Messages in this thread |  | | Date | Tue, 23 Jan 2024 20:33:09 +0100 | | Subject | Re: [PATCH v1 00/11] mm/memory: optimize fork() with PTE-mapped THP | | From | David Hildenbrand <> |
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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.
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.
-- Cheers,
David / dhildenb
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