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    Subject[PATCH v2 2/2] mm: compaction: fix the possible deadlock when isolating hugetlb pages
    Date
    When trying to isolate a migratable pageblock, it can contain several
    normal pages or several hugetlb pages (e.g. CONT-PTE 64K hugetlb on arm64)
    in a pageblock. That means we may hold the lru lock of a normal page to
    continue to isolate the next hugetlb page by isolate_or_dissolve_huge_page()
    in the same migratable pageblock.

    However in the isolate_or_dissolve_huge_page(), it may allocate a new hugetlb
    page and dissolve the old one by alloc_and_dissolve_hugetlb_folio() if the
    hugetlb's refcount is zero. That means we can still enter the direct compaction
    path to allocate a new hugetlb page under the current lru lock, which
    may cause possible deadlock.

    To avoid this possible deadlock, we should release the lru lock when trying
    to isolate a hugetbl page. Moreover it does not make sense to take the lru
    lock to isolate a hugetlb, which is not in the lru list.

    Fixes: 369fa227c219 ("mm: make alloc_contig_range handle free hugetlb pages")
    Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
    Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
    Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
    ---
    Changes from v1:
    - Collect reviewed tags. Thanks Mike and Vlastimil.
    ---
    mm/compaction.c | 5 +++++
    1 file changed, 5 insertions(+)

    diff --git a/mm/compaction.c b/mm/compaction.c
    index 7e645cdfc2e9..3df076716691 100644
    --- a/mm/compaction.c
    +++ b/mm/compaction.c
    @@ -894,6 +894,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
    }

    if (PageHuge(page) && cc->alloc_contig) {
    + if (locked) {
    + unlock_page_lruvec_irqrestore(locked, flags);
    + locked = NULL;
    + }
    +
    ret = isolate_or_dissolve_huge_page(page, &cc->migratepages);

    /*
    --
    2.27.0
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