| Message ID | 20230830095011.1228673-2-ryan.roberts@arm.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Optimize mmap_exit for large folios | expand |
On Wed, Aug 30, 2023 at 10:50:07AM +0100, Ryan Roberts wrote: > Like page_remove_rmap() but batch-removes the rmap for a range of pages > belonging to a folio. This can provide a small speedup due to less > manipuation of the various counters. But more crucially, if removing the > rmap for all pages of a folio in a batch, there is no need to > (spuriously) add it to the deferred split list, which saves significant > cost when there is contention for the split queue lock. > > All contained pages are accounted using the order-0 folio (or base page) > scheme. > > page_remove_rmap() is refactored so that it forwards to > folio_remove_rmap_range() for !compound cases, and both functions now > share a common epilogue function. The intention here is to avoid > duplication of code. What would you think to doing it like this instead? This probably doesn't even compile and it's definitely not sanity checked; just trying to get across an idea of the shape of this code. I think this is more like what DavidH was asking for (but he's on holiday this week so won't be able to confirm). diff --git a/include/linux/rmap.h b/include/linux/rmap.h index a3825ce81102..d442d1e5425d 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -202,6 +202,8 @@ void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr, struct vm_area_struct *, bool compound); void page_remove_rmap(struct page *, struct vm_area_struct *, bool compound); +void folio_remove_rmap_range(struct folio *folio, struct page *page, + int nr, struct vm_area_struct *vma); void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *, unsigned long address, rmap_t flags); diff --git a/mm/rmap.c b/mm/rmap.c index ec7f8e6c9e48..2592be47452e 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1380,24 +1380,26 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma, } /** - * page_remove_rmap - take down pte mapping from a page - * @page: page to remove mapping from - * @vma: the vm area from which the mapping is removed - * @compound: uncharge the page as compound or small page + * folio_remove_rmap_range - Take down PTE mappings from a range of pages. + * @folio: Folio containing all pages in range. + * @page: First page in range to unmap. + * @nr: Number of pages to unmap. -1 to unmap a PMD. + * @vma: The VM area containing the range. * - * The caller needs to hold the pte lock. + * All pages in the range must belong to the same VMA & folio. + * + * Context: Caller holds the pte lock. */ -void page_remove_rmap(struct page *page, struct vm_area_struct *vma, - bool compound) +void folio_remove_rmap_range(struct folio *folio, struct page *page, + int pages, struct vm_area_struct *vma) { - struct folio *folio = page_folio(page); atomic_t *mapped = &folio->_nr_pages_mapped; + int nr_unmapped = 0; + int nr_mapped = 0; int nr = 0, nr_pmdmapped = 0; bool last; enum node_stat_item idx; - VM_BUG_ON_PAGE(compound && !PageHead(page), page); - /* Hugetlb pages are not counted in NR_*MAPPED */ if (unlikely(folio_test_hugetlb(folio))) { /* hugetlb pages are always mapped with pmds */ @@ -1405,14 +1407,25 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, return; } - /* Is page being unmapped by PTE? Is this its last map to be removed? */ - if (likely(!compound)) { - last = atomic_add_negative(-1, &page->_mapcount); - nr = last; - if (last && folio_test_large(folio)) { - nr = atomic_dec_return_relaxed(mapped); - nr = (nr < COMPOUND_MAPPED); + /* Are we taking down a PMD mapping? */ + if (likely(pages > 0)) { + VM_WARN_ON_ONCE(page < folio_page(folio, 0) || + page + pages > folio_page(folio, + folio_nr_pages(folio))); + while (pages) { + /* Is this the page's last map to be removed? */ + last = atomic_add_negative(-1, &page->_mapcount); + if (last) + nr_unmapped++; + pages--; + page++; } + + /* Pages still mapped if folio mapped entirely */ + nr_mapped = atomic_sub_return_relaxed(nr_unmapped, mapped); + if (nr_mapped >= COMPOUND_MAPPED) + nr_unmapped = 0; + } else if (folio_test_pmd_mappable(folio)) { /* That test is redundant: it's for safety or to optimize out */ @@ -1441,18 +1454,19 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, idx = NR_FILE_PMDMAPPED; __lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped); } + if (nr) { idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED; __lruvec_stat_mod_folio(folio, idx, -nr); /* - * Queue anon THP for deferred split if at least one - * page of the folio is unmapped and at least one page - * is still mapped. + * Queue large anon folio for deferred split if at least one + * page of the folio is unmapped and at least one page is still + * mapped. */ - if (folio_test_pmd_mappable(folio) && folio_test_anon(folio)) - if (!compound || nr < nr_pmdmapped) - deferred_split_folio(folio); + if (folio_test_large(folio) && folio_test_anon(folio) && + nr_mapped) + deferred_split_folio(folio); } /* @@ -1466,6 +1480,20 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, munlock_vma_folio(folio, vma, compound); } +/** + * page_remove_rmap - take down pte mapping from a page + * @page: page to remove mapping from + * @vma: the vm area from which the mapping is removed + * @compound: uncharge the page as compound or small page + * + * The caller needs to hold the pte lock. + */ +void page_remove_rmap(struct page *page, struct vm_area_struct *vma, + bool compound) +{ + folio_remove_rmap_range(page_folio(page), page, compound ? -1 : 1, vma); +} + /* * @arg: enum ttu_flags will be passed to this argument */
On 30/08/2023 15:51, Matthew Wilcox wrote: > On Wed, Aug 30, 2023 at 10:50:07AM +0100, Ryan Roberts wrote: >> Like page_remove_rmap() but batch-removes the rmap for a range of pages >> belonging to a folio. This can provide a small speedup due to less >> manipuation of the various counters. But more crucially, if removing the >> rmap for all pages of a folio in a batch, there is no need to >> (spuriously) add it to the deferred split list, which saves significant >> cost when there is contention for the split queue lock. >> >> All contained pages are accounted using the order-0 folio (or base page) >> scheme. >> >> page_remove_rmap() is refactored so that it forwards to >> folio_remove_rmap_range() for !compound cases, and both functions now >> share a common epilogue function. The intention here is to avoid >> duplication of code. > > What would you think to doing it like this instead? This probably doesn't > even compile and it's definitely not sanity checked; just trying to get > across an idea of the shape of this code. I think this is more like > what DavidH was asking for (but he's on holiday this week so won't be > able to confirm). I think it was actually Yu Zhou who was arguing for something more like this? I don't love it, because as I've expressed previously, I don't think an API that operates on a "range" of pages has any business manipulating a PMD mapping, because it can't be subdivided. So my preference is for what I've already got. But certainly your proposal to use nr=-1 to mean compound is the nicest way I've seen since you no longer make nr redundant in this case. Given you and Yu both have a lot more experience with this code than me, I'll follow your advice and do it this way for the next rev. Thanks, Ryan > > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h > index a3825ce81102..d442d1e5425d 100644 > --- a/include/linux/rmap.h > +++ b/include/linux/rmap.h > @@ -202,6 +202,8 @@ void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr, > struct vm_area_struct *, bool compound); > void page_remove_rmap(struct page *, struct vm_area_struct *, > bool compound); > +void folio_remove_rmap_range(struct folio *folio, struct page *page, > + int nr, struct vm_area_struct *vma); > > void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *, > unsigned long address, rmap_t flags); > diff --git a/mm/rmap.c b/mm/rmap.c > index ec7f8e6c9e48..2592be47452e 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1380,24 +1380,26 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma, > } > > /** > - * page_remove_rmap - take down pte mapping from a page > - * @page: page to remove mapping from > - * @vma: the vm area from which the mapping is removed > - * @compound: uncharge the page as compound or small page > + * folio_remove_rmap_range - Take down PTE mappings from a range of pages. > + * @folio: Folio containing all pages in range. > + * @page: First page in range to unmap. > + * @nr: Number of pages to unmap. -1 to unmap a PMD. > + * @vma: The VM area containing the range. > * > - * The caller needs to hold the pte lock. > + * All pages in the range must belong to the same VMA & folio. > + * > + * Context: Caller holds the pte lock. > */ > -void page_remove_rmap(struct page *page, struct vm_area_struct *vma, > - bool compound) > +void folio_remove_rmap_range(struct folio *folio, struct page *page, > + int pages, struct vm_area_struct *vma) > { > - struct folio *folio = page_folio(page); > atomic_t *mapped = &folio->_nr_pages_mapped; > + int nr_unmapped = 0; > + int nr_mapped = 0; > int nr = 0, nr_pmdmapped = 0; > bool last; > enum node_stat_item idx; > > - VM_BUG_ON_PAGE(compound && !PageHead(page), page); > - > /* Hugetlb pages are not counted in NR_*MAPPED */ > if (unlikely(folio_test_hugetlb(folio))) { > /* hugetlb pages are always mapped with pmds */ > @@ -1405,14 +1407,25 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, > return; > } > > - /* Is page being unmapped by PTE? Is this its last map to be removed? */ > - if (likely(!compound)) { > - last = atomic_add_negative(-1, &page->_mapcount); > - nr = last; > - if (last && folio_test_large(folio)) { > - nr = atomic_dec_return_relaxed(mapped); > - nr = (nr < COMPOUND_MAPPED); > + /* Are we taking down a PMD mapping? */ > + if (likely(pages > 0)) { > + VM_WARN_ON_ONCE(page < folio_page(folio, 0) || > + page + pages > folio_page(folio, > + folio_nr_pages(folio))); > + while (pages) { > + /* Is this the page's last map to be removed? */ > + last = atomic_add_negative(-1, &page->_mapcount); > + if (last) > + nr_unmapped++; > + pages--; > + page++; > } > + > + /* Pages still mapped if folio mapped entirely */ > + nr_mapped = atomic_sub_return_relaxed(nr_unmapped, mapped); > + if (nr_mapped >= COMPOUND_MAPPED) > + nr_unmapped = 0; > + > } else if (folio_test_pmd_mappable(folio)) { > /* That test is redundant: it's for safety or to optimize out */ > > @@ -1441,18 +1454,19 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, > idx = NR_FILE_PMDMAPPED; > __lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped); > } > + > if (nr) { > idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED; > __lruvec_stat_mod_folio(folio, idx, -nr); > > /* > - * Queue anon THP for deferred split if at least one > - * page of the folio is unmapped and at least one page > - * is still mapped. > + * Queue large anon folio for deferred split if at least one > + * page of the folio is unmapped and at least one page is still > + * mapped. > */ > - if (folio_test_pmd_mappable(folio) && folio_test_anon(folio)) > - if (!compound || nr < nr_pmdmapped) > - deferred_split_folio(folio); > + if (folio_test_large(folio) && folio_test_anon(folio) && > + nr_mapped) > + deferred_split_folio(folio); > } > > /* > @@ -1466,6 +1480,20 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, > munlock_vma_folio(folio, vma, compound); > } > > +/** > + * page_remove_rmap - take down pte mapping from a page > + * @page: page to remove mapping from > + * @vma: the vm area from which the mapping is removed > + * @compound: uncharge the page as compound or small page > + * > + * The caller needs to hold the pte lock. > + */ > +void page_remove_rmap(struct page *page, struct vm_area_struct *vma, > + bool compound) > +{ > + folio_remove_rmap_range(page_folio(page), page, compound ? -1 : 1, vma); > +} > + > /* > * @arg: enum ttu_flags will be passed to this argument > */
On 30.08.23 17:42, Ryan Roberts wrote: > On 30/08/2023 15:51, Matthew Wilcox wrote: >> On Wed, Aug 30, 2023 at 10:50:07AM +0100, Ryan Roberts wrote: >>> Like page_remove_rmap() but batch-removes the rmap for a range of pages >>> belonging to a folio. This can provide a small speedup due to less >>> manipuation of the various counters. But more crucially, if removing the >>> rmap for all pages of a folio in a batch, there is no need to >>> (spuriously) add it to the deferred split list, which saves significant >>> cost when there is contention for the split queue lock. >>> >>> All contained pages are accounted using the order-0 folio (or base page) >>> scheme. >>> >>> page_remove_rmap() is refactored so that it forwards to >>> folio_remove_rmap_range() for !compound cases, and both functions now >>> share a common epilogue function. The intention here is to avoid >>> duplication of code. >> >> What would you think to doing it like this instead? This probably doesn't >> even compile and it's definitely not sanity checked; just trying to get >> across an idea of the shape of this code. I think this is more like >> what DavidH was asking for (but he's on holiday this week so won't be >> able to confirm). > > I think it was actually Yu Zhou who was arguing for something more like this? I think so, not me. ... but the second variant is certainly shorter.
diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 51cc21ebb568..f2e5af3c0e7f 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -202,6 +202,8 @@ void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr, struct vm_area_struct *, bool compound); void page_remove_rmap(struct page *, struct vm_area_struct *, bool compound); +void folio_remove_rmap_range(struct folio *folio, struct page *page, + int nr, struct vm_area_struct *vma); void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *, unsigned long address, rmap_t flags); diff --git a/mm/rmap.c b/mm/rmap.c index ec7f8e6c9e48..5ea51c70ecd6 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1379,6 +1379,94 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma, folio_add_file_rmap_range(folio, page, nr_pages, vma, compound); } +/** + * __remove_rmap_finish - common operations when taking down a mapping. + * @folio: Folio containing all pages taken down. + * @vma: The VM area containing the range. + * @compound: True if pages were taken down from PMD or false if from PTE(s). + * @nr_unmapped: Number of pages within folio that are now unmapped. + * @nr_mapped: Number of pages within folio that are still mapped. + */ +static void __remove_rmap_finish(struct folio *folio, + struct vm_area_struct *vma, bool compound, + int nr_unmapped, int nr_mapped) +{ + enum node_stat_item idx; + + if (nr_unmapped) { + idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED; + __lruvec_stat_mod_folio(folio, idx, -nr_unmapped); + + /* + * Queue large anon folio for deferred split if at least one + * page of the folio is unmapped and at least one page is still + * mapped. + */ + if (folio_test_large(folio) && + folio_test_anon(folio) && nr_mapped) + deferred_split_folio(folio); + } + + /* + * It would be tidy to reset folio_test_anon mapping when fully + * unmapped, but that might overwrite a racing page_add_anon_rmap + * which increments mapcount after us but sets mapping before us: + * so leave the reset to free_pages_prepare, and remember that + * it's only reliable while mapped. + */ + + munlock_vma_folio(folio, vma, compound); +} + +/** + * folio_remove_rmap_range - Take down PTE mappings from a range of pages. + * @folio: Folio containing all pages in range. + * @page: First page in range to unmap. + * @nr: Number of pages to unmap. + * @vma: The VM area containing the range. + * + * All pages in the range must belong to the same VMA & folio. They must be + * mapped with PTEs, not a PMD. + * + * Context: Caller holds the pte lock. + */ +void folio_remove_rmap_range(struct folio *folio, struct page *page, + int nr, struct vm_area_struct *vma) +{ + atomic_t *mapped = &folio->_nr_pages_mapped; + int nr_unmapped = 0; + int nr_mapped = 0; + bool last; + + if (unlikely(folio_test_hugetlb(folio))) { + VM_WARN_ON_FOLIO(1, folio); + return; + } + + VM_WARN_ON_ONCE(page < &folio->page || + page + nr > (&folio->page + folio_nr_pages(folio))); + + if (!folio_test_large(folio)) { + /* Is this the page's last map to be removed? */ + last = atomic_add_negative(-1, &page->_mapcount); + nr_unmapped = last; + } else { + for (; nr != 0; nr--, page++) { + /* Is this the page's last map to be removed? */ + last = atomic_add_negative(-1, &page->_mapcount); + if (last) + nr_unmapped++; + } + + /* Pages still mapped if folio mapped entirely */ + nr_mapped = atomic_sub_return_relaxed(nr_unmapped, mapped); + if (nr_mapped >= COMPOUND_MAPPED) + nr_unmapped = 0; + } + + __remove_rmap_finish(folio, vma, false, nr_unmapped, nr_mapped); +} + /** * page_remove_rmap - take down pte mapping from a page * @page: page to remove mapping from @@ -1405,15 +1493,13 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, return; } - /* Is page being unmapped by PTE? Is this its last map to be removed? */ + /* Is page being unmapped by PTE? */ if (likely(!compound)) { - last = atomic_add_negative(-1, &page->_mapcount); - nr = last; - if (last && folio_test_large(folio)) { - nr = atomic_dec_return_relaxed(mapped); - nr = (nr < COMPOUND_MAPPED); - } - } else if (folio_test_pmd_mappable(folio)) { + folio_remove_rmap_range(folio, page, 1, vma); + return; + } + + if (folio_test_pmd_mappable(folio)) { /* That test is redundant: it's for safety or to optimize out */ last = atomic_add_negative(-1, &folio->_entire_mapcount); @@ -1441,29 +1527,8 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma, idx = NR_FILE_PMDMAPPED; __lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped); } - if (nr) { - idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED; - __lruvec_stat_mod_folio(folio, idx, -nr); - - /* - * Queue anon THP for deferred split if at least one - * page of the folio is unmapped and at least one page - * is still mapped. - */ - if (folio_test_pmd_mappable(folio) && folio_test_anon(folio)) - if (!compound || nr < nr_pmdmapped) - deferred_split_folio(folio); - } - - /* - * It would be tidy to reset folio_test_anon mapping when fully - * unmapped, but that might overwrite a racing page_add_anon_rmap - * which increments mapcount after us but sets mapping before us: - * so leave the reset to free_pages_prepare, and remember that - * it's only reliable while mapped. - */ - munlock_vma_folio(folio, vma, compound); + __remove_rmap_finish(folio, vma, compound, nr, nr_pmdmapped - nr); } /*
Like page_remove_rmap() but batch-removes the rmap for a range of pages belonging to a folio. This can provide a small speedup due to less manipuation of the various counters. But more crucially, if removing the rmap for all pages of a folio in a batch, there is no need to (spuriously) add it to the deferred split list, which saves significant cost when there is contention for the split queue lock. All contained pages are accounted using the order-0 folio (or base page) scheme. page_remove_rmap() is refactored so that it forwards to folio_remove_rmap_range() for !compound cases, and both functions now share a common epilogue function. The intention here is to avoid duplication of code. Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> --- include/linux/rmap.h | 2 + mm/rmap.c | 125 ++++++++++++++++++++++++++++++++----------- 2 files changed, 97 insertions(+), 30 deletions(-)