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.tatas...@soleen.com> > Cc: sta...@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
