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    Subject[PATCH] mm: avoid blocking lock_page() in kcompactd
    Date
    We observed kcompactd hung at __lock_page():

    INFO: task kcompactd0:57 blocked for more than 120 seconds.
    Not tainted 4.19.56.x86_64 #1
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    kcompactd0 D 0 57 2 0x80000000
    Call Trace:
    ? __schedule+0x236/0x860
    schedule+0x28/0x80
    io_schedule+0x12/0x40
    __lock_page+0xf9/0x120
    ? page_cache_tree_insert+0xb0/0xb0
    ? update_pageblock_skip+0xb0/0xb0
    migrate_pages+0x88c/0xb90
    ? isolate_freepages_block+0x3b0/0x3b0
    compact_zone+0x5f1/0x870
    kcompactd_do_work+0x130/0x2c0
    ? __switch_to_asm+0x35/0x70
    ? __switch_to_asm+0x41/0x70
    ? kcompactd_do_work+0x2c0/0x2c0
    ? kcompactd+0x73/0x180
    kcompactd+0x73/0x180
    ? finish_wait+0x80/0x80
    kthread+0x113/0x130
    ? kthread_create_worker_on_cpu+0x50/0x50
    ret_from_fork+0x35/0x40

    which faddr2line maps to:

    migrate_pages+0x88c/0xb90:
    lock_page at include/linux/pagemap.h:483
    (inlined by) __unmap_and_move at mm/migrate.c:1024
    (inlined by) unmap_and_move at mm/migrate.c:1189
    (inlined by) migrate_pages at mm/migrate.c:1419

    Sometimes kcompactd eventually got out of this situation, sometimes not.

    I think for memory compaction, it is a best effort to migrate the pages,
    so it doesn't have to wait for I/O to complete. It is fine to call
    trylock_page() here, which is pretty much similar to
    buffer_migrate_lock_buffers().

    Given MIGRATE_SYNC_LIGHT is used on compaction path, just relax the
    check for it.

    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: linux-mm@kvack.org
    Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
    ---
    mm/migrate.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/mm/migrate.c b/mm/migrate.c
    index 86873b6f38a7..df60026779d2 100644
    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -1010,7 +1010,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
    bool is_lru = !__PageMovable(page);

    if (!trylock_page(page)) {
    - if (!force || mode == MIGRATE_ASYNC)
    + if (!force || mode == MIGRATE_ASYNC
    + || mode == MIGRATE_SYNC_LIGHT)
    goto out;

    /*
    --
    2.21.1
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