linux-kernel - Re: [PATCH 10/25] Calculate the alloc

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Message-ID: <20090421103709.GR12713@csn.ul.ie>
Date:	Tue, 21 Apr 2009 11:37:10 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 10/25] Calculate the alloc_flags for allocation only
	once

On Tue, Apr 21, 2009 at 07:12:57PM +0900, KOSAKI Motohiro wrote:
> > > > +	/* Avoid recursion of direct reclaim */
> > > > +	if (p->flags & PF_MEMALLOC)
> > > > +		goto nopage;
> > > > +
> > > 
> > > Again. old code doesn't only check PF_MEMALLOC, but also check TIF_MEMDIE.
> > > 
> > 
> > But a direct reclaim will have PF_MEMALLOC set and doesn't care about
> > the value of TIF_MEMDIE with respect to recursion.
> > 
> > There is still a check made for TIF_MEMDIE for setting ALLOC_NO_WATERMARKS
> > in gfp_to_alloc_flags() so that flag is still being taken care of.
> 
> Do you mean this is intentional change?
> I only said there is changelog-less behavior change.
> 

Yes, it's intentional.

> old code is here.
> PF_MEMALLOC and TIF_MEMDIE makes goto nopage. it avoid reclaim.

PF_MEMALLOC avoiding reclaim makes sense but TIF_MEMDIE should be
allowed to reclaim. I called it out a bit better in the changelog now.

> -------------------------------------------------------------------------
> rebalance:
>         if (((p->flags & PF_MEMALLOC) || unlikely(test_thread_flag(TIF_MEMDIE)))
>                         && !in_interrupt()) {
>                 if (!(gfp_mask & __GFP_NOMEMALLOC)) {
> nofail_alloc:
>                         /* go through the zonelist yet again, ignoring mins */
>                         page = get_page_from_freelist(gfp_mask, nodemask, order,
>                                 zonelist, high_zoneidx, ALLOC_NO_WATERMARKS);
>                         if (page)
>                                 goto got_pg;
>                         if (gfp_mask & __GFP_NOFAIL) {
>                                 congestion_wait(WRITE, HZ/50);
>                                 goto nofail_alloc;
>                         }
>                 }
>                 goto nopage;
>         }
> -------------------------------------------------------------------------
> 
> 
> but I don't oppose this change if it is your intentional.
> 

The changelog now reads
=====

Factor out the mapping between GFP and alloc_flags only once. Once factored
out, it only needs to be calculated once but some care must be taken.

[neilb@...e.de says]
As the test:

-       if (((p->flags & PF_MEMALLOC) ||
        unlikely(test_thread_flag(TIF_MEMDIE)))
-                       && !in_interrupt()) {
-               if (!(gfp_mask & __GFP_NOMEMALLOC)) {

has been replaced with a slightly weaker one:

+       if (alloc_flags & ALLOC_NO_WATERMARKS) {

Without care, this would allow recursion into the allocator via direct
reclaim. This patch ensures we do not recurse when PF_MEMALLOC is set
but TF_MEMDIE callers are now allowed to directly reclaim where they
would have been prevented in the past.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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