On Mon 19-11-18 21:44:41, 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. I would add the following for the "problem statement". Feel free to reuse per your preference: " An elevated reference count, however, stands in the way of migration and forces it to fail with a bad timing. This is especially a problem for memory offlining which retries for ever (or until the operation is terminated from userspace) because a heavy refault workload can trigger essentially an endless loop of migration failures. Therefore __migration_entry_wait is essentially harmful for the even it is waiting for. " > 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 > 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(). I would appreciate this would be more explicit about the existence of the elevated-ref-count problem but it reduces it to a tiny time window compared to the whole time the waiter is blocked. So a great improvement. > 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! and I can imagine a bad one. Do we really have to be so clever here? The unlock_page went away in the name of performance (a978d6f521063) and I would argue that this is a slow path where this is just not worth it. > 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 <hughd@xxxxxxxxxx> The patch looks good to me - quite ugly but it doesn't make the existing code much worse. With the problem described Vlastimil fixed, feel free to add Acked-by: Michal Hocko <mhocko@xxxxxxxx> And thanks for a prompt patch. This is something I've been chasing for quite some time. __migration_entry_wait came to my radar only recently because this is an extremely volatile area. -- Michal Hocko SUSE Labs
