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

[Chris Goldsworthy]

On 2021-03-10 08:14, 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>
> ---

Hi Minchan,

This all looks good to me - feel free to add a Reviewed-by from me.

Thanks,

Chris.

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