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

Michal Hocko
2020-09-02T14:20:11Z

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?

>
> 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?

> 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