On 2 Feb 2023, at 10:31, Matthew Wilcox wrote: > 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. What is the logic of using PMD as the basic counting unit? Why not use PTE or PUG? I just cannot understand the goal of doing this. > > [ 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. ] -- Best Regards, Yan, Zi
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