, flush;
+ DECLARE_WAITQUEUE(wait, current);

-start:
- count = LAST_PKMAP;
- /* Find an empty entry */
- for (;;) {
- last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
- if (!last_pkmap_nr) {
- flush_all_zero_pkmaps();
- count = LAST_PKMAP;
- }
- if (!pkmap_count[last_pkmap_nr])
- break; /* Found a usable entry */
- if (--count)
- continue;
+restart:
+ for (i = 0; i < LAST_PKMAP; i++) {
+ pos = atomic_inc_return(&pkmap_hand) % LAST_PKMAP;
+ flush = pkmap_try_free(pos);
+ if (flush >= 0)
+ goto got_one;
+ }
+
+ /*
+ * wait for somebody else to unmap their entries
+ */
+ __set_current_state(TASK_UNINTERRUPTIBLE);
+ add_wait_queue(&pkmap_map_wait, &wait);
+ schedule();
+ remove_wait_queue(&pkmap_map_wait, &wait);
+
+ goto restart;
+
+got_one:
+ if (flush) {
+#if 0
+ flush_tlb_kernel_range(PKMAP_ADDR(pos), PKMAP_ADDR(pos+1));
+#else
+ int pos2 = (pos + 1) % LAST_PKMAP;
+ int nr;
+ int entries[TLB_BATCH];

/*
- * Sleep for somebody else to unmap their entries
+ * For those architectures that cannot help but flush the
+ * whole TLB, flush some more entries to make it worthwhile.
+ * Scan ahead of the hand to minimise search distances.
*/
- {
- DECLARE_WAITQUEUE(wait, current);
+ for (i = 0, nr = 0; i < LAST_PKMAP && nr < TLB_BATCH;
+ i++, pos2 = (pos2 + 1) % LAST_PKMAP) {

- __set_current_state(TASK_UNINTERRUPTIBLE);
- add_wait_queue(&pkmap_map_wait, &wait);
- spin_unlock(&kmap_lock);
- schedule();
- remove_wait_queue(&pkmap_map_wait, &wait);
- spin_lock(&kmap_lock);
-
- /* Somebody else might have mapped it while we slept */
- if (page_address(page))
- return (unsigned long)page_address(page);
+ flush = pkmap_try_free(pos2);
+ if (flush < 0)
+ continue;
+
+ if (!flush) {
+ atomic_t *counter = &pkmap_count[pos2];
+ VM_BUG_ON(atomic_read(counter) != 0);
+ atomic_set(counter, 2);
+ pkmap_put(counter);
+ } else
+ entries[nr++] = pos2;
+ }
+ flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));

- /* Re-start */
- goto start;
+ for (i = 0; i < nr; i++) {
+ atomic_t *counter = &pkmap_count[entries[i]];
+ VM_BUG_ON(atomic_read(counter) != 0);
+ atomic_set(counter, 2);
+ pkmap_put(counter);
}
+#endif
}
- vaddr = PKMAP_ADDR(last_pkmap_nr);
- set_pte_at(&init_mm, vaddr,
- &(pkmap_page_table[last_pkmap_nr]), mk_pte(page, kmap_prot));
+ return pos;
+}
+
+static unsigned long pkmap_insert(struct page *page)
+{
+ int pos = pkmap_get_free();
+ unsigned long vaddr = PKMAP_ADDR(pos);
+ pte_t *ptep = &pkmap_page_table[pos];
+ pte_t entry = mk_pte(page, kmap_prot);
+ atomic_t *counter = &pkmap_count[pos];
+
+ VM_BUG_ON(atomic_read(counter) != 0);

- pkmap_count[last_pkmap_nr] = 1;
- set_page_address(page, (void *)vaddr);
+ set_pte_at(&init_mm, vaddr, ptep, entry);
+ if (unlikely(!__set_page_address(page, (void *)vaddr, pos))) {
+ /*
+ * concurrent pkmap_inserts for this page -
+ * the other won the race, release this entry.
+ *
+ * we can still clear the pte without a tlb flush since
+ * it couldn't have been used yet.
+ */
+ pte_clear(&init_mm, vaddr, ptep);
+ VM_BUG_ON(atomic_read(counter) != 0);
+ atomic_set(counter, 2);
+ pkmap_put(counter);
+ vaddr = 0;
+ } else
+ atomic_set(counter, 2);

return vaddr;
}

-void fastcall *kmap_high(struct page *page)
+fastcall void *kmap_high(struct page *page)
{
unsigned long vaddr;
-
- /*
- * For highmem pages, we can't trust "virtual" until
- * after we have the lock.
- *
- * We cannot call this from interrupts, as it may block
- */
- spin_lock(&kmap_lock);
+again:
vaddr = (unsigned long)page_address(page);
+ if (vaddr) {
+ atomic_t *counter = &pkmap_count[PKMAP_NR(vaddr)];
+ if (atomic_inc_not_zero(counter)) {
+ /*
+ * atomic_inc_not_zero implies a (memory) barrier on success
+ * so page address will be reloaded.
+ */
+ unsigned long vaddr2 = (unsigned long)page_address(page);
+ if (likely(vaddr == vaddr2))
+ return (void *)vaddr;
+
+ /*
+ * Oops, we got someone else.
+ *
+ * This can happen if we get preempted after
+ * page_address() and before atomic_inc_not_zero()
+ * and during that preemption this slot is freed and
+ * reused.
+ */
+ pkmap_put(counter);
+ goto again;
+ }
+ }
+
+ vaddr = pkmap_insert(page);
if (!vaddr)
- vaddr = map_new_virtual(page);
- pkmap_count[PKMAP_NR(vaddr)]++;
- BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 2);
- spin_unlock(&kmap_lock);
- return (void*) vaddr;
+ goto again;
+
+ return (void *)vaddr;
}

EXPORT_SYMBOL(kmap_high);

-void fastcall kunmap_high(struct page *page)
+fastcall void kunmap_high(struct page *page)
{
- unsigned long vaddr;
- unsigned long nr;
- int need_wakeup;
-
- spin_lock(&kmap_lock);
- vaddr = (unsigned long)page_address(page);
+ unsigned long vaddr = (unsigned long)page_address(page);
BUG_ON(!vaddr);
- nr = PKMAP_NR(vaddr);
-
- /*
- * A count must never go down to zero
- * without a TLB flush!
- */
- need_wakeup = 0;
- switch (--pkmap_count[nr]) {
- case 0:
- BUG();
- case 1:
- /*
- * Avoid an unnecessary wake_up() function call.
- * The common case is pkmap_count[] == 1, but
- * no waiters.
- * The tasks queued in the wait-queue are guarded
- * by both the lock in the wait-queue-head and by
- * the kmap_lock. As the kmap_lock is held here,
- * no need for the wait-queue-head's lock. Simply
- * test if the queue is empty.
- */
- need_wakeup = waitqueue_active(&pkmap_map_wait);
- }
- spin_unlock(&kmap_lock);
-
- /* do wake-up, if needed, race-free outside of the spin lock */
- if (need_wakeup)
- wake_up(&pkmap_map_wait);
+ pkmap_put(&pkmap_count[PKMAP_NR(vaddr)]);
}

EXPORT_SYMBOL(kunmap_high);
+
#endif

#if defined(HASHED_PAGE_VIRTUAL)
@@ -217,19 +262,13 @@ EXPORT_SYMBOL(kunmap_high);
#define PA_HASH_ORDER 7

/*
- * Describes one page->virtual association
+ * Describes one page->virtual address association.
*/
-struct page_address_map {
+static struct page_address_map {
struct page *page;
void *virtual;
struct list_head list;
-};
-
-/*
- * page_address_map freelist, allocated from page_address_maps.
- */
-static struct list_head page_address_pool; /* freelist */
-static spinlock_t pool_lock; /* protects page_address_pool */
+} page_address_maps[LAST_PKMAP];

/*
* Hash table bucket
@@ -244,91 +283,123 @@ static struct page_address_slot *page_sl
return &page_address_htable[hash_ptr(page, PA_HASH_ORDER)];
}

-void *page_address(struct page *page)
+static void *__page_address(struct page_address_slot *pas, struct page *page)
{
- unsigned long flags;
- void *ret;
- struct page_address_slot *pas;
-
- if (!PageHighMem(page))
- return lowmem_page_address(page);
+ void *ret = NULL;

- pas = page_slot(page);
- ret = NULL;
- spin_lock_irqsave(&pas->lock, flags);
if (!list_empty(&pas->lh)) {
struct page_address_map *pam;

list_for_each_entry(pam, &pas->lh, list) {
if (pam->page == page) {
ret = pam->virtual;
- goto done;
+ break;
}
}
}
-done:
+
+ return ret;
+}
+
+void *page_address(struct page *page)
+{
+ unsigned long flags;
+ void *ret;
+ struct page_address_slot *pas;
+
+ if (!PageHighMem(page))
+ return lowmem_page_address(page);
+
+ pas = page_slot(page);
+ spin_lock_irqsave(&pas->lock, flags);
+ ret = __page_address(pas, page);
spin_unlock_irqrestore(&pas->lock, flags);
return ret;
}

EXPORT_SYMBOL(page_address);

-void set_page_address(struct page *page, void *virtual)
+static int __set_page_address(struct page *page, void *virtual, int pos)
{
+ int ret = 0;
unsigned long flags;
struct page_address_slot *pas;
struct page_address_map *pam;

- BUG_ON(!PageHighMem(page));
+ VM_BUG_ON(!PageHighMem(page));
+ VM_BUG_ON(atomic_read(&pkmap_count[pos]) != 0);
+ VM_BUG_ON(pos < 0 || pos >= LAST_PKMAP);

pas = page_slot(page);
- if (virtual) { /* Add */
- BUG_ON(list_empty(&page_address_pool));
+ pam = &page_address_maps[pos];

- spin_lock_irqsave(&pool_lock, flags);
- pam = list_entry(page_address_pool.next,
- struct page_address_map, list);
- list_del(&pam->list);
- spin_unlock_irqrestore(&pool_lock, flags);
-
- pam->page = page;
- pam->virtual = virtual;
-
- spin_lock_irqsave(&pas->lock, flags);
- list_add_tail(&pam->list, &pas->lh);
- spin_unlock_irqrestore(&pas->lock, flags);
- } else { /* Remove */
- spin_lock_irqsave(&pas->lock, flags);
- list_for_each_entry(pam, &pas->lh, list) {
- if (pam->page == page) {
- list_del(&pam->list);
- spin_unlock_irqrestore(&pas->lock, flags);
- spin_lock_irqsave(&pool_lock, flags);
- list_add_tail(&pam->list, &page_address_pool);
- spin_unlock_irqrestore(&pool_lock, flags);
- goto done;
- }
+ spin_lock_irqsave(&pas->lock, flags);
+ if (virtual) { /* add */
+ VM_BUG_ON(!list_empty(&pam->list));
+
+ if (!__page_address(pas, page)) {
+ pam->page = page;
+ pam->virtual = virtual;
+ list_add_tail(&pam->list, &pas->lh);
+ ret = 1;
+ }
+ } else { /* remove */
+ if (!list_empty(&pam->list)) {
+ list_del_init(&pam->list);
+ ret = 1;
}
- spin_unlock_irqrestore(&pas->lock, flags);
}
-done:
- return;
+ spin_unlock_irqrestore(&pas->lock, flags);
+
+ return ret;
}

-static struct page_address_map page_address_maps[LAST_PKMAP];
+int set_page_address(struct page *page, void *virtual)
+{
+ /*
+ * set_page_address is not supposed to be called when using
+ * hashed virtual addresses.
+ */
+ BUG();
+ return 0;
+}

-void __init page_address_init(void)
+void __init __page_address_init(void)
{
int i;

- INIT_LIST_HEAD(&page_address_pool);
for (i = 0; i < ARRAY_SIZE(page_address_maps); i++)
- list_add(&page_address_maps[i].list, &page_address_pool);
+ INIT_LIST_HEAD(&page_address_maps[i].list);
+
for (i = 0; i < ARRAY_SIZE(page_address_htable); i++) {
INIT_LIST_HEAD(&page_address_htable[i].lh);
spin_lock_init(&page_address_htable[i].lock);
}
- spin_lock_init(&pool_lock);
+}
+
+#elif defined (CONFIG_HIGHMEM) /* HASHED_PAGE_VIRTUAL */
+
+static int __set_page_address(struct page *page, void *virtual, int pos)
+{
+ return set_page_address(page, virtual);
}

#endif /* defined(CONFIG_HIGHMEM) && !defined(WANT_PAGE_VIRTUAL) */
+
+#if defined(CONFIG_HIGHMEM) || defined(HASHED_PAGE_VIRTUAL)
+
+void __init page_address_init(void)
+{
+#ifdef CONFIG_HIGHMEM
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(pkmap_count); i++)
+ atomic_set(&pkmap_count[i], 1);
+#endif
+
+#ifdef HASHED_PAGE_VIRTUAL
+ __page_address_init();
+#endif
+}
+
+#endif


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