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

[David Hildenbrand <david@redhat.com>]

On 10.08.23 23:54, Matthew Wilcox wrote:
> On Thu, Aug 10, 2023 at 05:48:19PM -0400, Peter Xu wrote:
>>> Yes, that comment from Hugh primarily discusses how we could possibly
>>> optimize the loop, and if relying on folio_nr_pages_mapped() to reduce the
>>> iterations would be racy. As far as I can see, there are cases where "it
>>> would be certainly a bad idea" :)
>>
>> Is the race described about mapcount being changed right after it's read?
>> Are you aware of anything specific that will be broken, and will be fixed
>> with this patch?
> 
> The problem is that people check the mapcount while holding no locks;
> not the PTL, not the page lock.  So it's an unfixable race.
> 
>> Having a total mapcount does sound helpful if partial folio is common
>> indeed.
>>
>> I'm curious whether that'll be so common after the large anon folio work -
>> isn't it be sad if partial folio will be a norm?  It sounds to me that's
>> the case when small page sizes should be used.. and it's prone to waste?
> 
> The problem is that entire_mapcount isn't really entire_mapcount.
> It's pmd_mapcount.  I have had thoughts about using it as entire_mapcount,
> but it gets gnarly when people do partial unmaps.  So the _usual_ case
> ends up touching every struct page.  Which sucks.  Also it's one of the
> things which stands in the way of shrinking struct page.

Right, so one current idea is to have a single total_mapcount and look 
into removing the subpage mapcounts (which will require first removing 
_nr_pages_mapped, because that's still one of the important users).

Until we get there, also rmap code has to do eventually "more tracking" 
and might, unfortunately, end up slower.

> 
> But it's kind of annoying to explain all of this to you individually.
> There have been hundreds of emails about it over the last months on
> this mailing list.  It would be nice if you could catch up instead of
> jumping in.

To be fair, a lot of the details are not readily available and in the 
heads of selected people :)

Peter, if you're interested, we can discuss the current plans, issues 
and ideas offline!

-- 
Cheers,

David / dhildenb