Re: [PATCH 1/3] btrfs: Continue write in case of can_not_nocow

[Chris Mason]

On Fri, Feb 26, 2016 at 10:41:31AM +0500, Roman Mamedov wrote:
> On Wed, 6 Jan 2016 19:00:17 +0800
> Zhao Lei  wrote:
> 
> > btrfs failed in xfstests btrfs/080 with -o nodatacow.
> > 
> > Can be reproduced by following script:
> >   DEV=/dev/vdg
> >   MNT=/mnt/tmp
> > 
> >   umount $DEV &>/dev/null
> >   mkfs.btrfs -f $DEV
> >   mount -o nodatacow $DEV $MNT
> > 
> >   dd if=/dev/zero of=$MNT/test bs=1 count=2048 &
> >   btrfs subvolume snapshot -r $MNT $MNT/test_snap &
> >   wait
> >   --
> >   We can see dd failed on NO_SPACE.
> > 
> > Reason:
> >   __btrfs_buffered_write should run cow write when no_cow impossible,
> >   and current code is designed with above logic.
> >   But check_can_nocow() have 2 type of return value(0 and <0) on
> >   can_not_no_cow, and current code only continue write on first case,
> >   the second case happened in doing subvolume.
> > 
> > Fix:
> >   Continue write when check_can_nocow() return 0 and <0.
> > 
> > Signed-off-by: Zhao Lei 
> 
> Guys please don't forget about this patch. It solves real problem for people,
> http://www.spinics.net/lists/linux-btrfs/msg51276.html
> http://www.spinics.net/lists/linux-btrfs/msg51819.html
> but it's not in 4.4.1, not in 4.4.2... and now not in 4.4.3

Dave already has it queued for the next merge window.  Thanks!

-chris
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Re: [PATCH 1/3] btrfs: Continue write in case of can_not_nocow

[Roman Mamedov]

On Wed, 6 Jan 2016 19:00:17 +0800
Zhao Lei  wrote:

> btrfs failed in xfstests btrfs/080 with -o nodatacow.
> 
> Can be reproduced by following script:
>   DEV=/dev/vdg
>   MNT=/mnt/tmp
> 
>   umount $DEV &>/dev/null
>   mkfs.btrfs -f $DEV
>   mount -o nodatacow $DEV $MNT
> 
>   dd if=/dev/zero of=$MNT/test bs=1 count=2048 &
>   btrfs subvolume snapshot -r $MNT $MNT/test_snap &
>   wait
>   --
>   We can see dd failed on NO_SPACE.
> 
> Reason:
>   __btrfs_buffered_write should run cow write when no_cow impossible,
>   and current code is designed with above logic.
>   But check_can_nocow() have 2 type of return value(0 and <0) on
>   can_not_no_cow, and current code only continue write on first case,
>   the second case happened in doing subvolume.
> 
> Fix:
>   Continue write when check_can_nocow() return 0 and <0.
> 
> Signed-off-by: Zhao Lei 

Guys please don't forget about this patch. It solves real problem for people,
http://www.spinics.net/lists/linux-btrfs/msg51276.html
http://www.spinics.net/lists/linux-btrfs/msg51819.html
but it's not in 4.4.1, not in 4.4.2... and now not in 4.4.3

> ---
>  fs/btrfs/file.c | 37 +
>  1 file changed, 17 insertions(+), 20 deletions(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 977e715..11fd981 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1516,27 +1516,24 @@ static noinline ssize_t __btrfs_buffered_write(struct 
> file *file,
>  
>   reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
>  
> - if (BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> -  BTRFS_INODE_PREALLOC)) {
> - ret = check_can_nocow(inode, pos, _bytes);
> - if (ret < 0)
> - break;
> - if (ret > 0) {
> - /*
> -  * For nodata cow case, no need to reserve
> -  * data space.
> -  */
> - only_release_metadata = true;
> - /*
> -  * our prealloc extent may be smaller than
> -  * write_bytes, so scale down.
> -  */
> - num_pages = DIV_ROUND_UP(write_bytes + offset,
> -  PAGE_CACHE_SIZE);
> - reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
> - goto reserve_metadata;
> - }
> + if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
> +   BTRFS_INODE_PREALLOC)) &&
> + check_can_nocow(inode, pos, _bytes) > 0) {
> + /*
> +  * For nodata cow case, no need to reserve
> +  * data space.
> +  */
> + only_release_metadata = true;
> + /*
> +  * our prealloc extent may be smaller than
> +  * write_bytes, so scale down.
> +  */
> + num_pages = DIV_ROUND_UP(write_bytes + offset,
> +  PAGE_CACHE_SIZE);
> + reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
> + goto reserve_metadata;
>   }
> +
>   ret = btrfs_check_data_free_space(inode, pos, write_bytes);
>   if (ret < 0)
>   break;


-- 
With respect,
Roman


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