linux-kernel - Re: [PATCHv2 2/5] vmscan: Kill hibernation specific reclaim logic and unify it

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Date:	Mon, 2 Nov 2009 20:03:52 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	"linux-mm" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCHv2 2/5] vmscan: Kill hibernation specific reclaim logic and unify it

On Monday 02 November 2009, KOSAKI Motohiro wrote:
> > > Then, This patch changed shrink_all_memory() to only the wrapper function of 
> > > do_try_to_free_pages(). it bring good reviewability and debuggability, and solve 
> > > above problems.
> > > 
> > > side note: Reclaim logic unificication makes two good side effect.
> > >  - Fix recursive reclaim bug on shrink_all_memory().
> > >    it did forgot to use PF_MEMALLOC. it mean the system be able to stuck into deadlock.
> > >  - Now, shrink_all_memory() got lockdep awareness. it bring good debuggability.
> > 
> > As I said previously, I don't really see a reason to keep shrink_all_memory().
> > 
> > Do you think that removing it will result in performance degradation?
> 
> Hmm...
> Probably, I misunderstood your mention. I thought you suggested to kill
> all hibernation specific reclaim code. I did. It's no performance degression.
> (At least, I didn't observe)
> 
> But, if you hope to kill shrink_all_memory() function itsef, the short answer is,
> it's impossible.
> 
> Current VM reclaim code need some preparetion to caller, and there are existing in
> both alloc_pages_slowpath() and try_to_free_pages(). We can't omit its preparation.

Well, my grepping for 'shrink_all_memory' throughout the entire kernel source
code seems to indicate that hibernate_preallocate_memory() is the only current
user of it.  I may be wrong, but I doubt it, unless some new users have been
added since 2.6.31.

In case I'm not wrong, it should be safe to drop it from
hibernate_preallocate_memory(), because it's there for performance reasons
only.  Now, since hibernate_preallocate_memory() appears to be the only user of
it, it should be safe to drop it entirely.

> Please see following shrink_all_memory() code. it's pretty small. it only have
> few vmscan preparation. I don't think it is hard to maintainance.

No, it's not, but I'm really not sure it's worth keeping.

Thanks,
Rafael


> =====================================================
> unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
> {
>         struct reclaim_state reclaim_state;
>         struct scan_control sc = {
>                 .gfp_mask = GFP_HIGHUSER_MOVABLE,
>                 .may_swap = 1,
>                 .may_unmap = 1,
>                 .may_writepage = 1,
>                 .nr_to_reclaim = nr_to_reclaim,
>                 .hibernation_mode = 1,
>                 .swappiness = vm_swappiness,
>                 .order = 0,
>                 .isolate_pages = isolate_pages_global,
>         };
>         struct zonelist * zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
>         struct task_struct *p = current;
>         unsigned long nr_reclaimed;
> 
>         p->flags |= PF_MEMALLOC;
>         lockdep_set_current_reclaim_state(sc.gfp_mask);
>         reclaim_state.reclaimed_slab = 0;
>         p->reclaim_state = &reclaim_state;
> 
>         nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
> 
>         p->reclaim_state = NULL;
>         lockdep_clear_current_reclaim_state();
>         p->flags &= ~PF_MEMALLOC;
> 
>         return nr_reclaimed;
> }
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