Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v3 2/3] mm: disable LRU pagevec during the migration temporarily | Date | Fri, 12 Mar 2021 10:00:28 +0100 |
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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@kernel.org> > --- > 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
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