Re: [PATCH 1/2] mm: disable LRU pagevec during the migration temporarily

David Hildenbrand
2021-03-04T08:10:20Z

On 03.03.21 21:23, Minchan Kim wrote:
> On Wed, Mar 03, 2021 at 01:49:36PM +0100, Michal Hocko wrote:
>> On Tue 02-03-21 13:09:48, 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 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.
>>
>> Have you seen any improvement on the CMA allocation success rate?
>
> Unfortunately, the cma alloc failure rate with reasonable margin
> of error is really hard to reproduce under real workload.
> That's why I measured the soft metric instead of direct cma fail
> under real workload(I don't want to make some adhoc artificial
> benchmark and keep tunes system knobs until it could show
> extremly exaggerated result to convice patch effect).
>
> Please say if you belive this work is pointless unless there is
> stable data under reproducible scenario. I am happy to drop it.

Do you have *some* application that triggers such a high retry count?

I'd love to run it along with virtio-mem and report the actual
allocation success rate / necessary retries. That could give an
indication of how helpful your work would be.

Anything that improves the reliability of alloc_contig_range() is of
high interest to me. If it doesn't increase the reliability but merely
does some internal improvements (less retries), it might still be
valuable, but not that important.

--
Thanks,

David / dhildenb