linux-kernel - Re: [PATCH v2] mm/memory_hotplug: drain per-cpu pages again during memory offline

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Message-ID: <20200904124219.GB4610@dhcp22.suse.cz>
Date:   Fri, 4 Sep 2020 14:42:19 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        osalvador@...e.de, richard.weiyang@...il.com, vbabka@...e.cz,
        rientjes@...gle.com
Subject: Re: [PATCH v2] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

On Thu 03-09-20 12:31:36, Andrew Morton wrote:
> On Thu, 3 Sep 2020 19:36:26 +0200 David Hildenbrand <david@...hat.com> wrote:
> 
> > (still on vacation, back next week on Tuesday)
> > 
> > I didn't look into discussions in v1, but to me this looks like we are
> > trying to hide an actual bug by implementing hacks in the caller
> > (repeated calls to drain_all_pages()). What about alloc_contig_range()
> > users - you get more allocation errors just because PCP code doesn't
> > play along.
> > 
> > There *is* strong synchronization with the page allocator - however,
> > there seems to be one corner case race where we allow to allocate pages
> > from isolated pageblocks.
> > 
> > I want that fixed instead if possible, otherwise this is just an ugly
> > hack to make the obvious symptoms (offlining looping forever) disappear.
> > 
> > If that is not possible easily, I'd much rather want to see all
> > drain_all_pages() calls being moved to the caller and have the expected
> > behavior documented instead of specifying "there is no strong
> > synchronization with the page allocator" - which is wrong in all but PCP
> > cases (and there only in one possible race?).
> > 
> 
> It's a two-line hack which fixes a bug in -stable kernels, so I'm
> inclined to proceed with it anyway.  We can undo it later on as part of
> a better fix, OK?

Agreed. http://lkml.kernel.org/r/20200904070235.GA15277@dhcp22.suse.cz
for reference.
-- 
Michal Hocko
SUSE Labs