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    SubjectRe: [RFC PATCH] mm: silence soft lockups from unlock_page
    On 07/23, Linus Torvalds wrote:
    >
    > But I'll walk over my patch mentally one more time. Here's the current
    > version, anyway.

    Both patches look correct to me, feel free to add

    Reviewed-by: Oleg Nesterov <oleg@redhat.com>

    > @@ -1013,18 +1014,40 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
    > if (wait_page->bit_nr != key->bit_nr)
    > return 0;
    >
    > + /* Stop walking if it's locked */
    > + if (wait->flags & WQ_FLAG_EXCLUSIVE) {
    > + if (test_and_set_bit(key->bit_nr, &key->page->flags))
    > + return -1;
    > + } else {
    > + if (test_bit(key->bit_nr, &key->page->flags))
    > + return -1;
    > + }

    not sure this makes any sense, but this looks like another user of
    trylock_page_bit_common(), see the patch below on top of 1/2.

    Oleg.

    --- mm/filemap.c~ 2020-07-24 17:09:34.728133846 +0200
    +++ mm/filemap.c 2020-07-24 17:23:52.279185374 +0200
    @@ -1000,6 +1000,14 @@
    wait_queue_entry_t wait;
    };

    +static int trylock_page_bit_common(struct page *page, int bit_nr,
    + struct wait_queue_entry *wait)
    +{
    + return wait->flags & WQ_FLAG_EXCLUSIVE ?
    + !test_and_set_bit(bit_nr, &page->flags) :
    + !test_bit(bit_nr, &page->flags);
    +}
    +
    static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync, void *arg)
    {
    int ret;
    @@ -1015,13 +1023,8 @@
    return 0;

    /* Stop walking if it's locked */
    - if (wait->flags & WQ_FLAG_EXCLUSIVE) {
    - if (test_and_set_bit(key->bit_nr, &key->page->flags))
    - return -1;
    - } else {
    - if (test_bit(key->bit_nr, &key->page->flags))
    - return -1;
    - }
    + if (!trylock_page_bit_common(key->page, key->bit_nr, wait))
    + return -1;

    /*
    * Let the waiter know we have done the page flag
    @@ -1126,14 +1129,6 @@
    */
    };

    -static inline int trylock_page_bit_common(struct page *page, int bit_nr,
    - enum behavior behavior)
    -{
    - return behavior == EXCLUSIVE ?
    - !test_and_set_bit(bit_nr, &page->flags) :
    - !test_bit(bit_nr, &page->flags);
    -}
    -
    static inline int wait_on_page_bit_common(wait_queue_head_t *q,
    struct page *page, int bit_nr, int state, enum behavior behavior)
    {
    @@ -1170,7 +1165,7 @@
    */
    spin_lock_irq(&q->lock);
    SetPageWaiters(page);
    - if (!trylock_page_bit_common(page, bit_nr, behavior))
    + if (!trylock_page_bit_common(page, bit_nr, wait))
    __add_wait_queue_entry_tail(q, wait);
    spin_unlock_irq(&q->lock);

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