linux-kernel - Re: [PATCH] mm/vmscan: Do not block forever at shrink_inactive

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Message-ID: <20140520063330.GI18954@dastard>
Date:	Tue, 20 May 2014 16:33:30 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, riel@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	kamezawa.hiroyu@...fujitsu.com, linux-kernel@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [PATCH] mm/vmscan: Do not block forever at
 shrink_inactive_list().

On Mon, May 19, 2014 at 11:03:11PM -0700, Andrew Morton wrote:
> On Mon, 19 May 2014 22:59:15 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Tue, 20 May 2014 10:44:49 +1000 Dave Chinner <david@...morbit.com> wrote:
> > 
> > > @@ -258,14 +258,23 @@ xfs_bmapi_allocate_worker(
> > >  	struct xfs_bmalloca	*args = container_of(work,
> > >  						struct xfs_bmalloca, work);
> > >  	unsigned long		pflags;
> > > +	unsigned long		new_pflags = PF_FSTRANS;
> > >  
> > > -	/* we are in a transaction context here */
> > > -	current_set_flags_nested(&pflags, PF_FSTRANS);
> > > +	/*
> > > +	 * we are in a transaction context here, but may also be doing work
> > > +	 * in kswapd context, and hence we may need to inherit that state
> > > +	 * temporarily to ensure that we don't block waiting for memory reclaim
> > > +	 * in any way.
> > > +	 */
> > > +	if (args->kswapd)
> > > +		new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> > 
> > So current_is_kswapd() returns true for a thread which is not kswapd. 
> > That's a bit smelly.
> > 
> > Should this thread really be incrementing KSWAPD_INODESTEAL instead of
> > PGINODESTEAL, for example?  current_is_kswapd() does a range of things,
> > only one(?) of which you actually want.
> > 
> > It would be cleaner to create a new PF_ flag to select just that
> > behavior.  That's a better model than telling the world "I am magic and
> > special".
> 
> Or a new __GFP_FLAG.

Sure - and with that XFS will need another PF_ flag to tell the
memory allocator to set the new __GFP_FLAG on every allocation done
in that kworker task context, just like it uses PF_FSTRANS to ensure
that __GFP_NOFS is set for all the allocations in that kworker
context....

Cheers,

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