Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

Peter Xu
2023-08-11T15:20:04Z

On Fri, Aug 11, 2023 at 12:27:13AM +0200, David Hildenbrand wrote:
> On 10.08.23 23:48, Matthew Wilcox wrote:
> > On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
> > > AFAICS if that patch was all correct (while I'm not yet sure..), you can
> > > actually fit your new total mapcount field into page 1 so even avoid the
> > > extra cacheline access. You can have a look: the trick is refcount for
> > > tail page 1 is still seems to be free on 32 bits (if that was your worry
> > > before). Then it'll be very nice if to keep Hugh's counter all in tail 1.
> >
> > No, refcount must be 0 on all tail pages. We rely on this in many places
> > in the MM.
>
> Very right.

Obviously I could have missed this in the past.. can I ask for an example
explaining why refcount will be referenced before knowing it's a head?

>
> One could theoretically
>
> 1) Move the compound/entire mapcount to page[2]
> 2) Make hugetlb stop using the entire mapcount and only the total
> mapcount.
> 3) Then leave total_mapcount and nr_pages_mapped in page[1]
> 4) Make page_mapcount() use total_mapcount for hugetlb.
>
> When (un)mapping a PMD-mapped THP, we would go to page[2]. Otherwise, only
> page[1].
>
> The entire mapcount, similarly to nr_pages_mapped, primarily serves to get
> memory stats right; well, and to implement page_mapcount() for THP.
>
> But I'm not 100% sure yet if the overhead from having nr_pages_mapped in
> page[2] is significant enough at this point.

Worth trying.

Besides the cachelines, cmpxchg should also lock the memory bus and
meanwhile affect the whole processor pipelines in some way. Maybe that
effect is amplified too when it runs in a loop with one more counter.

--
Peter Xu