From: Zi Yan ziy@nvidia.com Subject: mm/compaction: count pages and stop correctly during page isolation

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.

[ziy@nvidia.com: v3] Link: https://lkml.kernel.org/r/20201030183809.3616803-1-zi.yan@sent.com Link: https://lkml.kernel.org/r/20201029200435.3386066-1-zi.yan@sent.com Fixes: 1da2f328fa64 ("cmm,thp,compaction,cma: allow THP migration for CMA allocations") Signed-off-by: Zi Yan ziy@nvidia.com Reviewed-by: Yang Shi shy828301@gmail.com Acked-by: Vlastimil Babka vbabka@suse.cz Cc: Rik van Riel riel@surriel.com Cc: Michal Hocko mhocko@kernel.org Cc: Mel Gorman mgorman@techsingularity.net Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/compaction.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/mm/compaction.c~mm-compaction-count-pages-and-stop-correctly-during-page-isolation +++ a/mm/compaction.c @@ -1012,8 +1012,8 @@ isolate_migratepages_block(struct compac

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_success: * 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 compac if (!pfn) break;

- if (cc->nr_migratepages == COMPACT_CLUSTER_MAX) + if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX) break; }

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