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Re: [PATCH] xfs: flush the range before zero range conversion

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH] xfs: flush the range before zero range conversion
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 25 Sep 2014 22:01:55 +1000
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1411585591-55975-1-git-send-email-bfoster@xxxxxxxxxx>
References: <1411585591-55975-1-git-send-email-bfoster@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Sep 24, 2014 at 03:06:31PM -0400, Brian Foster wrote:
> XFS currently discards delalloc blocks within the target range of a zero
> range request. Unaligned start and end offsets are zeroed through the
> page cache and the internal, aligned blocks are converted to unwritten
> extents.
> 
> If EOF is page aligned and covered by a delayed allocation extent. The
> inode size is not updated until I/O completion. If a zero range request
> discards a delalloc range that covers page aligned EOF as such, the
> inode size update never occurs. For example:
> 
> $ rm -f /mnt/file
> $ xfs_io -fc "pwrite 0 64k" -c "zero 60k 4k" /mnt/file
> $ stat -c "%s" /mnt/file
> 65536
> $ umount /mnt
> $ mount <dev> /mnt
> $ stat -c "%s" /mnt/file
> 61440
> 
> Update xfs_zero_file_space() to flush the range rather than discard
> delalloc blocks to ensure that inode size updates occur appropriately.
> 
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
> 
> I suppose we could be more clever here and only flush the range in this
> particular scenario, but I'm not sure if there's a major benefit there.

Punching the delalloc range rather than flushing the file
was done intentionally - this was added primarily for speeding up
the zeroing of large VM image files. i.e. it's an extent
manipulation operation rather than a data Io operation. Flushing the
file defeats the primary reason for the operation existing.

We can easily detect this situation and just zero the last block in
the file directly after punching out all the delalloc state. This
should happen anyway when the region to be zeroed is not page
aligned....

> FWIW, this implicitly addresses the indlen==0 assert failures described
> in the xfs_bmap_del_extent() rfc, but doesn't necessarily mean we
> shouldn't fix that code IMO.

We punch delalloc extents elsewhere, so that still needs fixing.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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