On Fri 04-09-20 11:14:47, 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.
>
> Fixes: c52e75935f8d ("mm: remove extra drain pages on pcp list")
>
> Signed-off-by: Pavel Tatashin <pasha.tatas...@soleen.com>
> Cc: sta...@vger.kernel.org
> Acked-by: David Rientjes <rient...@google.com>
> Acked-by: Vlastimil Babka <vba...@suse.cz>
Already acked the mmotm version but let's add it here as well.
Acked-by: Michal Hocko <mho...@suse.com>
> ---
> mm/memory_hotplug.c | 14 ++++++++++++++
> mm/page_isolation.c | 8 ++++++++
> 2 files changed, 22 insertions(+)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index e9d5ab5d3ca0..b11a269e2356 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1575,6 +1575,20 @@ 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.
> + *
> + * Forward progress should be still guaranteed because
> + * pages on the pcp list can only belong to MOVABLE_ZONE
> + * because has_unmovable_pages explicitly checks for
> + * PageBuddy on freed pages on other zones.
> + */
> + if (ret)
> + drain_all_pages(zone);
> } while (ret);
>
> /* Ok, all of our target is isolated.
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 242c03121d73..63a3db10a8c0 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, unsigned long
> nr_pages)
> * pageblocks we may have modified and return -EBUSY to caller. This
> * prevents two threads from simultaneously working on overlapping ranges.
> *
> + * Please note that there is no strong synchronization with the page
> allocator
> + * either. Pages might be freed while their page blocks are marked ISOLATED.
> + * In some cases pages might still end up on pcp lists and that would allow
> + * for their allocation even when they are in fact isolated already.
> Depending
> + * on how strong of a guarantee the caller needs drain_all_pages might be
> needed
> + * (e.g. __offline_pages will need to call it after check for isolated range
> for
> + * a next retry).
> + *
> * Return: the number of isolated pageblocks on success and -EBUSY if any
> part
> * of range cannot be isolated.
> */
> --
> 2.25.1
>
--
Michal Hocko
SUSE Labs
