linux-kernel - Re: [PATCH v5 1/5] mm,page_alloc: Bail out earlier on -ENOMEM in alloc_contig_migrate

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Message-ID: <YFIXElOEfMuVEOZh@dhcp22.suse.cz>
Date:   Wed, 17 Mar 2021 15:49:54 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm,page_alloc: Bail out earlier on -ENOMEM in
 alloc_contig_migrate_range

On Wed 17-03-21 15:42:43, David Hildenbrand wrote:
> On 17.03.21 15:05, Michal Hocko wrote:
> > On Wed 17-03-21 12:12:47, Oscar Salvador wrote:
> > > Currently, __alloc_contig_migrate_range can generate -EINTR, -ENOMEM or -EBUSY,
> > > and report them down the chain.
> > > The problem is that when migrate_pages() reports -ENOMEM, we keep going till we
> > > exhaust all the try-attempts (5 at the moment) instead of bailing out.
> > > 
> > > migrate_pages() bails out right away on -ENOMEM because it is considered a fatal
> > > error. Do the same here instead of keep going and retrying.
> > 
> > I suspect this is not really a real life problem, right? The allocation
> > would be more costly in the end but this is to be expected under a heavy
> > memory pressure.
> > 
> > That being said, bailing out early makes sense to me. But now that
> > you've made me look into the migrate_pages excellent error state reporting
> > I suspect we have a bug here. Note the
> > "Returns the number of pages that were not migrated, or an error code."
> > 
> > but I do not see putback_movable_pages for ret > 0 so it seems we might
> > leak some pages.
> 
> At least in __alloc_contig_migrate_range() we seem to always leave the loop
> with ret <= 0 and do a putback_movable_pages() with ret < 0.
> 
> Which code are you referring to?

OK, my bad. I have managed to confuse myself around the retry bailout
which indeed overrides the return value. So there is no bug. Sorry about
the noise but I still believe making migrate_pages less tricky with
error handling would be an improvement.

Thanks!
-- 
Michal Hocko
SUSE Labs