On 11/20/18 6:44 AM, Hugh Dickins wrote:
> [PATCH] mm: put_and_wait_on_page_locked() while page is migrated
>
> We have all assumed that it is essential to hold a page reference while
> waiting on a page lock: partly to guarantee that there is still a struct
> page when MEMORY_HOTREMOVE is configured, but also to protect against
> reuse of the struct page going to someone who then holds the page locked
> indefinitely, when the waiter can reasonably expect timely unlocking.
>
> But in fact, so long as wait_on_page_bit_common() does the put_page(),
> and is careful not to rely on struct page contents thereafter, there is
> no need to hold a reference to the page while waiting on it. That does
So there's still a moment where refcount is elevated, but hopefully
short enough, right? Let's see if it survives Baoquan's stress testing.
> mean that this case cannot go back through the loop: but that's fine for
> the page migration case, and even if used more widely, is limited by the
> "Stop walking if it's locked" optimization in wake_page_function().
>
> Add interface put_and_wait_on_page_locked() to do this, using negative
> value of the lock arg to wait_on_page_bit_common() to implement it.
> No interruptible or killable variant needed yet, but they might follow:
> I have a vague notion that reporting -EINTR should take precedence over
> return from wait_on_page_bit_common() without knowing the page state,
> so arrange it accordingly - but that may be nothing but pedantic.
>
> shrink_page_list()'s __ClearPageLocked(): that was a surprise! this
> survived a lot of testing before that showed up. It does raise the
> question: should is_page_cache_freeable() and __remove_mapping() now
> treat a PG_waiters page as if an extra reference were held? Perhaps,
> but I don't think it matters much, since shrink_page_list() already
> had to win its trylock_page(), so waiters are not very common there: I
> noticed no difference when trying the bigger change, and it's surely not
> needed while put_and_wait_on_page_locked() is only for page migration.
>
> Signed-off-by: Hugh Dickins <hu...@google.com>
> ---
...
> @@ -1100,6 +1111,17 @@ static inline int
> wait_on_page_bit_common(wait_queue_head_t *q,
> ret = -EINTR;
> break;
> }
> +
> + if (lock < 0) {
> + /*
> + * We can no longer safely access page->flags:
Hmm...
> + * even if CONFIG_MEMORY_HOTREMOVE is not enabled,
> + * there is a risk of waiting forever on a page reused
> + * for something that keeps it locked indefinitely.
> + * But best check for -EINTR above before breaking.
> + */
> + break;
> + }
> }
>
> finish_wait(q, wait);
... the code continues by:
if (thrashing) {
if (!PageSwapBacked(page))
So maybe we should not set 'thrashing' true when lock < 0?
Thanks!
Vlastimil
