linux-kernel - Re: [patch] mm, migrate: increment fail count on ENOMEM

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Date:	Tue, 24 May 2016 08:17:25 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm, migrate: increment fail count on ENOMEM

On Mon 23-05-16 16:32:56, Hugh Dickins wrote:
> On Mon, 23 May 2016, Andrew Morton wrote:
> > On Fri, 20 May 2016 15:31:21 +0200 Michal Hocko <mhocko@...nel.org> wrote:
> > > On Fri 20-05-16 15:19:12, Vlastimil Babka wrote:
> > > > On 05/20/2016 03:06 PM, Michal Hocko wrote:
> > > [...]
> > > > > Why don't we need also to count also retries?
> > > > 
> > > > We could, but not like you suggest.
> > > > 
> > > > > ---
> > > > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > > > index 53ab6398e7a2..ef9c5211ae3c 100644
> > > > > --- a/mm/migrate.c
> > > > > +++ b/mm/migrate.c
> > > > > @@ -1190,9 +1190,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> > > > >   			}
> > > > >   		}
> > > > >   	}
> > > > > +out:
> > > > >   	nr_failed += retry;
> > > > >   	rc = nr_failed;
> > > > 
> > > > This overwrites rc == -ENOMEM, which at least compaction needs to recognize.
> > > > But we could duplicate "nr_failed += retry" in the case -ENOMEM.
> > > 
> > > Right you are. So we should do
> > > ---
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 53ab6398e7a2..123fed94022b 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -1171,6 +1171,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> > >  
> > >  			switch(rc) {
> > >  			case -ENOMEM:
> > > +				nr_failed += retry + 1;
> > >  				goto out;
> > >  			case -EAGAIN:
> > >  				retry++;
> > > 	
> > > 
> > 
> > argh, this was lost.  Please resend as a real patch sometime?
> 
> It's not correct.  "retry" is reset to 0 each time around the
> loop, and it's only a meaningful number to add on to nr_failed, in
> the case when we've gone through the whole list: not in this "goto
> out" case. 

You are right! I've missed that, my bad.

> We could add another loop to count how many are left
> when we hit -ENOMEM, and add that on to nr_failed; but I'm not
> convinced that it's worth the bother.

Agreed!

Sorry about the noise!
-- 
Michal Hocko
SUSE Labs