On Sat, Nov 24, 2018 at 07:21:07PM -0800, Hugh Dickins wrote: > Waiting on a page migration entry has used wait_on_page_locked() all > along since 2006: but you cannot safely wait_on_page_locked() without > holding a reference to the page, and that extra reference is enough to > make migrate_page_move_mapping() fail with -EAGAIN, when a racing task > faults on the entry before migrate_page_move_mapping() gets there. > > And that failure is retried nine times, amplifying the pain when > trying to migrate a popular page. With a single persistent faulter, > migration sometimes succeeds; with two or three concurrent faulters, > success becomes much less likely (and the more the page was mapped, > the worse the overhead of unmapping and remapping it on each try). > > This is especially a problem for memory offlining, where the outer > level retries forever (or until terminated from userspace), because > a heavy refault workload can trigger an endless loop of migration > failures. wait_on_page_locked() is the wrong tool for the job. > > David Herrmann (but was he the first?) noticed this issue in 2014: > https://marc.info/?l=linux-mm&m=140110465608116&w=2 > > Tim Chen started a thread in August 2017 which appears relevant: > https://marc.info/?l=linux-mm&m=150275941014915&w=2 > where Kan Liang went on to implicate __migration_entry_wait(): > https://marc.info/?l=linux-mm&m=150300268411980&w=2 > and the thread ended up with the v4.14 commits: > 2554db916586 ("sched/wait: Break up long wake list walk") > 11a19c7b099f ("sched/wait: Introduce wakeup boomark in wake_up_page_bit") > > Baoquan He reported "Memory hotplug softlock issue" 14 November 2018: > https://marc.info/?l=linux-mm&m=154217936431300&w=2 > > 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 > 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. > > __migration_entry_wait() still has to take a brief reference to the > page, prior to calling put_and_wait_on_page_locked(): but now that it > is dropped before waiting, the chance of impeding page migration is > very much reduced. Should we perhaps disable preemption across this? > > shrink_page_list()'s __ClearPageLocked(): that was a surprise! This > survived a lot of testing before that showed up. PageWaiters may have > been set by wait_on_page_bit_common(), and the reference dropped, just > before shrink_page_list() succeeds in freezing its last page reference: > in such a case, unlock_page() must be used. Follow the suggestion from > Michal Hocko, just revert a978d6f52106 ("mm: unlockless reclaim") now: > that optimization predates PageWaiters, and won't buy much these days; > but we can reinstate it for the !PageWaiters case if anyone notices. > > It does raise the question: should vmscan.c's is_page_cache_freeable() > and __remove_mapping() now treat a PageWaiters 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 used for page migration. > > Reported-and-tested-by: Baoquan He <bhe@xxxxxxxxxx> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > Acked-by: Michal Hocko <mhocko@xxxxxxxx> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
