Re: bcache super block corruption with non 4k pages

[Stefan Bader]

On 26.07.2016 14:49, Kent Overstreet wrote:
> On Tue, Jul 26, 2016 at 02:32:31PM +0200, Stefan Bader wrote:
>> On 26.07.2016 12:21, Kent Overstreet wrote:
>>> On Tue, Jul 26, 2016 at 11:51:25AM +0200, Stefan Bader wrote:
>>>> On 21.07.2016 10:58, Stefan Bader wrote:
>>>>> I was pointed at the thread which seems to address the same after
>>>>> I wrote most of below text. Did not want to re-write this so please
>>>>> bear with the odd layout.
>>>>>
>>>>> https://www.redhat.com/archives/dm-devel/2016-June/msg00015.html
>>>>>
>>>>> Zhengyuan tries to fix the problem by relocating the superblock on
>>>>> disk. But I am not sure whether there is really any guarantee about
>>>>> how __bread fills data into the buffer_head. What if there is the next
>>>>> odd arch with 128K pages?
>>>>>
>>>>> So below is an attempt to be more generic. Still I don't feel completely
>>>>> happy with the way that a page moves (or is shared) between buffer_head
>>>>> and biovec. What I tried to outline below is to let the register functions
>>>>> allocate bio+biovec memory and use the in-memory sb_cache data to initialize
>>>>> the biovec buffer.
>>>>
>>>> Any opinions here? Also adding LKML as I don't seem to get through moderation on
>>>> dm-devel.
>>>
>>> The correct solution is to rip out the __bread() and just read the superblock by
>>> issuing a bio, the same way all the other IO in bcache is done.
>>>
>>> This is the way it's done in the bcache-dev branch - unfortunately, the patch
>>> that does that in bcache-dev is big and invasive and probably not worth the
>>> hassle to backport:
>>>
>>> https://evilpiepirate.org/git/linux-bcache.git/commit/?h=bcache-dev&id=303eb67bffad57b4d9e71523e7df04bf258e66d1
>>
>> I agree that this looks better and also rather large.
>>>
>>> Probably best to just do something small and localized.
>>>
>> So what did you think about the change I did? It seemed to be ok for 4K and 64K
>> at least and is rather small. And I believe that, compared to Zhengyuan's
>> approach this would have the benefit of not changing the superblock sector. So
>> it would be compatible with previously created superblocks.
> 
> Too ugly to live. Just kmalloc() 4k, allocate a bio on the stack, set it up, and
> submit it with submit_bio_wait(). Use virt_to_page(), don't bother with raw
> pages - you want 4k, not whatever the page size is.
> 

So here is another attempt which does half the proposed changes. And before you
point out that it looks still ugly, let me explain some reasons. The goal for me
would be to have something as small as possible to be acceptable as stable change.
And the part about putting a bio on the stack and using submit_bio_wait: I
believe you meant in read_super to replace the __bread. I was thinking about
that but in the end it seemed to make the change unnecessary big. Whether using
__bread or submit_bio_wait, in both cases and without needing to make more
changes on the write side, read_super has to return the in-memory and on-disk
variant of the superblock. So as long as nothing that is related to __bread is
leaked out of read_super, it is much better than what is there now. And I remain
as small as possible for the delta.

So there is one part of the patch which I find hard to do in a better manner but
is a bit ugly: and that is to sanely free the sb_disk_data blob on all error
paths but not on success when it is assigned to either cache or cached_dev.
Could possibly pass the address of the pointer and then clear it inside the
called functions. Not sure that would be much less strange...


-Stefan


From 391682e2329a6f8608bfc7628b6c8a0afa9a5d98 Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Date: Tue, 26 Jul 2016 18:47:21 +0200
Subject: [PATCH] bcache: read_super: handle architectures with more than 4k
 page size

There is no guarantee that the superblock which __bread returns in
a buffer_head starts at offset 0 when an architecture has bigger
pages than 4k (the used sector size).

This is the attempt to fix this with the minimum amount of change
by having a buffer allocated with kmalloc that holds the superblock
data as it is on disk.
This buffer can then be passed to bch_map_bio which will set up the
bio_vec correctly.

Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
---
 drivers/md/bcache/bcache.h |  2 ++
 drivers/md/bcache/super.c  | 61 ++++++++++++++++++++++++++--------------------
 2 files changed, 36 insertions(+), 27 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 6b420a5..3c48927 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -295,6 +295,7 @@ struct cached_dev {
 	struct cache_sb		sb;
 	struct bio		sb_bio;
 	struct bio_vec		sb_bv[1];
+	void			*sb_disk_data;
 	struct closure		sb_write;
 	struct semaphore	sb_write_mutex;
 
@@ -382,6 +383,7 @@ struct cache {
 	struct cache_sb		sb;
 	struct bio		sb_bio;
 	struct bio_vec		sb_bv[1];
+	void			*sb_disk_data;
 
 	struct kobject		kobj;
 	struct block_device	*bdev;
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index e169739..f017f69 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -62,7 +62,7 @@ struct workqueue_struct *bcache_wq;
 /* Superblock */
 
 static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
-			      struct page **res)
+			      void *sb_data)
 {
 	const char *err;
 	struct cache_sb *s;
@@ -191,8 +191,7 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 	sb->last_mount = get_seconds();
 	err = NULL;
 
-	get_page(bh->b_page);
-	*res = bh->b_page;
+	memcpy(sb_data, bh->b_data, SB_SIZE);
 err:
 	put_bh(bh);
 	return err;
@@ -206,15 +205,15 @@ static void write_bdev_super_endio(struct bio *bio)
 	closure_put(&dc->sb_write);
 }
 
-static void __write_super(struct cache_sb *sb, struct bio *bio)
+static void __write_super(struct cache_sb *sb, struct bio *bio, void *sb_data)
 {
-	struct cache_sb *out = page_address(bio->bi_io_vec[0].bv_page);
+	struct cache_sb *out = sb_data;
 	unsigned i;
 
 	bio->bi_iter.bi_sector	= SB_SECTOR;
 	bio->bi_rw		= REQ_SYNC|REQ_META;
 	bio->bi_iter.bi_size	= SB_SIZE;
-	bch_bio_map(bio, NULL);
+	bch_bio_map(bio, sb_data);
 
 	out->offset		= cpu_to_le64(sb->offset);
 	out->version		= cpu_to_le64(sb->version);
@@ -262,7 +261,7 @@ void bch_write_bdev_super(struct cached_dev *dc, struct closure *parent)
 	bio->bi_private = dc;
 
 	closure_get(cl);
-	__write_super(&dc->sb, bio);
+	__write_super(&dc->sb, bio, dc->sb_disk_data);
 
 	closure_return_with_destructor(cl, bch_write_bdev_super_unlock);
 }
@@ -308,7 +307,7 @@ void bcache_write_super(struct cache_set *c)
 		bio->bi_private = ca;
 
 		closure_get(cl);
-		__write_super(&ca->sb, bio);
+		__write_super(&ca->sb, bio, ca->sb_disk_data);
 	}
 
 	closure_return_with_destructor(cl, bcache_write_super_unlock);
@@ -1045,6 +1044,8 @@ void bch_cached_dev_release(struct kobject *kobj)
 {
 	struct cached_dev *dc = container_of(kobj, struct cached_dev,
 					     disk.kobj);
+
+	kfree(dc->sb_disk_data);
 	kfree(dc);
 	module_put(THIS_MODULE);
 }
@@ -1138,7 +1139,7 @@ static int cached_dev_init(struct cached_dev *dc, unsigned block_size)
 
 /* Cached device - bcache superblock */
 
-static void register_bdev(struct cache_sb *sb, struct page *sb_page,
+static void register_bdev(struct cache_sb *sb, void *sb_disk_data,
 				 struct block_device *bdev,
 				 struct cached_dev *dc)
 {
@@ -1152,9 +1153,8 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
 
 	bio_init(&dc->sb_bio);
 	dc->sb_bio.bi_max_vecs	= 1;
-	dc->sb_bio.bi_io_vec	= dc->sb_bio.bi_inline_vecs;
-	dc->sb_bio.bi_io_vec[0].bv_page = sb_page;
-	get_page(sb_page);
+	dc->sb_bio.bi_io_vec	= &dc->sb_bv[0];
+	dc->sb_disk_data	= sb_disk_data;
 
 	if (cached_dev_init(dc, sb->block_size << 9))
 		goto err;
@@ -1179,6 +1179,7 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
 	return;
 err:
 	pr_notice("error opening %s: %s", bdevname(bdev, name), err);
+	kfree(sb_disk_data);
 	bcache_device_stop(&dc->disk);
 }
 
@@ -1793,8 +1794,7 @@ void bch_cache_release(struct kobject *kobj)
 	for (i = 0; i < RESERVE_NR; i++)
 		free_fifo(&ca->free[i]);
 
-	if (ca->sb_bio.bi_inline_vecs[0].bv_page)
-		put_page(ca->sb_bio.bi_io_vec[0].bv_page);
+	kfree(ca->sb_disk_data);
 
 	if (!IS_ERR_OR_NULL(ca->bdev))
 		blkdev_put(ca->bdev, FMODE_READ|FMODE_WRITE|FMODE_EXCL);
@@ -1838,7 +1838,7 @@ static int cache_alloc(struct cache_sb *sb, struct cache *ca)
 	return 0;
 }
 
-static int register_cache(struct cache_sb *sb, struct page *sb_page,
+static int register_cache(struct cache_sb *sb, void *sb_disk_data,
 				struct block_device *bdev, struct cache *ca)
 {
 	char name[BDEVNAME_SIZE];
@@ -1851,16 +1851,17 @@ static int register_cache(struct cache_sb *sb, struct page *sb_page,
 
 	bio_init(&ca->sb_bio);
 	ca->sb_bio.bi_max_vecs	= 1;
-	ca->sb_bio.bi_io_vec	= ca->sb_bio.bi_inline_vecs;
-	ca->sb_bio.bi_io_vec[0].bv_page = sb_page;
-	get_page(sb_page);
+	ca->sb_bio.bi_io_vec	= &ca->sb_bv[0];
+	ca->sb_disk_data	= sb_disk_data;
 
 	if (blk_queue_discard(bdev_get_queue(ca->bdev)))
 		ca->discard = CACHE_DISCARD(&ca->sb);
 
 	ret = cache_alloc(sb, ca);
-	if (ret != 0)
+	if (ret != 0) {
+		err = "error calling cache_alloc";
 		goto err;
+	}
 
 	if (kobject_add(&ca->kobj, &part_to_dev(bdev->bd_part)->kobj, "bcache")) {
 		err = "error calling kobject_add";
@@ -1883,8 +1884,10 @@ out:
 	kobject_put(&ca->kobj);
 
 err:
-	if (err)
+	if (err) {
 		pr_notice("error opening %s: %s", bdevname(bdev, name), err);
+		kfree(sb_disk_data);
+	}
 
 	return ret;
 }
@@ -1935,13 +1938,14 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 	char *path = NULL;
 	struct cache_sb *sb = NULL;
 	struct block_device *bdev = NULL;
-	struct page *sb_page = NULL;
+	void *sb_disk_data = NULL;
 
 	if (!try_module_get(THIS_MODULE))
 		return -EBUSY;
 
 	if (!(path = kstrndup(buffer, size, GFP_KERNEL)) ||
-	    !(sb = kmalloc(sizeof(struct cache_sb), GFP_KERNEL)))
+	    !(sb = kmalloc(sizeof(struct cache_sb), GFP_KERNEL)) ||
+	    !(sb_disk_data = kmalloc(SB_SIZE, GFP_KERNEL)))
 		goto err;
 
 	err = "failed to open device";
@@ -1967,7 +1971,7 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 	if (set_blocksize(bdev, 4096))
 		goto err_close;
 
-	err = read_super(sb, bdev, &sb_page);
+	err = read_super(sb, bdev, sb_disk_data);
 	if (err)
 		goto err_close;
 
@@ -1977,20 +1981,23 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 			goto err_close;
 
 		mutex_lock(&bch_register_lock);
-		register_bdev(sb, sb_page, bdev, dc);
+		register_bdev(sb, sb_disk_data, bdev, dc);
+		sb_disk_data = NULL; /* Consumed or freed in register call */
 		mutex_unlock(&bch_register_lock);
 	} else {
 		struct cache *ca = kzalloc(sizeof(*ca), GFP_KERNEL);
 		if (!ca)
 			goto err_close;
 
-		if (register_cache(sb, sb_page, bdev, ca) != 0)
+		if (register_cache(sb, sb_disk_data, bdev, ca) != 0) {
+			sb_disk_data = NULL;
 			goto err_close;
+		}
+		sb_disk_data = NULL;
 	}
 out:
-	if (sb_page)
-		put_page(sb_page);
 	kfree(sb);
+	kfree(sb_disk_data);
 	kfree(path);
 	module_put(THIS_MODULE);
 	return ret;
-- 
1.9.1

Attachment: signature.asc
Description: OpenPGP digital signature