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Re: [PATCH] xfs: Fix overallocation in xfs_buf_allocate_memory()

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: Fix overallocation in xfs_buf_allocate_memory()
From: Jan Kara <jack@xxxxxxx>
Date: Wed, 6 Jun 2012 00:16:14 +0200
Cc: Jan Kara <jack@xxxxxxx>, xfs@xxxxxxxxxxx, Ben Myers <bpm@xxxxxxx>, Alex Elder <elder@xxxxxxxxxx>
In-reply-to: <20120605132852.GB22848@dastard>
References: <1338894490-12662-1-git-send-email-jack@xxxxxxx> <20120605132852.GB22848@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue 05-06-12 23:28:52, Dave Chinner wrote:
> On Tue, Jun 05, 2012 at 01:08:10PM +0200, Jan Kara wrote:
> > Commit 0e6e847f which introduced xfs_buf_allocate_memory() function has a 
> > bug
> > causing the function to overestimate the number of necessary pages.
> 
> I don't think that commit is responsible at all - bp->b_bn was not
> used at all originally - it was bp->b_file_offset that was used.
  Yes, sorry, I got confused by that patch.

> > The problem
> > is that xfs_buf_alloc() sets b_bn to -1
> 
> Right, and the change that was made in commit de1cbee (xfs: kill
> b_file_offset) changed that bp->b_file_offset to bp->b_bn, and that
> is where the bug was introduced. This means it's only been present
> in mainline since the 3.5-rc1 XFS merge....
> 
> > and thus effectively every buffer is
> > straddling a page boundary which causes xfs_buf_allocate_memory() to 
> > allocate
> > two pages and use vmalloc() for access which slows things down.
> 
> I did not notice this at all - it didn't cause me any problems or
> slowdowns that I could measure in any benchmark I ran, so I'm
> interested to know how you found it/noticed it....
  By luck ;) I take back the "slow down" part (although obviously the
vmalloc stuff is slower). I was tracking some soft lockup problem with XFS
and vmalloc in SUSE kernel and looked into vanilla sources where I found
this bug. I though it's causing also my problem but as you mention, that
got introduced only recently so it was a false alarm.

> > Fix the code to use correct block number.
> > 
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > ---
> >  fs/xfs/xfs_buf.c |    7 ++++---
> >  1 files changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index 172d3cc..b67cc83 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -296,6 +296,7 @@ xfs_buf_free(
> >  STATIC int
> >  xfs_buf_allocate_memory(
> >     xfs_buf_t               *bp,
> > +   xfs_daddr_t             blkno,
> >     uint                    flags)
> >  {
> >     size_t                  size;
> > @@ -334,8 +335,8 @@ xfs_buf_allocate_memory(
> >     }
> >  
> >  use_alloc_page:
> > -   start = BBTOB(bp->b_bn) >> PAGE_SHIFT;
> > -   end = (BBTOB(bp->b_bn + bp->b_length) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > +   start = BBTOB(blkno) >> PAGE_SHIFT;
> > +   end = (BBTOB(blkno + bp->b_length) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> >     page_count = end - start;
> >     error = _xfs_buf_get_pages(bp, page_count, flags);
> >     if (unlikely(error))
> > @@ -552,7 +553,7 @@ xfs_buf_get(
> >     if (unlikely(!new_bp))
> >             return NULL;
> >  
> > -   error = xfs_buf_allocate_memory(new_bp, flags);
> > +   error = xfs_buf_allocate_memory(new_bp, blkno, flags);
> >     if (error) {
> >             kmem_zone_free(xfs_buf_zone, new_bp);
> >             return NULL;
> 
> While that will fix the problem, I think that I fixed the
> underlying problem that required us to set bp->b_bn to -1 at
> initialisation in that same series that introduced this problem.
> That problem was that we were inserting buffers in a partially
> intialised state into the cache and so we couldn't allow IO to be
> started on them in the case of a lookup race before the final
> initialisation was done. We could detect that case by checking for
> bp->b_bn == -1 at any point in time.
> 
> We now don't insert the new buffer into the cache until it is fully
> initialised, so we don't need to initialise bp->b_bn to -1 anymore -
> it can be intialised to the correct block number, which we already
> pass to xfs_buf_alloc() for the cached case. Hence I think that's
> the better way to solve the problem. If this is done, then the
> xfs_buf_alloc() call in xfs_buf_get_uncached() needs to pass
> XFS_BUF_DADDR_NULL as the blkno rather than 0 as it currently
> does....
  OK, I'll redo the patch as you suggest. Thanks for having a look!

                                                                Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

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