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Re: [PATCH 3/6] xfs: bulkstat chunk-formatter has issues

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/6] xfs: bulkstat chunk-formatter has issues
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Tue, 4 Nov 2014 13:58:49 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1415105601-6455-4-git-send-email-david@xxxxxxxxxxxxx>
References: <1415105601-6455-1-git-send-email-david@xxxxxxxxxxxxx> <1415105601-6455-4-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.23 (2014-03-12)
On Tue, Nov 04, 2014 at 11:53:18PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> The loop construct has issues:
>       - clustidx is completely unused, so remove it.
>       - the loop tries to be smart by terminating when the
>         "freecount" tells it that all inodes are free. Just drop
>         it as in most cases we have to scan all inodes in the
>         chunk anyway.
>       - move the "user buffer left" condition check to the only
>         point where we consume space int eh user buffer.
>       - move the initialisation of agino out of the loop, leaving
>         just a simple loop control logic using the clusteridx.
> 
> Also, double handling of the user buffer variables leads to problems
> tracking the current state - use the cursor variables directly
> rather than keeping local copies and then having to update the
> cursor before returning.
> 
> cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_itable.c | 55 
> ++++++++++++++++++++---------------------------------
>  1 file changed, 21 insertions(+), 34 deletions(-)
> 
> diff --git a/fs/xfs/xfs_itable.c b/fs/xfs/xfs_itable.c
> index 50a3e59..ff31965 100644
> --- a/fs/xfs/xfs_itable.c
> +++ b/fs/xfs/xfs_itable.c
> @@ -283,59 +283,46 @@ xfs_bulkstat_ag_ichunk(
>       xfs_ino_t                       *lastino)
>  {
>       char                            __user **ubufp = acp->ac_ubuffer;
> -     int                             ubleft = acp->ac_ubleft;
> -     int                             ubelem = acp->ac_ubelem;
> -     int                             chunkidx, clustidx;
> +     int                             chunkidx;
>       int                             error = 0;
>       xfs_agino_t                     agino;
>  
> -     for (agino = irbp->ir_startino, chunkidx = clustidx = 0;
> -          XFS_BULKSTAT_UBLEFT(ubleft) &&
> -          irbp->ir_freecount < XFS_INODES_PER_CHUNK;
> -          chunkidx++, clustidx++, agino++) {
> -             int             fmterror;       /* bulkstat formatter result */
> +     agino = irbp->ir_startino;
> +     for (chunkidx = 0; chunkidx < XFS_INODES_PER_CHUNK;
> +          chunkidx++, agino++) {
> +             int             fmterror;
>               int             ubused;
>               xfs_ino_t       ino = XFS_AGINO_TO_INO(mp, agno, agino);
>  
> -             ASSERT(chunkidx < XFS_INODES_PER_CHUNK);
> -
>               /* Skip if this inode is free */
>               if (XFS_INOBT_MASK(chunkidx) & irbp->ir_free) {
>                       *lastino = ino;
>                       continue;
>               }
>  
> -             /*
> -              * Count used inodes as free so we can tell when the
> -              * chunk is used up.
> -              */
> -             irbp->ir_freecount++;
> -
>               /* Get the inode and fill in a single buffer */
>               ubused = statstruct_size;
> -             error = formatter(mp, ino, *ubufp, ubleft, &ubused, &fmterror);
> -             if (fmterror == BULKSTAT_RV_NOTHING) {
> -                     if (error && error != -ENOENT && error != -EINVAL) {
> -                             ubleft = 0;
> -                             break;
> -                     }
> -                     *lastino = ino;
> -                     continue;
> -             }
> -             if (fmterror == BULKSTAT_RV_GIVEUP) {
> -                     ubleft = 0;
> +             error = formatter(mp, ino, *ubufp, acp->ac_ubleft,
> +                               &ubused, &fmterror);
> +             if (fmterror == BULKSTAT_RV_GIVEUP ||
> +                 (error && error != -ENOENT && error != -EINVAL)) {
> +                     acp->ac_ubleft = 0;
>                       ASSERT(error);
>                       break;
>               }
> -             if (*ubufp)
> -                     *ubufp += ubused;
> -             ubleft -= ubused;
> -             ubelem++;
> +             if (fmterror == BULKSTAT_RV_NOTHING || error) {
> +                     *lastino = ino;
> +                     continue;
> +             }

Not introduced by this patch, but this looks like inconsistent error
handling. The structure of the code suggests we intend to carry on in
the event of ENOENT or EINVAL, but said errors can leak out of this
function if we never get back to the formatter call (e.g., we're at the
last inode or all subsequent inodes are free).

Brian

> +
> +             *ubufp += ubused;
> +             acp->ac_ubleft -= ubused;
> +             acp->ac_ubelem++;
>               *lastino = ino;
> -     }
>  
> -     acp->ac_ubleft = ubleft;
> -     acp->ac_ubelem = ubelem;
> +             if (acp->ac_ubleft < statstruct_size)
> +                     break;
> +     }
>  
>       return error;
>  }
> -- 
> 2.0.0
> 
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