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

Michal Hocko
2020-09-02T14:22:58Z

On Wed 02-09-20 16:01:17, Michal Hocko wrote:
> [Cc Mel and Vlastimil - I am still rummaging]
>
> 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.
> >
> > 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.

Still trying to wrap my head around this but I think this is not a
proper fix. It should be the page isolation to make sure no races are
possible with the page freeing path.

> > Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> > Cc: stable@vger.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

--
Michal Hocko
SUSE Labs