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

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Date:   Wed, 2 Sep 2020 10:26:44 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 01-09-20 08:46:15, Pavel Tatashin wrote:
> > There is a race during page offline that can lead to infinite loop:
> > a page never ends up on a buddy list and __offline_pages() keeps
> > retrying infinitely or until a termination signal is received.
> >
> > Thread#1 - a new process:
> >
> > load_elf_binary
> >  begin_new_exec
> >   exec_mmap
> >    mmput
> >     exit_mmap
> >      tlb_finish_mmu
> >       tlb_flush_mmu
> >        release_pages
> >         free_unref_page_list
> >          free_unref_page_prepare
> >           set_pcppage_migratetype(page, migratetype);
> >              // Set page->index migration type below  MIGRATE_PCPTYPES
> >
> > Thread#2 - hot-removes memory
> > __offline_pages
> >   start_isolate_page_range
> >     set_migratetype_isolate
> >       set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> >         Set migration type to MIGRATE_ISOLATE-> set
> >         drain_all_pages(zone);
> >              // drain per-cpu page lists to buddy allocator.
>
> It is not really clear to me how we could have passed
> has_unmovable_pages at this stage when the page is not PageBuddy. Is
> this because you are using Movable Zones?

Yes, we hot-remove memory from the movable zone.

>
> >
> > Thread#1 - continue
> >          free_unref_page_commit
> >            migratetype = get_pcppage_migratetype(page);
> >               // get old migration type
> >            list_add(&page->lru, &pcp->lists[migratetype]);
> >               // add new page to already drained pcp list
> >
> > Thread#2
> > Never drains pcp again, and therefore gets stuck in the loop.
> >
> > The fix is to try to drain per-cpu lists again after
> > check_pages_isolated_cb() fails.
>
> But this means that the page is not isolated and so it could be reused
> for something else. No?

The page is in a movable zone, has zero references, and the section is
isolated (i.e. set_pageblock_migratetype(page, MIGRATE_ISOLATE);) is
set. The page should be offlinable, but it is lost in a pcp list as
that list is never drained again after the first failure to migrate
all pages in the range.

>
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> > Cc: stable@...r.kernel.org
> > ---
> >  mm/memory_hotplug.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index e9d5ab5d3ca0..d6d54922bfce 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1575,6 +1575,15 @@ static int __ref __offline_pages(unsigned long start_pfn,
> >               /* check again */
> >               ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> >                                           NULL, check_pages_isolated_cb);
> > +             /*
> > +              * per-cpu pages are drained in start_isolate_page_range, but if
> > +              * there are still pages that are not free, make sure that we
> > +              * drain again, because when we isolated range we might
> > +              * have raced with another thread that was adding pages to
> > +              * pcp list.
> > +              */
> > +             if (ret)
> > +                     drain_all_pages(zone);
> >       } while (ret);
> >
> >       /* Ok, all of our target is isolated.
> > --
> > 2.25.1
> >
>
> --
> Michal Hocko
> SUSE Labs