linux-kernel - Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in page_cache

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Date:	Thu, 19 Mar 2015 18:14:39 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Neil Brown <neilb@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Sage Weil <sage@...tank.com>, Mark Fasheh <mfasheh@...e.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in
 page_cache_read

On Wed, Mar 18, 2015 at 03:55:28PM +0100, Michal Hocko wrote:
> On Wed 18-03-15 10:44:11, Rik van Riel wrote:
> > On 03/18/2015 10:09 AM, Michal Hocko wrote:
> > > page_cache_read has been historically using page_cache_alloc_cold to
> > > allocate a new page. This means that mapping_gfp_mask is used as the
> > > base for the gfp_mask. Many filesystems are setting this mask to
> > > GFP_NOFS to prevent from fs recursion issues. page_cache_read is,
> > > however, not called from the fs layer 
> > 
> > Is that true for filesystems that have directories in
> > the page cache?
> 
> I haven't found any explicit callers of filemap_fault except for ocfs2
> and ceph and those seem OK to me. Which filesystems you have in mind?

Just about every major filesystem calls filemap_fault through the
.fault callout.

C symbol: filemap_fault

  File           Function            Line
  0 9p/vfs_file.c  <global>             831 .fault = filemap_fault,
  1 9p/vfs_file.c  <global>             838 .fault = filemap_fault,
  2 btrfs/file.c   <global>            2081 .fault = filemap_fault,
  3 cifs/file.c    <global>            3242 .fault = filemap_fault,
  4 ext4/file.c    <global>             215 .fault = filemap_fault,
  5 f2fs/file.c    <global>              93 .fault = filemap_fault,
  6 fuse/file.c    <global>            2062 .fault = filemap_fault,
  7 gfs2/file.c    <global>             498 .fault = filemap_fault,
  8 nfs/file.c     <global>             653 .fault = filemap_fault,
  9 nilfs2/file.c  <global>             128 .fault = filemap_fault,
  a ubifs/file.c   <global>            1536 .fault = filemap_fault,
  b xfs/xfs_file.c <global>            1420 .fault = filemap_fault,


> Btw. how would that work as we already have GFP_KERNEL allocation few
> lines below?

GFP_KERNEL allocation for mappings is simply wrong. All mapping
allocations where the caller cannot pass a gfp_mask need to obey
the mapping_gfp_mask that is set by the mapping owner....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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