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SubjectRe: [PATCH v3 1/2] mm/compaction: count pages and stop correctly during page isolation.
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On 10/30/20 7:38 PM, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
>
> 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@nvidia.com>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> Cc: <stable@vger.kernel.org>

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

> ---
> 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;
> }
>
>

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