Re: [PATCH v3 2/2] mm/compaction: stop isolation if too many pages are isolated and we have pages to migrate.

[Vlastimil Babka <vbabka@suse.cz>]

On 10/30/20 7:38 PM, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> In isolate_migratepages_block, if we have too many isolated pages and
> nr_migratepages is not zero, we should try to migrate what we have
> without wasting time on isolating.

As you CC stable, there should be a stronger reason (strictly speaking the 
problem should have been observed in practice, but this is a simple patch, so 
they could accept it), so I suggest Andrew adds the following paragraph:

In theory it's possible that multiple parallel compactions will cause 
too_many_isolated() to become true even if each has isolated less than 
COMPACT_CLUSTER_MAX, and loop forever in the while loop. Bailing immediately 
prevents that.

> Fixes: 1da2f328fa64 (“mm,thp,compaction,cma: allow THP migration for CMA allocations”)
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Cc: <stable@vger.kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>   mm/compaction.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 3e834ac402f1..4d237a7c3830 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -817,6 +817,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   	 * delay for some time until fewer pages are isolated
>   	 */
>   	while (unlikely(too_many_isolated(pgdat))) {
> +		/* stop isolation if there are still pages not migrated */
> +		if (cc->nr_migratepages)
> +			return 0;
> +
>   		/* async migration should just abort */
>   		if (cc->mode == MIGRATE_ASYNC)
>   			return 0;
>