Re: [PATCH stable-5.4 6/6] btrfs: allow btrfs_truncate_block() to fallback to nocow for data space reservation

[Greg KH]

On Thu, Oct 08, 2020 at 06:59:54PM +0800, Anand Jain wrote:
> From: Qu Wenruo <wqu@xxxxxxxx>
> 
> commit 6d4572a9d71d5fc2affee0258d8582d39859188c upstream.
> 
> [BUG]
> When the data space is exhausted, even if the inode has NOCOW attribute,
> we will still refuse to truncate unaligned range due to ENOSPC.
> 
> The following script can reproduce it pretty easily:
>   #!/bin/bash
> 
>   dev=/dev/test/test
>   mnt=/mnt/btrfs
> 
>   umount $dev &> /dev/null
>   umount $mnt &> /dev/null
> 
>   mkfs.btrfs -f $dev -b 1G
>   mount -o nospace_cache $dev $mnt
>   touch $mnt/foobar
>   chattr +C $mnt/foobar
> 
>   xfs_io -f -c "pwrite -b 4k 0 4k" $mnt/foobar > /dev/null
>   xfs_io -f -c "pwrite -b 4k 0 1G" $mnt/padding &> /dev/null
>   sync
> 
>   xfs_io -c "fpunch 0 2k" $mnt/foobar
>   umount $mnt
> 
> Currently this will fail at the fpunch part.
> 
> [CAUSE]
> Because btrfs_truncate_block() always reserves space without checking
> the NOCOW attribute.
> 
> Since the writeback path follows NOCOW bit, we only need to bother the
> space reservation code in btrfs_truncate_block().
> 
> [FIX]
> Make btrfs_truncate_block() follow btrfs_buffered_write() to try to
> reserve data space first, and fall back to NOCOW check only when we
> don't have enough space.
> 
> Such always-try-reserve is an optimization introduced in
> btrfs_buffered_write(), to avoid expensive btrfs_check_can_nocow() call.
> 
> This patch will export check_can_nocow() as btrfs_check_can_nocow(), and
> use it in btrfs_truncate_block() to fix the problem.
> 
> Reported-by: Martin Doucha <martin.doucha@xxxxxxxx>
> Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
> Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> Reviewed-by: David Sterba <dsterba@xxxxxxxx>
> Signed-off-by: David Sterba <dsterba@xxxxxxxx>
> Signed-off-by: Anand Jain <anand.jain@xxxxxxxxxx>
>  Conflicts:
> 	fs/btrfs/ctree.h
> 	fs/btrfs/file.c
> 	fs/btrfs/inode.c

Why are these Conflicts: lines here?

Anyway, fixed up, and all queued up now, thansk!

greg k-h