[PATCH] Btrfs: make sure there is always room for generation number

[PATCH] Btrfs: make sure there is always room for generation number

[Zhihui Zhang]

io_ctl_set_generation() assumes that the generation number shares
the same page with inline CRCs. Let's make sure this is always true.

Signed-off-by: Zhihui Zhang <zzhsuny@gmail.com>
---
 fs/btrfs/free-space-cache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index d5f80cb300be..9aa0fbf16515 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -300,9 +300,9 @@ static int io_ctl_init(struct btrfs_io_ctl *io_ctl, struct inode *inode,
 	if (btrfs_ino(BTRFS_I(inode)) != BTRFS_FREE_INO_OBJECTID)
 		check_crcs = 1;
 
-	/* Make sure we can fit our crcs into the first page */
+	/* Make sure we can fit our crcs and generation # into the first page */
 	if (write && check_crcs &&
-	    (num_pages * sizeof(u32)) >= PAGE_SIZE)
+	    (num_pages * sizeof(u32) + sizeof(u64)) > PAGE_SIZE)
 		return -ENOSPC;
 
 	memset(io_ctl, 0, sizeof(struct btrfs_io_ctl));
-- 
2.17.1

Re: [PATCH] Btrfs: make sure there is always room for generation number

[David Sterba]

On Mon, Jul 02, 2018 at 08:00:54PM -0400, Zhihui Zhang wrote:
> io_ctl_set_generation() assumes that the generation number shares
> the same page with inline CRCs. Let's make sure this is always true.

Have you found this by code inspection or is it a fix for a bug that can
be reproduced?

Re: [PATCH] Btrfs: make sure there is always room for generation number

[David Sterba]

On Mon, Jul 02, 2018 at 08:00:54PM -0400, Zhihui Zhang wrote:
> io_ctl_set_generation() assumes that the generation number shares
> the same page with inline CRCs. Let's make sure this is always true.
> 
> Signed-off-by: Zhihui Zhang <zzhsuny@gmail.com>

Reviewed-by: David Sterba <dsterba@suse.com>

Re: [PATCH] Btrfs: make sure there is always room for generation number

[Zhihui Zhang]

code inspection. If I understand it correctly, we are off by 4 bytes
because CRC is 32 bits and generation is 64 bits. But this bug might
never hit in practice.

thanks,

-Zhihui

On Wed, Jul 4, 2018 at 11:21 AM, David Sterba <dsterba@suse.cz> wrote:
> On Mon, Jul 02, 2018 at 08:00:54PM -0400, Zhihui Zhang wrote:
>> io_ctl_set_generation() assumes that the generation number shares
>> the same page with inline CRCs. Let's make sure this is always true.
>>
>> Signed-off-by: Zhihui Zhang <zzhsuny@gmail.com>
>
> Reviewed-by: David Sterba <dsterba@suse.com>