Re: + mm-memory_hotplug-drain-per-cpu-pages-again-during-memory-offline.patch added to -mm tree

Re: + mm-memory_hotplug-drain-per-cpu-pages-again-during-memory-offline.patch added to -mm tree

[Michal Hocko]

On Thu 03-09-20 12:29:01, Andrew Morton wrote:
> From: Pavel Tatashin <pasha.tatashin@soleen.com>
> 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@soleen.com
> Fixes: c52e75935f8d ("mm: remove extra drain pages on pcp list")
> Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> Acked-by: David Rientjes <rientjes@google.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Wei Yang <richard.weiyang@gmail.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 
>  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@soleen.com are
> 
> mm-memory_hotplug-drain-per-cpu-pages-again-during-memory-offline.patch

-- 
Michal Hocko
SUSE Labs