On Wed, 21 Nov 2018, Michal Hocko wrote: > 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. > " Okay, I do have a lot written from way back when I prepared the now-abandoned migration_waitqueue patch internally, but I'll factor in what you say above when I get there - in particular, you highlight the memory offlining aspect, as in this mailthread: which is very helpful, because it's outside my experience so I won't have mentioned it - thanks. I just know that there's some important linkage to do, to the August 2017 WQ_FLAG_BOOKMARK discussion: so it's a research and editing job I have to work myself up to at the right moment. > > > 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. Fair enough, I'll do so. (But that's a bit like when we say we've attached something and then forget to do so: please check that I've been honest when I do post.) > > > 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. Do we really have to be so clever here? That's a good question: now we have PG_waiters, we probably do not need to bother with this cleverness, and it would save me from having to expand on that comment as I was asked. I'll try going back to a simple unlock_page() there: and can always restore the __ClearPageLocked if a reviewer demands, or 0-day notices regression, > > > 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> Thanks! > > 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. You are very gracious to describe a patch promised six months ago as "prompt". But it does help me a lot to have it fixing a real problem for someone (thank you Baoquan) - well, it fixed a real problem for us internally too, but very nice to gather more backing for it like this. Hugh
