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From: Roman Gushchin <guro@fb.com>
To: Joonsoo Kim <js1304@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, Rik van Riel <riel@surriel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
	Qian Cai <cai@lca.pw>, Mel Gorman <mgorman@techsingularity.net>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations
Date: Thu, 12 Mar 2020 10:07:15 -0700	[thread overview]
Message-ID: <20200312170715.GA5764@carbon.DHCP.thefacebook.com> (raw)
In-Reply-To: <CAAmzW4MMV8jgboSgHizUoH6wuuztCTJRY7AGN95869rrfH++=Q@mail.gmail.com>

On Thu, Mar 12, 2020 at 05:56:34PM +0900, Joonsoo Kim wrote:
> 2020년 3월 12일 (목) 오전 11:40, Roman Gushchin <guro@fb.com>님이 작성:
> >
> > On Thu, Mar 12, 2020 at 10:41:28AM +0900, Joonsoo Kim wrote:
> > > Hello, Roman.
> >
> > Hello, Joonsoo!
> >
> > >
> > > 2020년 3월 12일 (목) 오전 2:35, Roman Gushchin <guro@fb.com>님이 작성:
> > > >
> > > > On Wed, Mar 11, 2020 at 09:51:07AM +0100, Vlastimil Babka wrote:
> > > > > On 3/6/20 9:01 PM, Rik van Riel wrote:
> > > > > > Posting this one for Roman so I can deal with any upstream feedback and
> > > > > > create a v2 if needed, while scratching my head over the next piece of
> > > > > > this puzzle :)
> > > > > >
> > > > > > ---8<---
> > > > > >
> > > > > > From: Roman Gushchin <guro@fb.com>
> > > > > >
> > > > > > Currently a cma area is barely used by the page allocator because
> > > > > > it's used only as a fallback from movable, however kswapd tries
> > > > > > hard to make sure that the fallback path isn't used.
> > > > >
> > > > > Few years ago Joonsoo wanted to fix these kinds of weird MIGRATE_CMA corner
> > > > > cases by using ZONE_MOVABLE instead [1]. Unfortunately it was reverted due to
> > > > > unresolved bugs. Perhaps the idea could be resurrected now?
> > > >
> > > > Hi Vlastimil!
> > > >
> > > > Thank you for this reminder! I actually looked at it and also asked Joonsoo in private
> > > > about the state of this patch(set). As I understand, Joonsoo plans to resubmit
> > > > it later this year.
> > > >
> > > > What Rik and I are suggesting seems to be much simpler, however it's perfectly
> > > > possible that Joonsoo's solution is preferable long-term.
> > > >
> > > > So if the proposed patch looks ok for now, I'd suggest to go with it and return
> > > > to this question once we'll have a new version of ZONE_MOVABLE solution.
> > >
> > > Hmm... utilization is not the only matter for CMA user. The more
> > > important one is
> > > success guarantee of cma_alloc() and this patch would have a bad impact on it.
> > >
> > > A few years ago, I have tested this kind of approach and found that increasing
> > > utilization increases cma_alloc() failure. Reason is that the page
> > > allocated with
> > > __GFP_MOVABLE, especially, by sb_bread(), is sometimes pinned by someone.
> > >
> > > Until now, cma memory isn't used much so this problem doesn't occur easily.
> > > However, with this patch, it would happen.
> >
> > Sure, but the whole point of cma is to be able to use the cma area
> > effectively by movable pages. Otherwise we can just reserve it and
> > have 100% reliability.
> 
> I agree with that cma area should be used effectively. However, cma_alloc()
> failure is functional failure in embedded system so we need to approach
> this problem more carefully. At least, to control the behaviour, configurable
> option (default is current behaviour) would be necessary.

Do we know who can test it? Adding a configuration option is a last resort
measure, I really hope we can avoid it.

> 
> > So I'd focus on fixing page migration issues, rather than trying
> > to keep it empty most of the time.
> 
> Great! Fixing page migration issue is always good thing!
> 
> > Btw, I've fixed two issues, which dramatically increased the success
> > rate of 1 GB page allocation in my case:
> >
> > https://patchwork.kernel.org/patch/11420997/
> > https://lore.kernel.org/patchwork/patch/1202868/
> >
> > (They both are on the way to upstream, but not there yet)
> >
> > Can you, please, pull them and try?
> 
> Unfortunately, I lose my test setup for this problem so cannot try it now.
> I will try it as soon as possible.

Thanks! Looking forward to it...

> 
> Anyway, AFAIR, I saw the problem while journal is continually working
> on ext4. Have you checked this case?

My ext4 fix sounds very similar to what you're describing, but it's hard to
say for sure.

Thanks!


  reply	other threads:[~2020-03-12 17:07 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-06 20:01 [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations Rik van Riel
2020-03-07 22:38 ` Andrew Morton
2020-03-08 13:23   ` Rik van Riel
2020-03-11 17:58     ` Vlastimil Babka
2020-03-11 22:58       ` Roman Gushchin
2020-03-11 23:03         ` Vlastimil Babka
2020-03-11 23:21           ` Roman Gushchin
2020-03-11  8:51 ` Vlastimil Babka
2020-03-11 10:13   ` Joonsoo Kim
2020-03-11 17:41     ` Vlastimil Babka
2020-03-11 17:35   ` Roman Gushchin
2020-03-12  1:41     ` Joonsoo Kim
2020-03-12  2:39       ` Roman Gushchin
2020-03-12  8:56         ` Joonsoo Kim
2020-03-12 17:07           ` Roman Gushchin [this message]
2020-03-13  7:44             ` Joonsoo Kim
2020-04-02  2:13       ` Andrew Morton
2020-04-02  2:53         ` Roman Gushchin
2020-04-02  5:43           ` Joonsoo Kim
2020-04-02 19:42             ` Roman Gushchin
2020-04-03  4:34               ` Joonsoo Kim
2020-04-03 17:50                 ` Roman Gushchin
2020-04-02  3:05         ` Roman Gushchin
2020-03-21  0:49 ` Minchan Kim

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