On Thu 03-09-20 12:29:01, Andrew Morton wrote: > From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> > Subject: mm/memory_hotplug: drain per-cpu pages again during memory offline > > 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. > > Link: https://lkml.kernel.org/r/20200903140032.380431-1-pasha.tatashin@xxxxxxxxxx > Fixes: c52e75935f8d ("mm: remove extra drain pages on pcp list") > Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx> > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Oscar Salvador <osalvador@xxxxxxx> > Cc: Wei Yang <richard.weiyang@xxxxxxxxx> > Cc: David Hildenbrand <david@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> > --- > > mm/memory_hotplug.c | 14 ++++++++++++++ > mm/page_isolation.c | 8 ++++++++ > 2 files changed, 22 insertions(+) > > --- a/mm/memory_hotplug.c~mm-memory_hotplug-drain-per-cpu-pages-again-during-memory-offline > +++ a/mm/memory_hotplug.c > @@ -1575,6 +1575,20 @@ static int __ref __offline_pages(unsigne > /* 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. > --- a/mm/page_isolation.c~mm-memory_hotplug-drain-per-cpu-pages-again-during-memory-offline > +++ a/mm/page_isolation.c > @@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, un > * 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. > */ > _ > > Patches currently in -mm which might be from pasha.tatashin@xxxxxxxxxx are > > mm-memory_hotplug-drain-per-cpu-pages-again-during-memory-offline.patch -- Michal Hocko SUSE Labs
