| Message ID | 20240125193227.444072-14-david@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/memory: optimize fork() with PTE-mapped THP | expand |
On 25.01.24 20:32, David Hildenbrand wrote: > Let's implement PTE batching when consecutive (present) PTEs map > consecutive pages of the same large folio, and all other PTE bits besides > the PFNs are equal. > > We will optimize folio_pte_batch() separately, to ignore selected > PTE bits. This patch is based on work by Ryan Roberts. > > Use __always_inline for __copy_present_ptes() and keep the handling for > single PTEs completely separate from the multi-PTE case: we really want > the compiler to optimize for the single-PTE case with small folios, to > not degrade performance. > > Note that PTE batching will never exceed a single page table and will > always stay within VMA boundaries. > > Further, processing PTE-mapped THP that maybe pinned and have > PageAnonExclusive set on at least one subpage should work as expected, > but there is room for improvement: We will repeatedly (1) detect a PTE > batch (2) detect that we have to copy a page (3) fall back and allocate a > single page to copy a single page. For now we won't care as pinned pages > are a corner case, and we should rather look into maintaining only a > single PageAnonExclusive bit for large folios. > > Signed-off-by: David Hildenbrand <david@redhat.com> > --- > include/linux/pgtable.h | 31 +++++++++++ > mm/memory.c | 112 +++++++++++++++++++++++++++++++++------- > 2 files changed, 124 insertions(+), 19 deletions(-) > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h > index 351cd9dc7194f..891ed246978a4 100644 > --- a/include/linux/pgtable.h > +++ b/include/linux/pgtable.h > @@ -650,6 +650,37 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres > } > #endif > > +#ifndef wrprotect_ptes > +/** > + * wrprotect_ptes - Write-protect consecutive pages that are mapped to a > + * contiguous range of addresses. > + * @mm: Address space to map the pages into. > + * @addr: Address the first page is mapped at. > + * @ptep: Page table pointer for the first entry. > + * @nr: Number of pages to write-protect. > + * > + * May be overridden by the architecture; otherwise, implemented as a simple > + * loop over ptep_set_wrprotect(). > + * > + * Note that PTE bits in the PTE range besides the PFN can differ. For example, > + * some PTEs might already be write-protected. > + * > + * Context: The caller holds the page table lock. The pages all belong > + * to the same folio. The PTEs are all in the same PMD. > + */ After writing documentation for another two such functions, I'll change this to: /** * wrprotect_ptes - Write-protect PTEs that map consecutive pages of the same * folio. * @mm: Address space the pages are mapped into. * @addr: Address the first page is mapped at. * @ptep: Page table pointer for the first entry. * @nr: Number of entries to write-protect. * * May be overridden by the architecture; otherwise, implemented as a simple * loop over ptep_set_wrprotect(). * * Note that PTE bits in the PTE range besides the PFN can differ. For example, * some PTEs might be write-protected. * * Context: The caller holds the page table lock. The PTEs map consecutive * pages that belong to the same folio. The PTEs are all in the same PMD. */
On 25/01/2024 19:32, David Hildenbrand wrote: > Let's implement PTE batching when consecutive (present) PTEs map > consecutive pages of the same large folio, and all other PTE bits besides > the PFNs are equal. > > We will optimize folio_pte_batch() separately, to ignore selected > PTE bits. This patch is based on work by Ryan Roberts. > > Use __always_inline for __copy_present_ptes() and keep the handling for > single PTEs completely separate from the multi-PTE case: we really want > the compiler to optimize for the single-PTE case with small folios, to > not degrade performance. > > Note that PTE batching will never exceed a single page table and will > always stay within VMA boundaries. > > Further, processing PTE-mapped THP that maybe pinned and have > PageAnonExclusive set on at least one subpage should work as expected, > but there is room for improvement: We will repeatedly (1) detect a PTE > batch (2) detect that we have to copy a page (3) fall back and allocate a > single page to copy a single page. For now we won't care as pinned pages > are a corner case, and we should rather look into maintaining only a > single PageAnonExclusive bit for large folios. > > Signed-off-by: David Hildenbrand <david@redhat.com> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com> > --- > include/linux/pgtable.h | 31 +++++++++++ > mm/memory.c | 112 +++++++++++++++++++++++++++++++++------- > 2 files changed, 124 insertions(+), 19 deletions(-) > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h > index 351cd9dc7194f..891ed246978a4 100644 > --- a/include/linux/pgtable.h > +++ b/include/linux/pgtable.h > @@ -650,6 +650,37 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres > } > #endif > > +#ifndef wrprotect_ptes > +/** > + * wrprotect_ptes - Write-protect consecutive pages that are mapped to a > + * contiguous range of addresses. > + * @mm: Address space to map the pages into. > + * @addr: Address the first page is mapped at. > + * @ptep: Page table pointer for the first entry. > + * @nr: Number of pages to write-protect. > + * > + * May be overridden by the architecture; otherwise, implemented as a simple > + * loop over ptep_set_wrprotect(). > + * > + * Note that PTE bits in the PTE range besides the PFN can differ. For example, > + * some PTEs might already be write-protected. > + * > + * Context: The caller holds the page table lock. The pages all belong > + * to the same folio. The PTEs are all in the same PMD. > + */ > +static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep, unsigned int nr) > +{ > + for (;;) { > + ptep_set_wrprotect(mm, addr, ptep); > + if (--nr == 0) > + break; > + ptep++; > + addr += PAGE_SIZE; > + } > +} > +#endif > + > /* > * On some architectures hardware does not set page access bit when accessing > * memory page, it is responsibility of software setting this bit. It brings > diff --git a/mm/memory.c b/mm/memory.c > index 729ca4d6a820c..4d1be89a01ee0 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -930,15 +930,15 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma > return 0; > } > > -static inline void __copy_present_pte(struct vm_area_struct *dst_vma, > +static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma, > struct vm_area_struct *src_vma, pte_t *dst_pte, pte_t *src_pte, > - pte_t pte, unsigned long addr) > + pte_t pte, unsigned long addr, int nr) > { > struct mm_struct *src_mm = src_vma->vm_mm; > > /* If it's a COW mapping, write protect it both processes. */ > if (is_cow_mapping(src_vma->vm_flags) && pte_write(pte)) { > - ptep_set_wrprotect(src_mm, addr, src_pte); > + wrprotect_ptes(src_mm, addr, src_pte, nr); > pte = pte_wrprotect(pte); > } > > @@ -950,26 +950,93 @@ static inline void __copy_present_pte(struct vm_area_struct *dst_vma, > if (!userfaultfd_wp(dst_vma)) > pte = pte_clear_uffd_wp(pte); > > - set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte); > + set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr); > +} > + > +/* > + * Detect a PTE batch: consecutive (present) PTEs that map consecutive > + * pages of the same folio. > + * > + * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN. > + */ > +static inline int folio_pte_batch(struct folio *folio, unsigned long addr, > + pte_t *start_ptep, pte_t pte, int max_nr) > +{ > + 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_next_pfn(pte); > + pte_t *ptep = start_ptep + 1; > + > + VM_WARN_ON_FOLIO(!pte_present(pte), folio); > + > + while (ptep != end_ptep) { > + pte = ptep_get(ptep); > + > + 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) > + break; > + > + expected_pte = pte_next_pfn(expected_pte); > + ptep++; > + } > + > + return ptep - start_ptep; > } > > /* > - * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page > - * is required to copy this pte. > + * Copy one present PTE, trying to batch-process subsequent PTEs that map > + * consecutive pages of the same folio by copying them as well. > + * > + * Returns -EAGAIN if one preallocated page is required to copy the next PTE. > + * Otherwise, returns the number of copied PTEs (at least 1). > */ > static inline int > -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > pte_t *dst_pte, pte_t *src_pte, pte_t pte, unsigned long addr, > - int *rss, struct folio **prealloc) > + int max_nr, int *rss, struct folio **prealloc) > { > struct page *page; > struct folio *folio; > + int err, nr; > > page = vm_normal_page(src_vma, addr, pte); > if (unlikely(!page)) > goto copy_pte; > > folio = page_folio(page); > + > + /* > + * If we likely have to copy, just don't bother with batching. Make > + * sure that the common "small folio" case is as fast as possible > + * by keeping the batching logic separate. > + */ > + if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) { > + nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr); > + folio_ref_add(folio, nr); > + if (folio_test_anon(folio)) { > + if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, > + nr, src_vma))) { > + folio_ref_sub(folio, nr); > + return -EAGAIN; > + } > + rss[MM_ANONPAGES] += nr; > + VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio); > + } else { > + folio_dup_file_rmap_ptes(folio, page, nr); > + rss[mm_counter_file(page)] += nr; > + } > + __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte, > + addr, nr); > + return nr; > + } > + > folio_get(folio); > if (folio_test_anon(folio)) { > /* > @@ -981,8 +1048,9 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) { > /* Page may be pinned, we have to copy. */ > folio_put(folio); > - return copy_present_page(dst_vma, src_vma, dst_pte, src_pte, > - addr, rss, prealloc, page); > + err = copy_present_page(dst_vma, src_vma, dst_pte, src_pte, > + addr, rss, prealloc, page); > + return err ? err : 1; > } > rss[MM_ANONPAGES]++; > VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio); > @@ -992,8 +1060,8 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > } > > copy_pte: > - __copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, pte, addr); > - return 0; > + __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte, addr, 1); > + return 1; > } > > static inline struct folio *folio_prealloc(struct mm_struct *src_mm, > @@ -1030,10 +1098,11 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > pte_t *src_pte, *dst_pte; > pte_t ptent; > spinlock_t *src_ptl, *dst_ptl; > - int progress, ret = 0; > + int progress, max_nr, ret = 0; > int rss[NR_MM_COUNTERS]; > swp_entry_t entry = (swp_entry_t){0}; > struct folio *prealloc = NULL; > + int nr; > > again: > progress = 0; > @@ -1064,6 +1133,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > arch_enter_lazy_mmu_mode(); > > do { > + nr = 1; > + > /* > * We are holding two locks at this point - either of them > * could generate latencies in another task on another CPU. > @@ -1100,9 +1171,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > */ > WARN_ON_ONCE(ret != -ENOENT); > } > - /* copy_present_pte() will clear `*prealloc' if consumed */ > - ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, > - ptent, addr, rss, &prealloc); > + /* copy_present_ptes() will clear `*prealloc' if consumed */ > + max_nr = (end - addr) / PAGE_SIZE; > + ret = copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, > + ptent, addr, max_nr, rss, &prealloc); > /* > * If we need a pre-allocated page for this pte, drop the > * locks, allocate, and try again. > @@ -1119,8 +1191,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > folio_put(prealloc); > prealloc = NULL; > } > - progress += 8; > - } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end); > + nr = ret; > + progress += 8 * nr; > + } while (dst_pte += nr, src_pte += nr, addr += PAGE_SIZE * nr, > + addr != end); > > arch_leave_lazy_mmu_mode(); > pte_unmap_unlock(orig_src_pte, src_ptl); > @@ -1141,7 +1215,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, > prealloc = folio_prealloc(src_mm, src_vma, addr, false); > if (!prealloc) > return -ENOMEM; > - } else if (ret) { > + } else if (ret < 0) { > VM_WARN_ON_ONCE(1); > } >
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index 351cd9dc7194f..891ed246978a4 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -650,6 +650,37 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres } #endif +#ifndef wrprotect_ptes +/** + * wrprotect_ptes - Write-protect consecutive pages that are mapped to a + * contiguous range of addresses. + * @mm: Address space to map the pages into. + * @addr: Address the first page is mapped at. + * @ptep: Page table pointer for the first entry. + * @nr: Number of pages to write-protect. + * + * May be overridden by the architecture; otherwise, implemented as a simple + * loop over ptep_set_wrprotect(). + * + * Note that PTE bits in the PTE range besides the PFN can differ. For example, + * some PTEs might already be write-protected. + * + * Context: The caller holds the page table lock. The pages all belong + * to the same folio. The PTEs are all in the same PMD. + */ +static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr, + pte_t *ptep, unsigned int nr) +{ + for (;;) { + ptep_set_wrprotect(mm, addr, ptep); + if (--nr == 0) + break; + ptep++; + addr += PAGE_SIZE; + } +} +#endif + /* * On some architectures hardware does not set page access bit when accessing * memory page, it is responsibility of software setting this bit. It brings diff --git a/mm/memory.c b/mm/memory.c index 729ca4d6a820c..4d1be89a01ee0 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -930,15 +930,15 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma return 0; } -static inline void __copy_present_pte(struct vm_area_struct *dst_vma, +static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, pte_t *dst_pte, pte_t *src_pte, - pte_t pte, unsigned long addr) + pte_t pte, unsigned long addr, int nr) { struct mm_struct *src_mm = src_vma->vm_mm; /* If it's a COW mapping, write protect it both processes. */ if (is_cow_mapping(src_vma->vm_flags) && pte_write(pte)) { - ptep_set_wrprotect(src_mm, addr, src_pte); + wrprotect_ptes(src_mm, addr, src_pte, nr); pte = pte_wrprotect(pte); } @@ -950,26 +950,93 @@ static inline void __copy_present_pte(struct vm_area_struct *dst_vma, if (!userfaultfd_wp(dst_vma)) pte = pte_clear_uffd_wp(pte); - set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte); + set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr); +} + +/* + * Detect a PTE batch: consecutive (present) PTEs that map consecutive + * pages of the same folio. + * + * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN. + */ +static inline int folio_pte_batch(struct folio *folio, unsigned long addr, + pte_t *start_ptep, pte_t pte, int max_nr) +{ + 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_next_pfn(pte); + pte_t *ptep = start_ptep + 1; + + VM_WARN_ON_FOLIO(!pte_present(pte), folio); + + while (ptep != end_ptep) { + pte = ptep_get(ptep); + + 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) + break; + + expected_pte = pte_next_pfn(expected_pte); + ptep++; + } + + return ptep - start_ptep; } /* - * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page - * is required to copy this pte. + * Copy one present PTE, trying to batch-process subsequent PTEs that map + * consecutive pages of the same folio by copying them as well. + * + * Returns -EAGAIN if one preallocated page is required to copy the next PTE. + * Otherwise, returns the number of copied PTEs (at least 1). */ static inline int -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, pte_t *dst_pte, pte_t *src_pte, pte_t pte, unsigned long addr, - int *rss, struct folio **prealloc) + int max_nr, int *rss, struct folio **prealloc) { struct page *page; struct folio *folio; + int err, nr; page = vm_normal_page(src_vma, addr, pte); if (unlikely(!page)) goto copy_pte; folio = page_folio(page); + + /* + * If we likely have to copy, just don't bother with batching. Make + * sure that the common "small folio" case is as fast as possible + * by keeping the batching logic separate. + */ + if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) { + nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr); + folio_ref_add(folio, nr); + if (folio_test_anon(folio)) { + if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, + nr, src_vma))) { + folio_ref_sub(folio, nr); + return -EAGAIN; + } + rss[MM_ANONPAGES] += nr; + VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio); + } else { + folio_dup_file_rmap_ptes(folio, page, nr); + rss[mm_counter_file(page)] += nr; + } + __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte, + addr, nr); + return nr; + } + folio_get(folio); if (folio_test_anon(folio)) { /* @@ -981,8 +1048,9 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) { /* Page may be pinned, we have to copy. */ folio_put(folio); - return copy_present_page(dst_vma, src_vma, dst_pte, src_pte, - addr, rss, prealloc, page); + err = copy_present_page(dst_vma, src_vma, dst_pte, src_pte, + addr, rss, prealloc, page); + return err ? err : 1; } rss[MM_ANONPAGES]++; VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio); @@ -992,8 +1060,8 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, } copy_pte: - __copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, pte, addr); - return 0; + __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte, addr, 1); + return 1; } static inline struct folio *folio_prealloc(struct mm_struct *src_mm, @@ -1030,10 +1098,11 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, pte_t *src_pte, *dst_pte; pte_t ptent; spinlock_t *src_ptl, *dst_ptl; - int progress, ret = 0; + int progress, max_nr, ret = 0; int rss[NR_MM_COUNTERS]; swp_entry_t entry = (swp_entry_t){0}; struct folio *prealloc = NULL; + int nr; again: progress = 0; @@ -1064,6 +1133,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, arch_enter_lazy_mmu_mode(); do { + nr = 1; + /* * We are holding two locks at this point - either of them * could generate latencies in another task on another CPU. @@ -1100,9 +1171,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, */ WARN_ON_ONCE(ret != -ENOENT); } - /* copy_present_pte() will clear `*prealloc' if consumed */ - ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, - ptent, addr, rss, &prealloc); + /* copy_present_ptes() will clear `*prealloc' if consumed */ + max_nr = (end - addr) / PAGE_SIZE; + ret = copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, + ptent, addr, max_nr, rss, &prealloc); /* * If we need a pre-allocated page for this pte, drop the * locks, allocate, and try again. @@ -1119,8 +1191,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, folio_put(prealloc); prealloc = NULL; } - progress += 8; - } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end); + nr = ret; + progress += 8 * nr; + } while (dst_pte += nr, src_pte += nr, addr += PAGE_SIZE * nr, + addr != end); arch_leave_lazy_mmu_mode(); pte_unmap_unlock(orig_src_pte, src_ptl); @@ -1141,7 +1215,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, prealloc = folio_prealloc(src_mm, src_vma, addr, false); if (!prealloc) return -ENOMEM; - } else if (ret) { + } else if (ret < 0) { VM_WARN_ON_ONCE(1); }
Let's implement PTE batching when consecutive (present) PTEs map consecutive pages of the same large folio, and all other PTE bits besides the PFNs are equal. We will optimize folio_pte_batch() separately, to ignore selected PTE bits. This patch is based on work by Ryan Roberts. Use __always_inline for __copy_present_ptes() and keep the handling for single PTEs completely separate from the multi-PTE case: we really want the compiler to optimize for the single-PTE case with small folios, to not degrade performance. Note that PTE batching will never exceed a single page table and will always stay within VMA boundaries. Further, processing PTE-mapped THP that maybe pinned and have PageAnonExclusive set on at least one subpage should work as expected, but there is room for improvement: We will repeatedly (1) detect a PTE batch (2) detect that we have to copy a page (3) fall back and allocate a single page to copy a single page. For now we won't care as pinned pages are a corner case, and we should rather look into maintaining only a single PageAnonExclusive bit for large folios. Signed-off-by: David Hildenbrand <david@redhat.com> --- include/linux/pgtable.h | 31 +++++++++++ mm/memory.c | 112 +++++++++++++++++++++++++++++++++------- 2 files changed, 124 insertions(+), 19 deletions(-)