Re: [PATCH] xfs: Fix rounding in xfs_alloc_fix_len()

[Brian Foster <bfoster@redhat.com>]

On Wed, Jun 04, 2014 at 11:48:13AM +0200, Jan Kara wrote:
> Rounding in xfs_alloc_fix_len() is wrong. As the comment states, the
> result should be a number of a form (k*prod+mod) however due to sign
> mistake the result is different. As a result allocations on raid arrays
> could be misaligned in some cases.
> 
> This also seems to fix occasional assertion failure:
> 	XFS_WANT_CORRUPTED_GOTO(rlen <= flen, error0)
> in xfs_alloc_ag_vextent_size().
> 

Do you happen to have a reproducer for this?

The meaning of args->prod (the structure definition comment calls it the
prod value) is not clear to me. I see that we set it to an extent
size hint if one exists (in xfs_bmap_btalloc()), so I'll go with that.
args->mod then becomes the modulo of the file offset against that
alignment hint.

> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/xfs/xfs_alloc.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_alloc.c b/fs/xfs/xfs_alloc.c
> index c1cf6a336a72..6a0281b16451 100644
> --- a/fs/xfs/xfs_alloc.c
> +++ b/fs/xfs/xfs_alloc.c
> @@ -257,14 +257,12 @@ xfs_alloc_fix_len(

We get here and take the extent length, mod against the alignment and
compare to the mod of the offset. 

>  	k = rlen % args->prod;
>  	if (k == args->mod)
>  		return;
> -	if (k > args->mod) {
> -		if ((int)(rlen = rlen - k - args->mod) < (int)args->minlen)
> -			return;
> -	} else {
> -		if ((int)(rlen = rlen - args->prod - (args->mod - k)) <
> -		    (int)args->minlen)
> -			return;
> -	}
> +	if (k > args->mod)
> +		rlen = rlen - (k - args->mod);

If the length mod is greater than the offset mod, reduce the length by
the delta of the mods.

> +	else
> +		rlen = rlen - args->prod + (args->mod - k);

Otherwise (length mod is less than offset mod), reduce by a full
alignment size and add back the difference to match the offset mod.

This seems correct to me.

> +	if ((int)rlen < (int)args->minlen)
> +		return;
>  	ASSERT(rlen >= args->minlen);
>  	ASSERT(rlen <= args->maxlen);

The rlen >= minlen assert seems kind of pointless here, but what about
changing both instances of these two asserts to the following:

	ASSERT(rlen >= args->minlen && rlen <= args->maxlen);

... and add a new one after the length adjustment along the lines of:

	ASSERT((rlen % args->prod) == args->mod);

Thoughts? Would this have caught the problem you've found earlier?

Brian

>  	args->len = rlen;
> -- 
> 1.8.1.4
> 
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