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

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Date:	Fri, 20 Mar 2015 14:48:20 +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 Thu, Mar 19, 2015 at 01:44:41PM +0100, Michal Hocko wrote:
> On Thu 19-03-15 18:14:39, Dave Chinner wrote:
> > 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.
> 
> That is right but the callback is called from the VM layer where we
> obviously do not take any fs locks (we are holding only mmap_sem
> for reading).
> Those who call filemap_fault directly (ocfs2 and ceph) and those
> who call the callback directly: qxl_ttm_fault, radeon_ttm_fault,
> kernfs_vma_fault, shm_fault seem to be safe from the reclaim recursion
> POV. radeon_ttm_fault takes a lock for reading but that one doesn't seem
> to be used from the reclaim context.
> 
> Or did I miss your point? Are you concerned about some fs overloading
> filemap_fault and do some locking before delegating to filemap_fault?

The latter:

https://git.kernel.org/cgit/linux/kernel/git/dgc/linux-xfs.git/commit/?h=xfs-mmap-lock&id=de0e8c20ba3a65b0f15040aabbefdc1999876e6b

> > 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....
> 
> Hmm, I thought this is true only when the function might be called from
> the fs path.

How do you know in, say, mpage_readpages, you aren't being called
from a fs path that holds locks? e.g. we can get there from ext4
doing readdir, so it is holding an i_mutex lock at that point.

Many other paths into mpages_readpages don't hold locks, but there
are some that do, and those that do need functionals like this to
obey the mapping_gfp_mask because it is set appropriately for the
allocation context of the inode that owns the mapping....

Cheers,

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