Re: [PATCH v4 02/14] xfs: always tail align maxlen allocations

[John Garry <john.g.garry@oracle.com>]

On 23/08/2024 17:31, Darrick J. Wong wrote:

sorry for the slow reply...

> On Tue, Aug 13, 2024 at 04:36:26PM +0000, John Garry wrote:
>> From: Dave Chinner <dchinner@redhat.com>
>>>> When we do a large allocation, the core free space allocation code
>> assumes that args->maxlen is aligned to args->prod/args->mod. hence
>> if we get a maximum sized extent allocated, it does not do tail
>> alignment of the extent.
>>
>> However, this assumes that nothing modifies args->maxlen between the
>> original allocation context setup and trimming the selected free
>> space extent to size. This assumption has recently been found to be
>> invalid - xfs_alloc_space_available() modifies args->maxlen in low
>> space situations - and there may be more situations we haven't yet
>> found like this.
>>
>> Force aligned allocation introduces the requirement that extents are
>> correctly tail aligned, resulting in this occasional latent
>> alignment failure to be reclassified from an unimportant curiousity
>> to a must-fix bug.
>>
>> Removing the assumption about args->maxlen allocations always being
>> tail aligned is trivial, and should not impact anything because
>> args->maxlen for inodes with extent size hints configured are
>> already aligned. Hence all this change does it avoid weird corner
>> cases that would have resulted in unaligned extent sizes by always
>> trimming the extent down to an aligned size.
>>
>> Signed-off-by: Dave Chinner <dchinner@redhat.com>
>> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org> [provisional on v1 series comment]
> 
> Still provisional -- neither the original patch author nor the submitter
> have answered my question from June:
> 
> IOWs, we always trim rlen, unless there is no alignment (prod==1) or
> rlen is less than mod.  For a forcealign file, it should never be the
> case that minlen < mod because we'll have returned ENOSPC, right?

For forcealign, mod == 0, so naturally that (minlen < mod) would not 
happen. We want to alloc a multiple of align only, which is in prod.

If we consider minlen < prod, then that should not happen either as we 
would have returned ENOSPC. In xfs_bmap_select_minlen() we rounddown 
blen by args->alignment, and if that is less than the ap->minlen (1), 
i.e. if after rounddown we have 0, then we return ENOSPC for forcealign. 
So then minlen would not be less than prod after selecting minlen in 
xfs_bmap_select_minlen().

I hope that I am answering the question asked...

Thanks,
John

> 
> --D
> 
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>> ---
>>   fs/xfs/libxfs/xfs_alloc.c | 12 +++++-------
>>   1 file changed, 5 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
>> index d559d992c6ef..bf08b9e9d9ac 100644
>> --- a/fs/xfs/libxfs/xfs_alloc.c
>> +++ b/fs/xfs/libxfs/xfs_alloc.c
>> @@ -433,20 +433,18 @@ xfs_alloc_compute_diff(
>>    * Fix up the length, based on mod and prod.
>>    * len should be k * prod + mod for some k.
>>    * If len is too small it is returned unchanged.
>> - * If len hits maxlen it is left alone.
>>    */
>> -STATIC void
>> +static void
>>   xfs_alloc_fix_len(
>> -	xfs_alloc_arg_t	*args)		/* allocation argument structure */
>> +	struct xfs_alloc_arg	*args)
>>   {
>> -	xfs_extlen_t	k;
>> -	xfs_extlen_t	rlen;
>> +	xfs_extlen_t		k;
>> +	xfs_extlen_t		rlen = args->len;
>>   
>>   	ASSERT(args->mod < args->prod);
>> -	rlen = args->len;
>>   	ASSERT(rlen >= args->minlen);
>>   	ASSERT(rlen <= args->maxlen);
>> -	if (args->prod <= 1 || rlen < args->mod || rlen == args->maxlen ||
>> +	if (args->prod <= 1 || rlen < args->mod ||
>>   	    (args->mod == 0 && rlen < args->prod))
>>   		return;
>>   	k = rlen % args->prod;
>> -- 
>> 2.31.1
>>
>>