Re: [PATCH] mm/compaction: count pages and stop correctly during page isolation.

[Vlastimil Babka]

On 10/30/20 3:49 PM, Michal Hocko wrote:
> On Fri 30-10-20 10:35:43, Zi Yan wrote:
> > On 30 Oct 2020, at 9:36, Michal Hocko wrote:
> >
> > > On Fri 30-10-20 08:20:50, Zi Yan wrote:
> > >> On 30 Oct 2020, at 5:43, Michal Hocko wrote:
> > >>
> > >>> [Cc Vlastimil]
> > >>>
> > >>> On Thu 29-10-20 16:04:35, Zi Yan wrote:
> > >>>
> > >>> Does thp_nr_pages work for __PageMovable pages?
> > >>
> > >> Yes. It is the same as compound_nr() but compiled
> > >> to 1 when THP is not enabled.
> > >
> > > I am sorry but I do not follow. First of all the implementation of the
> > > two is different and also I was asking about __PageMovable which should
> > > never be THP IIRC. Can they be compound though?
> >
> > __PageMovable, non-lru movable pages, can be compound and thp_nr_page cannot
> > be used for it, since when THP is off, thp_nr_page will return the wrong number.
> > I got confused by its name, sorry.
>
> OK, this matches my understanding. Good we are on the same page.
>
> > But __PageMovable is irrelevant to this patch, since we are using
> > __isolate_lru_page to isolate pages. non-lru __PageMovable should not appear
> > after isolate_succes. thp_nr_pages can be used here.
>
> But this is still not clear to me. __PageMovable pages are isolated by
> isolate_movable_page and then jump to isolate_succes. Does that somehow
> changes the nature of the page being compound or tat thp_nr_page would
> start working on those pages.

Agreed that page movable can appear after isolate_success. compound_nr() should 
work for both.
Note that too_many_isolated() doesn't see __PageMovable isolated pages, as they 
are not counted as NR_ISOLATED_FILE/NR_ISOLATED_ANON, AFAIK. So in that sense 
they are irrelevant to the bug at hand... for now.