Re: Memory hotplug softlock issue

[Michal Hocko]

On Mon 19-11-18 12:34:09, Hugh Dickins wrote:
> On Mon, 19 Nov 2018, Michal Hocko wrote:
> > On Mon 19-11-18 15:10:16, Michal Hocko wrote:
> > [...]
> > > In other words. Why cannot we do the following?
> > 
> > Baoquan, this is certainly not the right fix but I would be really
> > curious whether it makes the problem go away.
> > 
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index f7e4bfdc13b7..7ccab29bcf9a 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -324,19 +324,9 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
> > >  		goto out;
> > >  
> > >  	page = migration_entry_to_page(entry);
> > > -
> > > -	/*
> > > -	 * Once page cache replacement of page migration started, page_count
> > > -	 * *must* be zero. And, we don't want to call wait_on_page_locked()
> > > -	 * against a page without get_page().
> > > -	 * So, we use get_page_unless_zero(), here. Even failed, page fault
> > > -	 * will occur again.
> > > -	 */
> > > -	if (!get_page_unless_zero(page))
> > > -		goto out;
> > >  	pte_unmap_unlock(ptep, ptl);
> > > -	wait_on_page_locked(page);
> > > -	put_page(page);
> > > +	page_lock(page);
> > > +	page_unlock(page);
> > >  	return;
> > >  out:
> > >  	pte_unmap_unlock(ptep, ptl);
> 
> Thanks for Cc'ing me. I did mention precisely this issue two or three
> times at LSF/MM this year, and claimed then that I would post the fix.

I've had a recollection about some issue I just couldn't remember what
was it exactly. Tried to make Vlastimil to remember but failed there as
well ;)

> I'm glad that I delayed, what I had then (migration_waitqueue instead
> of using page_waitqueue) was not wrong, but what I've been using the
> last couple of months is rather better (and can be put to use to solve
> similar problems in collapsing pages on huge tmpfs. but we don't need
> to get into that at this time): put_and_wait_on_page_locked().
> 
> What I have not yet done is verify it on latest kernel, and research
> the interested Cc list (Linus and Tim Chen come immediately to mind),
> and write the commit comment. I have some testing to do on the latest
> kernel today, so I'll throw put_and_wait_on_page_locked() in too,
> and post tomorrow I hope.

Cool, it seems that Baoquan has a reliable test case to trigger the
pathological case.
-- 
Michal Hocko
SUSE Labs