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

[Greg KH <greg@kroah.com>]

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