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

[Vlastimil Babka]

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:
> 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 "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>

For the record, anyone backporting this to older kernels should make
sure to also include 605ca5ede764 ("mm/huge_memory.c: reorder operations
in __split_huge_page_tail()") or they are in for a lot of fun, like me.

Long story [1] short, Konstantin was correct in 605ca5ede764 changelog,
although it wasn't the main known issue he was fixing:

  clear_compound_head() also must be called before unfreezing page
  reference because after successful get_page_unless_zero() might follow
  put_page() which needs correct compound_head().

Which is exactly what happens in __migration_entry_wait():

        if (!get_page_unless_zero(page))
                goto out;
        pte_unmap_unlock(ptep, ptl);
        put_and_wait_on_page_locked(page); -> does put_page(page)

while waiting on the THP split (which inserts those migration entries)
to finish. Before put_and_wait_on_page_locked() it would wait first, and
only then do put_page() on a page that's no longer tail page, so it
would work out despite the dangerous get_page_unless_zero() on a tail
page. Now it doesn't :)

Now if only 605ca5ede764 had a CC:stable and a Fixes: tag... Machine
Learning won this round though, because 605ca5ede764 was added to 4.14
stable by Sasha...

[1] https://bugzilla.suse.com/show_bug.cgi?id=1119962#c16