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    Subject[PATCH 5.9 209/255] mm/compaction: count pages and stop correctly during page isolation
    Date
    From: Zi Yan <ziy@nvidia.com>

    commit 38935861d85a4d9a353d1dd5a156c97700e2765d upstream.

    In isolate_migratepages_block, when cc->alloc_contig is true, we are
    able to isolate compound pages. But nr_migratepages and nr_isolated did
    not count compound pages correctly, causing us to isolate more pages
    than we thought.

    So 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
    Fixes: 1da2f328fa64 ("cmm,thp,compaction,cma: allow THP migration for CMA allocations")
    Signed-off-by: Zi Yan <ziy@nvidia.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    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>
    Link: https://lkml.kernel.org/r/20201029200435.3386066-1-zi.yan@sent.com
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/mm/compaction.c
    +++ b/mm/compaction.c
    @@ -1013,8 +1013,8 @@ 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
    @@ -1022,7 +1022,7 @@ 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;
    @@ -1133,7 +1133,7 @@ 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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