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

[Hugh Dickins]

On Mon, 7 Jun 2021, Yu Xu wrote:
> 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.

No, it would not be necessary to plumb through a return value from
__migration_entry_wait(): I didn't mean that this GUP cond_resched()
should be done only for the migration case, but (I guess) on any path
where handle_mm_fault() returns "success" for a retry, yet the retry
of follow_page_mask() fails.

But now that I look, I see there is already a cond_resched() there!

So I'm puzzled as to how your cond_resched() in __migration_entry_wait()
appeared to help - well, you never actually said that it helped, but I
assume that it did, or you wouldn't have bothered to send that patch?

It's irrelevant, now that we've admitted there should be a
"page = compound_head(page)" in there, and you have said that helps,
and that's the patch we want to send now.  But it troubles me, to be
unable to explain it.  Two cond_resched()s are not twice as good as one.

> 
> > 
> >>
> >> 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?

I was rather expecting you to send the patch: with your s.o.b, not mine.
You could say "Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>" if you like.

And I suggest that you put that "page = compound_head(page);" line
immediately after the "page = migration_entry_to_page(entry);" line,
so as not to interfere with the comment above get_page_unless_zero().

(No need for a comment on the compound_head(): it's self-explanatory.)

I did meanwhile research other callers of migration_entry_to_page():
it had been on my mind, that others might need a compound_head() too,
and perhaps it should be done inside migration_entry_to_page() itself.

But so far as I can tell (I don't really know about the s390 one),
the others are okay, and it would just be unnecessary overhead
(in particular, the mm_counter() stuff looks correct on a tail).

I *think* the right Fixes tag would be
Fixes: ba98828088ad ("thp: add option to setup migration entries during PMD split")
though I'm not sure of that; it's probably good enough.

(With all this direction, I did wonder if it would be kinder just to
send a patch myself, but using some of your comments: but I didn't
understand "conner" in your description, so couldn't do that.)

Thanks!
Hugh