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

[Matthew Wilcox]

On Tue, Jun 01, 2021 at 09:55:56AM -0700, 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?

You do realise you're strengthening my case for folios by suggesting
that, don't you?  ;-)

I was going to suggest that it won't make any difference because the
page reference count is frozen, but the freezing happens after the call
to unmap_page(), so it may make a difference.