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 14:57:10 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	NeilBrown <neilb@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	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 02:55:58PM +0100, Michal Hocko wrote:
> @@ -2701,13 +2701,24 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address,
>  {
>  	struct vm_fault vmf;
>  	int ret;
> +	struct address_space *mapping = vma->vm_file->f_mapping;
> +	gfp_t mapping_gfp;
>  
>  	vmf.virtual_address = (void __user *)(address & PAGE_MASK);
>  	vmf.pgoff = pgoff;
>  	vmf.flags = flags;
>  	vmf.page = NULL;
>  
> +	/*
> +	 * Some filesystems always drop __GFP_FS to prevent from reclaim
> +	 * recursion back to FS code. This is not the case here because
> +	 * we are at the top of the call chain. Add GFP_FS flags to prevent
> +	 * from premature OOM killer.
> +	 */
> +	mapping_gfp = mapping_gfp_mask(mapping);
> +	mapping_set_gfp_mask(mapping, mapping_gfp | __GFP_FS | __GFP_IO);
>  	ret = vma->vm_ops->fault(vma, &vmf);
> +	mapping_set_gfp_mask(mapping, mapping_gfp);

Urk! The inode owns the mapping and makes these decisions, not the
page fault path. These mapping flags may be set for reasons you
don't expect or know about (e.g. a subsystem specific shrinker
constraint) so paths like this have no business clearing flags they
don't own.

cheers,

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