[PATCH stable-5.15.y] btrfs: force v2 space cache usage for subpage mount

[PATCH stable-5.15.y] btrfs: force v2 space cache usage for subpage mount

[Qu Wenruo]

commit 9f73f1aef98b2fa7252c0a89be64840271ce8ea0 upstream.

[BUG]
For a 4K sector sized btrfs with v1 cache enabled and only mounted on
systems with 4K page size, if it's mounted on subpage (64K page size)
systems, it can cause the following warning on v1 space cache:

 BTRFS error (device dm-1): csum mismatch on free space cache
 BTRFS warning (device dm-1): failed to load free space cache for block group 84082688, rebuilding it now

Although not a big deal, as kernel can rebuild it without problem, such
warning will bother end users, especially if they want to switch the
same btrfs seamlessly between different page sized systems.

[CAUSE]
V1 free space cache is still using fixed PAGE_SIZE for various bitmap,
like BITS_PER_BITMAP.

Such hard-coded PAGE_SIZE usage will cause various mismatch, from v1
cache size to checksum.

Thus kernel will always reject v1 cache with a different PAGE_SIZE with
csum mismatch.

[FIX]
Although we should fix v1 cache, it's already going to be marked
deprecated soon.

And we have v2 cache based on metadata (which is already fully subpage
compatible), and it has almost everything superior than v1 cache.

So just force subpage mount to use v2 cache on mount.

Reported-by: Matt Corallo <blnxfsl@bluematt.me>
CC: stable@vger.kernel.org # 5.15+
Link: https://lore.kernel.org/linux-btrfs/61aa27d1-30fc-c1a9-f0f4-9df544395ec3@bluematt.me/
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/disk-io.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index f1f7dbfa6ecd..8cbed2f08d1b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3415,6 +3415,17 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	}
 
 	if (sectorsize != PAGE_SIZE) {
+		/*
+		 * V1 space cache has some hardcoded PAGE_SIZE usage, and is
+		 * going to be deprecated.
+		 *
+		 * Force to use v2 cache for subpage case.
+		 */
+		btrfs_clear_opt(fs_info->mount_opt, SPACE_CACHE);
+		btrfs_set_and_info(fs_info, FREE_SPACE_TREE,
+			"forcing free space tree for sector size %u with page size %lu",
+			sectorsize, PAGE_SIZE);
+
 		btrfs_warn(fs_info,
 		"read-write for sector size %u with page size %lu is experimental",
 			   sectorsize, PAGE_SIZE);
-- 
2.36.0

Re: [PATCH stable-5.15.y] btrfs: force v2 space cache usage for subpage mount

[Greg KH]

On Mon, May 09, 2022 at 05:28:56PM +0800, Qu Wenruo wrote:
> commit 9f73f1aef98b2fa7252c0a89be64840271ce8ea0 upstream.
> 
> [BUG]
> For a 4K sector sized btrfs with v1 cache enabled and only mounted on
> systems with 4K page size, if it's mounted on subpage (64K page size)
> systems, it can cause the following warning on v1 space cache:
> 
>  BTRFS error (device dm-1): csum mismatch on free space cache
>  BTRFS warning (device dm-1): failed to load free space cache for block group 84082688, rebuilding it now
> 
> Although not a big deal, as kernel can rebuild it without problem, such
> warning will bother end users, especially if they want to switch the
> same btrfs seamlessly between different page sized systems.
> 
> [CAUSE]
> V1 free space cache is still using fixed PAGE_SIZE for various bitmap,
> like BITS_PER_BITMAP.
> 
> Such hard-coded PAGE_SIZE usage will cause various mismatch, from v1
> cache size to checksum.
> 
> Thus kernel will always reject v1 cache with a different PAGE_SIZE with
> csum mismatch.
> 
> [FIX]
> Although we should fix v1 cache, it's already going to be marked
> deprecated soon.
> 
> And we have v2 cache based on metadata (which is already fully subpage
> compatible), and it has almost everything superior than v1 cache.
> 
> So just force subpage mount to use v2 cache on mount.
> 
> Reported-by: Matt Corallo <blnxfsl@bluematt.me>
> CC: stable@vger.kernel.org # 5.15+
> Link: https://lore.kernel.org/linux-btrfs/61aa27d1-30fc-c1a9-f0f4-9df544395ec3@bluematt.me/
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/disk-io.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)

Now queued up, thanks.

greg k-h