On 23/01/2024 19:33, David Hildenbrand wrote: > On 23.01.24 20:15, Ryan Roberts wrote: >> On 22/01/2024 19:41, David Hildenbrand wrote: >>> Now that the rmap overhaul[1] is upstream that provides a clean interface >>> for rmap batching, let's implement PTE batching during fork when processing >>> PTE-mapped THPs. >>> >>> This series is partially based on Ryan's previous work[2] to implement >>> cont-pte support on arm64, but its a complete rewrite based on [1] to >>> optimize all architectures independent of any such PTE bits, and to >>> use the new rmap batching functions that simplify the code and prepare >>> for further rmap accounting changes. >>> >>> We collect consecutive PTEs that map consecutive pages of the same large >>> folio, making sure that the other PTE bits are compatible, and (a) adjust >>> the refcount only once per batch, (b) call rmap handling functions only >>> once per batch and (c) perform batch PTE setting/updates. >>> >>> While this series should be beneficial for adding cont-pte support on >>> ARM64[2], it's one of the requirements for maintaining a total mapcount[3] >>> for large folios with minimal added overhead and further changes[4] that >>> build up on top of the total mapcount. >> >> I'm currently rebasing my contpte work onto this series, and have hit a problem. >> I need to expose the "size" of a pte (pte_size()) and skip forward to the start >> of the next (cont)pte every time through the folio_pte_batch() loop. But >> pte_next_pfn() only allows advancing by 1 pfn; I need to advance by nr pfns: >> >> >> static inline int folio_pte_batch(struct folio *folio, unsigned long addr, >> pte_t *start_ptep, pte_t pte, int max_nr, bool *any_writable) >> { >> unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio); >> const pte_t *end_ptep = start_ptep + max_nr; >> pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte)); >> - pte_t *ptep = start_ptep + 1; >> + pte_t *ptep = start_ptep; >> + int vfn, nr, i; >> bool writable; >> >> if (any_writable) >> *any_writable = false; >> >> VM_WARN_ON_FOLIO(!pte_present(pte), folio); >> >> + vfn = addr >> PAGE_SIZE; >> + nr = pte_size(pte); >> + nr = ALIGN_DOWN(vfn + nr, nr) - vfn; >> + ptep += nr; >> + >> while (ptep != end_ptep) { >> + pte = ptep_get(ptep); >> nr = pte_size(pte); >> if (any_writable) >> writable = !!pte_write(pte); >> pte = __pte_batch_clear_ignored(pte); >> >> if (!pte_same(pte, expected_pte)) >> break; >> >> /* >> * Stop immediately once we reached the end of the folio. In >> * corner cases the next PFN might fall into a different >> * folio. >> */ >> - if (pte_pfn(pte) == folio_end_pfn) >> + if (pte_pfn(pte) >= folio_end_pfn) >> break; >> >> if (any_writable) >> *any_writable |= writable; >> >> - expected_pte = pte_next_pfn(expected_pte); >> - ptep++; >> + for (i = 0; i < nr; i++) >> + expected_pte = pte_next_pfn(expected_pte); >> + ptep += nr; >> } >> >> return ptep - start_ptep; >> } >> >> >> So I'm wondering if instead of enabling pte_next_pfn() for all the arches, >> perhaps its actually better to expose pte_pgprot() for all the arches. Then we >> can be much more flexible about generating ptes with pfn_pte(pfn, pgprot). >> >> What do you think? > > The pte_pgprot() stuff is just nasty IMHO. I dunno; we have pfn_pte() which takes a pfn and a pgprot. It seems reasonable that we should be able to do the reverse. > > Likely it's best to simply convert pte_next_pfn() to something like > pte_advance_pfns(). The we could just have > > #define pte_next_pfn(pte) pte_advance_pfns(pte, 1) > > That should be fairly easy to do on top (based on PFN_PTE_SHIFT). And only 3 > archs (x86-64, arm64, and powerpc) need slight care to replace a hardcoded "1" > by an integer we pass in. I thought we agreed powerpc was safe to just define PFN_PTE_SHIFT? But, yeah, the principle works I guess. I guess I can do this change along with my series. >