Re: Folio mapcount

[Matthew Wilcox <willy@infradead.org>]

On Wed, Feb 01, 2023 at 07:45:17PM -0800, Mike Kravetz wrote:
> On 01/24/23 18:13, Matthew Wilcox wrote:
> > Once we get to the part of the folio journey where we have 
> > one-pointer-per-page, we can't afford to maintain per-page state.
> > Currently we maintain a per-page mapcount, and that will have to go. 
> > We can maintain extra state for a multi-page folio, but it has to be a
> > constant amount of extra state no matter how many pages are in the folio.
> > 
> > My proposal is that we maintain a single mapcount per folio, and its
> > definition is the number of (vma, page table) tuples which have a
> > reference to any pages in this folio.
> 
> Hi Matthew, finally took a look at this.  Can you clarify your definition of
> 'page table' here?  I think you are talking about all the entries within
> one page table page?  Is that correct?  It certainly makes sense in this
> context.
> 
> I have always thought of page table as the entire tree structure starting at
> *pgd in the mm_struct.  So, I was a bit confused.  But, I now see elsewhere
> that 'page table' may refer to either.

Yes, we're pretty sloppy about that.  What I had in mind was:

We have a large folio which is mapped at, say, (1.9MB - 2.1MB) in the
user address space.  There are thus multiple PTEs which map it and some
of those PTEs belong to one PMD and the rest belong to a second PMD.
It has a mapcount of 2 due to being mapped by PTE entries belonging to
two PMD tables.  If it were mapped at (2.1-2.3MB), it would have a
mapcount of 1 due to all its PTEs belonging to a single PMD table.

[ Mike & I spoke earlier this week about what should happen with mapcount
and a theoretical aligned 1GB THP that has its PUD mapping split into
PTE mappings.  Splitting a PMD to PTEs does not affect the mapcount
since all of the PTEs are now referenced from a single PMD table instead
of from a PMD entry.  But splitting a PUD to PTEs should increment the
mapcount by 511 since the folio is now referenced from 512 PMD tables. ]