Re: [PATCH] mm, thp: relax migration wait when failed to get tail page

[Yu Xu]

On 6/2/21 11:57 PM, Hugh Dickins wrote:
> On Wed, 2 Jun 2021, Yu Xu wrote:
> > On 6/2/21 12:55 AM, Hugh Dickins wrote:
> > > On Wed, 2 Jun 2021, Xu Yu wrote:
> > >
> > > > We notice that hung task happens in a conner but practical scenario when
> > > > CONFIG_PREEMPT_NONE is enabled, as follows.
> > > >
> > > > Process 0                       Process 1                     Process
> > > > 2..Inf
> > > > split_huge_page_to_list
> > > >       unmap_page
> > > >           split_huge_pmd_address
> > > >                                   __migration_entry_wait(head)
> > > >                                                                 __migration_entry_wait(tail)
> > > >       remap_page (roll back)
> > > >           remove_migration_ptes
> > > >               rmap_walk_anon
> > > >                   cond_resched
> > > >
> > > > Where __migration_entry_wait(tail) is occurred in kernel space, e.g.,
> > > > copy_to_user, which will immediately fault again without rescheduling,
> > > > and thus occupy the cpu fully.
> > > >
> > > > When there are too many processes performing __migration_entry_wait on
> > > > tail page, remap_page will never be done after cond_resched.
> > > >
> > > > This relaxes __migration_entry_wait on tail page, thus gives remap_page
> > > > a chance to complete.
> > > >
> > > > Signed-off-by: Gang Deng <gavin.dg@xxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
> > >
> > > Well caught: you're absolutely right that there's a bug there.
> > > But isn't cond_resched() just papering over the real bug, and
> > > what it should do is a "page = compound_head(page);" before the
> > > get_page_unless_zero()? How does that work out in your testing?
> >
> > compound_head works. The patched kernel is alive for hours under
> > our reproducer, which usually makes the vanilla kernel hung after
> > tens of minutes at most.
>
> Oh, that's good news, thanks.
>
> (It's still likely that a well-placed cond_resched() somewhere in
> mm/gup.c would also be a good idea, but none of us have yet got
> around to identifying where.)

We neither. If really have to do it outside of __migration_entry_wait,
return value of __migration_entry_wait is needed, and many related
functions have to updated, which may be undesirable.

> > If we use compound_head, the behavior of __migration_entry_wait(tail)
> > changes from "retry fault" to "prevent THP from being split". Is that
> > right?  Then which is preferred? If it were me, I would prefer "retry
> > fault".
>
> As Matthew remarked, you are asking very good questions, and split
> migration entries are difficult to think about.  But I believe you'll
> find it works out okay.
>
> The point of *put_and_* wait_on_page_locked() is that it does drop
> the page reference you acquired with get_page_unless_zero, as soon
> as the page is on the wait queue, before actually waiting.
>
> So splitting the THP is only prevented for a brief interval.  Now,
> it's true that if there are very many tasks faulting on portions
> of the huge page, in that interval between inserting the migration
> entries and freezing the huge page's refcount to 0, they can reduce
> the chance of splitting considerably.  But that's not an excuse for
> for doing get_page_unless_zero() on the wrong thing, as it was doing.

We finally come to your solution, i.e., compound_head.

In that case, who should resend the compound_head patch to this issue?
shall we do with your s.o.b?

> Hugh

--
Thanks,
Yu