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

David Hildenbrand
2023-08-10T22:28:07Z

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.

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.

--
Cheers,

David / dhildenb