linux-kernel - Re: [PATCH] mm,page_alloc: Update comment for last second allocation attempt.

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Message-Id: <201711032327.HGC39566.MLFQSFtOOJHFVO@I-love.SAKURA.ne.jp>
Date:   Fri, 3 Nov 2017 23:27:57 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, aarcange@...hat.com,
        hannes@...xchg.org
Subject: Re: [PATCH] mm,page_alloc: Update comment for last second allocation attempt.

Michal Hocko wrote:
> On Fri 03-11-17 23:08:35, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Fri 03-11-17 22:46:29, Tetsuo Handa wrote:
> > > [...]
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index c274960..547e9cb 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -3312,11 +3312,10 @@ void warn_alloc(gfp_t gfp_mask, nodemask_t *nodemask, const char *fmt, ...)
> > > >  	}
> > > >  
> > > >  	/*
> > > > -	 * Go through the zonelist yet one more time, keep very high watermark
> > > > -	 * here, this is only to catch a parallel oom killing, we must fail if
> > > > -	 * we're still under heavy pressure. But make sure that this reclaim
> > > > -	 * attempt shall not depend on __GFP_DIRECT_RECLAIM && !__GFP_NORETRY
> > > > -	 * allocation which will never fail due to oom_lock already held.
> > > > +	 * This allocation attempt must not depend on __GFP_DIRECT_RECLAIM &&
> > > > +	 * !__GFP_NORETRY allocation which will never fail due to oom_lock
> > > > +	 * already held. And since this allocation attempt does not sleep,
> > > > +	 * there is no reason we must use high watermark here.
> > > >  	 */
> > > >  	page = get_page_from_freelist((gfp_mask | __GFP_HARDWALL) &
> > > >  				      ~__GFP_DIRECT_RECLAIM, order,
> > > 
> > > Which patch does this depend on?
> > 
> > This patch is preparation for "mm,oom: Move last second allocation to inside
> > the OOM killer." patch in order to use changelog close to what you suggested.
> > That is, I will move this comment and get_page_from_freelist() together to
> > alloc_pages_before_oomkill(), after we recorded why using ALLOC_WMARK_HIGH.
> 
> Is it really worth a separate patch, though? Aren't you overcomplicating
> things again?

It is really worth a separate patch, for you don't want to include the paragraph
below into "mm,oom: Move last second allocation to inside the OOM killer." patch

  > __alloc_pages_may_oom() is doing last second allocation attempt using
  > ALLOC_WMARK_HIGH before calling out_of_memory(). This had two reasons.
  > 
  > The first reason is explained in the comment that it aims to catch
  > potential parallel OOM killing. But there is no longer parallel OOM
  > killing (in the sense that out_of_memory() is called "concurrently")
  > because we serialize out_of_memory() calls using oom_lock.
  > 
  > The second reason is explained by Andrea Arcangeli (who added that code)
  > that it aims to reduce the likelihood of OOM livelocks and be sure to
  > invoke the OOM killer. There was a risk of livelock or anyway of delayed
  > OOM killer invocation if ALLOC_WMARK_MIN is used, for relying on last
  > few pages which are constantly allocated and freed in the meantime will
  > not improve the situation.
  
  > But there is no longer possibility of OOM
  > livelocks or failing to invoke the OOM killer because we need to mask
  > __GFP_DIRECT_RECLAIM for last second allocation attempt because oom_lock
  > prevents __GFP_DIRECT_RECLAIM && !__GFP_NORETRY allocations which last
  > second allocation attempt indirectly involve from failing.
  
  I really fail to see how this has anything to do with the paragraph
  above. We are not talking about the reclaim for the last attempt. We are
  talking about reclaim that might have happened in _other_ context. Why
  don't you simply stick with the changelog which I've suggested and which
  is much more clear and easier to read.

while I want to avoid blindly copying or moving outdated comments.