Re: [PATCH 1/3] xfs: helper to set flags on uncached buffer reads

From: Brian Foster <bfoster@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com
Subject: Re: [PATCH 1/3] xfs: helper to set flags on uncached buffer reads
Date: Thu, 14 Jul 2016 06:52:47 -0400	[thread overview]
Message-ID: <20160714105247.GA4207@bfoster.bfoster> (raw)
In-Reply-To: <20160714000103.GR1922@dastard>

On Thu, Jul 14, 2016 at 10:01:04AM +1000, Dave Chinner wrote:
> On Wed, Jul 13, 2016 at 12:16:33PM -0400, Brian Foster wrote:
> > xfs_buf_read_uncached() allocates an uncached buffer and performs a read
> > in one go. As part of the upcoming buftarg I/O accounting mechanism,
> > some sites may need to set flags on a buffer before I/O submission.
> > 
> > Create a new helper to support the ability to set flags on a buffer
> > before it is submitted for I/O. This use case is the exception, so
> > create a wrapper for the original xfs_buf_read_uncached().
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> >  fs/xfs/xfs_buf.c | 20 +++++++++++++++++---
> >  fs/xfs/xfs_buf.h |  4 ++++
> >  2 files changed, 21 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index 4665ff6..f007713 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -693,13 +693,14 @@ xfs_buf_readahead_map(
> >   * buffer containing the disk contents or nothing.
> >   */
> >  int
> > -xfs_buf_read_uncached(
> > +xfs_buf_read_uncached_flags(
> >  	struct xfs_buftarg	*target,
> >  	xfs_daddr_t		daddr,
> >  	size_t			numblks,
> >  	int			flags,
> >  	struct xfs_buf		**bpp,
> > -	const struct xfs_buf_ops *ops)
> > +	const struct xfs_buf_ops *ops,
> > +	int			bflags)
> >  {
> 
> We already have a flags field being passed in. Why can't that be
> used to pass the XBF_NO_IOACCT flag? i.e:
> 

Oops, I read over that too quickly as I saw it being passed into
alloc_page() and assumed it was for memory allocation flags. It is in
fact for buffer flags.. we just use xb_to_gfp() to select GFP_* flags
based on the buffer flags. I'll drop this patch and fold the
xfs_buf_read_cached() fixup into patch 2. Thanks.

Brian

> >  	ASSERT(bp->b_map_count == 1);
> >  	bp->b_bn = XFS_BUF_DADDR_NULL;  /* always null for uncached buffers */
> >  	bp->b_maps[0].bm_bn = daddr;
> > -	bp->b_flags |= XBF_READ;
> > +	bp->b_flags |= XBF_READ | bflags;
> 
> 	bp->b_flags |= XBF_READ;
> 	bp->b_flags |= (flags & XBF_NO_IOACCT);
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
> 
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