Re: [PATCH v2] xfs: Honor FALLOC_FL_KEEP_SIZE when punching ends of files

[Calvin Owens]

On 03/21/2017 04:39 AM, Brian Foster wrote:
> On Sun, Mar 19, 2017 at 09:54:51PM -0700, Calvin Owens wrote:
> > When punching past EOF on XFS, fallocate(mode=PUNCH_HOLE|KEEP_SIZE) will
> > round the file size up to the nearest multiple of PAGE_SIZE:
> >
> >   calvinow@vm-disks/generic-xfs-1 ~$ dd if=/dev/urandom of=test bs=2048 count=1
> >   calvinow@vm-disks/generic-xfs-1 ~$ stat test
> >     Size: 2048            Blocks: 8          IO Block: 4096   regular file
> >   calvinow@vm-disks/generic-xfs-1 ~$ fallocate -n -l 2048 -o 2048 -p test
> >   calvinow@vm-disks/generic-xfs-1 ~$ stat test
> >     Size: 4096            Blocks: 8          IO Block: 4096   regular file
> >
> > Commit 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes") replaced
> > xfs_zero_remaining_bytes() with calls to iomap helpers. The new helpers
> > don't enforce that [pos,offset) lies strictly on [0,i_size) when being
> > called from xfs_free_file_space(), so by "leaking" these ranges into
> > xfs_zero_range() we get this buggy behavior.
> >
> > Fix this by reintroducing the checks xfs_zero_remaining_bytes() did
> > against i_size at the bottom of xfs_free_file_space().
> >
> > Reported-by: Aaron Gao <gzh@xxxxxx>
> > Fixes: 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes")
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> # 4.8+
> > Signed-off-by: Calvin Owens <calvinowens@xxxxxx>
> > ---
> >  fs/xfs/xfs_bmap_util.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > index 8b75dce..0796ebc 100644
> > --- a/fs/xfs/xfs_bmap_util.c
> > +++ b/fs/xfs/xfs_bmap_util.c
> > @@ -1309,6 +1309,17 @@ xfs_free_file_space(
> >  	}
> >
> >  	/*
> > +	 * Avoid doing I/O beyond eof - it's not necessary
> > +	 * since nothing can read beyond eof.  The space will
> > +	 * be zeroed when the file is extended anyway.
> > +	 */
>
> I'd suggest to update the comment below with this information and move
> the following bits down below it as well.

Will do.

> > +	if (offset >= XFS_ISIZE(ip))
> > +		return 0;
> > +
> > +	if ((offset + len) >= XFS_ISIZE(ip))
> > +		len = XFS_ISIZE(ip) - offset - 1;
> > +
>
> This looks like an off-by-one. Do you mean the following?
>
> 	if (offset + len > XFS_ISIZE(ip))
> 		len = XFS_ISIZE(ip) - offset;

It's not an off-by-one (it's self-consistent), but your way makes more
sense, I'll fix it ;)

Thanks,
Calvin

> Brian
>
> > +	/*
> >  	 * Now that we've unmap all full blocks we'll have to zero out any
> >  	 * partial block at the beginning and/or end.  xfs_zero_range is
> >  	 * smart enough to skip any holes, including those we just created.
> > --
> > 2.9.3
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> > the body of a message to majordomo@xxxxxxxxxxxxxxx
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html