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

[Calvin Owens <calvinowens@fb.com>]

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@fb.com>
>> Fixes: 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes")
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: <stable@vger.kernel.org> # 4.8+
>> Signed-off-by: Calvin Owens <calvinowens@fb.com>
>> ---
>>  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
>>
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