Re: [PATCH v3 2/3] mm: disable LRU pagevec during the migration temporarily

[David Hildenbrand]

On 10.03.21 17:14, Minchan Kim wrote:
> LRU pagevec holds refcount of pages until the pagevec are drained.
> It could prevent migration since the refcount of the page is greater
> than the expection in migration logic. To mitigate the issue,
> callers of migrate_pages drains LRU pagevec via migrate_prep or
> lru_add_drain_all before migrate_pages call.
>
> However, it's not enough because pages coming into pagevec after the
> draining call still could stay at the pagevec so it could keep
> preventing page migration. Since some callers of migrate_pages have
> retrial logic with LRU draining, the page would migrate at next trail
> but it is still fragile in that it doesn't close the fundamental race
> between upcoming LRU pages into pagvec and migration so the migration
> failure could cause contiguous memory allocation failure in the end.
>
> To close the race, this patch disables lru caches(i.e, pagevec)
> during ongoing migration until migrate is done.
>
> Since it's really hard to reproduce, I measured how many times
> migrate_pages retried with force mode(it is about a fallback to a
> sync migration) with below debug code.
>
> int migrate_pages(struct list_head *from, new_page_t get_new_page,
> 			..
> 			..
>
> if (rc && reason == MR_CONTIG_RANGE && pass > 2) {
>         printk(KERN_ERR, "pfn 0x%lx reason %d\n", page_to_pfn(page), rc);
>         dump_page(page, "fail to migrate");
> }
>
> The test was repeating android apps launching with cma allocation
> in background every five seconds. Total cma allocation count was
> about 500 during the testing. With this patch, the dump_page count
> was reduced from 400 to 30.
>
> The new interface is also useful for memory hotplug which currently
> drains lru pcp caches after each migration failure. This is rather
> suboptimal as it has to disrupt others running during the operation.
> With the new interface the operation happens only once. This is also in
> line with pcp allocator cache which are disabled for the offlining as
> well.
>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>   include/linux/swap.h |  3 ++
>   mm/memory_hotplug.c  |  3 +-
>   mm/mempolicy.c       |  4 ++-
>   mm/migrate.c         |  3 +-
>   mm/swap.c            | 79 ++++++++++++++++++++++++++++++++++++--------
>   5 files changed, 75 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 32f665b1ee85..a3e258335a7f 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -339,6 +339,9 @@ extern void lru_note_cost(struct lruvec *lruvec, bool file,
>   extern void lru_note_cost_page(struct page *);
>   extern void lru_cache_add(struct page *);
>   extern void mark_page_accessed(struct page *);
> +extern void lru_cache_disable(void);
> +extern void lru_cache_enable(void);
> +extern bool lru_cache_disabled(void);
>   extern void lru_add_drain(void);
>   extern void lru_add_drain_cpu(int cpu);
>   extern void lru_add_drain_cpu_zone(struct zone *zone);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 5ba51a8bdaeb..959f659ef085 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1611,6 +1611,7 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>   	 * in a way that pages from isolated pageblock are left on pcplists.
>   	 */
>   	zone_pcp_disable(zone);
> +	lru_cache_disable();

Did you also experiment which effects zone_pcp_disable() might have on 
alloc_contig_range() ?

Feels like both calls could be abstracted somehow and used in both 
(memory offlining/alloc_contig_range) cases. It's essentially disabling 
some kind of caching.


Looks sane to me, but I am not that experienced with migration code to 
give this a real RB.

--
Thanks,

David / dhildenb