Re: [PATCHi v2] mm: put_and_wait_on_page_locked() while page is migrated

From: Vlastimil Babka
Date: Tue Nov 27 2018 - 03:21:26 EST

On 11/26/18 8:27 PM, 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:
> Tim Chen started a thread in August 2017 which appears relevant:
> where Kan Liang went on to implicate __migration_entry_wait():
> 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:
> 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 "behavior"
> enum in place of "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>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>