Re: [PATCH v3 1/2] mm/compaction: count pages and stop correctly during page isolation.

[Vlastimil Babka]

On 10/30/20 7:38 PM, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
>
> In isolate_migratepages_block, when cc->alloc_contig is true, we are
> able to isolate compound pages, nr_migratepages and nr_isolated did not
> count compound pages correctly, causing us to isolate more pages than we
> thought. Count compound pages as the number of base pages they contain.
> Otherwise, we might be trapped in too_many_isolated while loop,
> since the actual isolated pages can go up to
> COMPACT_CLUSTER_MAX*512=16384, where COMPACT_CLUSTER_MAX is 32,
> since we stop isolation after cc->nr_migratepages reaches to
> COMPACT_CLUSTER_MAX.
>
> In addition, after we fix the issue above, cc->nr_migratepages could
> never be equal to COMPACT_CLUSTER_MAX if compound pages are isolated,
> thus page isolation could not stop as we intended. Change the isolation
> stop condition to >=.
>
> The issue can be triggered as follows:
> In a system with 16GB memory and an 8GB CMA region reserved by
> hugetlb_cma, if we first allocate 10GB THPs and mlock them
> (so some THPs are allocated in the CMA region and mlocked), reserving
> 6 1GB hugetlb pages via
> /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages will get stuck
> (looping in too_many_isolated function) until we kill either task.
> With the patch applied, oom will kill the application with 10GB THPs and
> let hugetlb page reservation finish.
>
> Fixes: 1da2f328fa64 (“mm,thp,compaction,cma: allow THP migration for CMA allocations”)
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>   mm/compaction.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ee1f8439369e..3e834ac402f1 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   
>   isolate_success:
>   		list_add(&page->lru, &cc->migratepages);
> -		cc->nr_migratepages++;
> -		nr_isolated++;
> +		cc->nr_migratepages += compound_nr(page);
> +		nr_isolated += compound_nr(page);
>   
>   		/*
>   		 * Avoid isolating too much unless this block is being
> @@ -1021,7 +1021,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   		 * or a lock is contended. For contention, isolate quickly to
>   		 * potentially remove one source of contention.
>   		 */
> -		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX &&
> +		if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX &&
>   		    !cc->rescan && !cc->contended) {
>   			++low_pfn;
>   			break;
> @@ -1132,7 +1132,7 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
>   		if (!pfn)
>   			break;
>   
> -		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
> +		if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX)
>   			break;
>   	}
>