| Message ID | 25a09a7a-81a9-e9c2-7567-c94ce18ac2@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm,thp,rmap: rework the use of subpages_mapcount | expand |
On Fri, Nov 18, 2022 at 01:14:17AM -0800, Hugh Dickins wrote: > Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? > Yes. Not by atomic64_t or cmpxchg games, those get difficult on 32-bit; > but if we slightly abuse subpages_mapcount by additionally demanding that > one bit be set there when the compound page is PMD-mapped, then a cascade > of two atomic ops is able to maintain the stats without bit_spin_lock. Yay! New home for PageDoubleMap()! :P > This is harder to reason about than when bit_spin_locked, but I believe > safe; and no drift in stats detected when testing. When there are racing > removes and adds, of course the sequence of operations is less well- > defined; but each operation on subpages_mapcount is atomically good. > What might be disastrous, is if subpages_mapcount could ever fleetingly > appear negative: but the pte lock (or pmd lock) these rmap functions are > called under, ensures that a last remove cannot race ahead of a first add. > > Continue to make an exception for hugetlb (PageHuge) pages, though that > exception can be easily removed by a further commit if necessary: leave > subpages_mapcount 0, don't bother with COMPOUND_MAPPED in its case, just > carry on checking compound_mapcount too in folio_mapped(), page_mapped(). > > Evidence is that this way goes slightly faster than the previous > implementation in all cases (pmds after ptes now taking around 103ms); > and relieves us of worrying about contention on the bit_spin_lock. > > Signed-off-by: Hugh Dickins <hughd@google.com> Jokes aside, looks neat. Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> As always few minor nits below. > --- > Documentation/mm/transhuge.rst | 7 +- > include/linux/mm.h | 19 ++++- > include/linux/rmap.h | 12 ++-- > mm/debug.c | 2 +- > mm/rmap.c | 124 +++++++-------------------------- > 5 files changed, 52 insertions(+), 112 deletions(-) > > diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst > index af4c9d70321d..ec3dc5b04226 100644 > --- a/Documentation/mm/transhuge.rst > +++ b/Documentation/mm/transhuge.rst > @@ -118,15 +118,14 @@ pages: > succeeds on tail pages. > > - map/unmap of PMD entry for the whole compound page increment/decrement > - ->compound_mapcount, stored in the first tail page of the compound page. > + ->compound_mapcount, stored in the first tail page of the compound page; > + and also increment/decrement ->subpages_mapcount (also in the first tail) > + by COMPOUND_MAPPED when compound_mapcount goes from -1 to 0 or 0 to -1. > > - map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount > on relevant sub-page of the compound page, and also increment/decrement > ->subpages_mapcount, stored in first tail page of the compound page, when > _mapcount goes from -1 to 0 or 0 to -1: counting sub-pages mapped by PTE. > - In order to have race-free accounting of sub-pages mapped, changes to > - sub-page ->_mapcount, ->subpages_mapcount and ->compound_mapcount are > - are all locked by bit_spin_lock of PG_locked in the first tail ->flags. > > split_huge_page internally has to distribute the refcounts in the head > page to the tail pages before clearing all PG_head/tail bits from the page > diff --git a/include/linux/mm.h b/include/linux/mm.h > index c9e46d4d46f2..a2bfb5e4be62 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -828,7 +828,16 @@ static inline int head_compound_mapcount(struct page *head) > } > > /* > - * Number of sub-pages mapped by PTE, does not include compound mapcount. > + * If a 16GB hugetlb page were mapped by PTEs of all of its 4kB sub-pages, > + * its subpages_mapcount would be 0x400000: choose the COMPOUND_MAPPED bit > + * above that range, instead of 2*(PMD_SIZE/PAGE_SIZE). Hugetlb currently > + * leaves subpages_mapcount at 0, but avoid surprise if it participates later. > + */ > +#define COMPOUND_MAPPED 0x800000 > +#define SUBPAGES_MAPPED (COMPOUND_MAPPED - 1) > + > +/* > + * Number of sub-pages mapped by PTE, plus COMPOUND_MAPPED if compound mapped. > * Must be called only on head of compound page. > */ > static inline int head_subpages_mapcount(struct page *head) > @@ -893,8 +902,12 @@ static inline int total_mapcount(struct page *page) > > static inline bool folio_large_is_mapped(struct folio *folio) > { > - return atomic_read(folio_mapcount_ptr(folio)) + > - atomic_read(folio_subpages_mapcount_ptr(folio)) >= 0; > + /* > + * Reading folio_mapcount_ptr() below could be omitted if hugetlb > + * participated in incrementing subpages_mapcount when compound mapped. > + */ > + return atomic_read(folio_mapcount_ptr(folio)) >= 0 || > + atomic_read(folio_subpages_mapcount_ptr(folio)) > 0; Maybe check folio_subpages_mapcount_ptr() first? It would avoid folio_mapcount_ptr() read for everything, but hugetlb. > } > > /** > diff --git a/include/linux/rmap.h b/include/linux/rmap.h > index 860f558126ac..bd3504d11b15 100644 > --- a/include/linux/rmap.h > +++ b/include/linux/rmap.h > @@ -204,14 +204,14 @@ void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *, > void hugepage_add_new_anon_rmap(struct page *, struct vm_area_struct *, > unsigned long address); > > -void page_dup_compound_rmap(struct page *page); > +static inline void __page_dup_rmap(struct page *page, bool compound) > +{ > + atomic_inc(compound ? compound_mapcount_ptr(page) : &page->_mapcount); > +} > > static inline void page_dup_file_rmap(struct page *page, bool compound) > { > - if (likely(!compound /* page is mapped by PTE */)) > - atomic_inc(&page->_mapcount); > - else > - page_dup_compound_rmap(page); > + __page_dup_rmap(page, compound); > } > > /** > @@ -260,7 +260,7 @@ static inline int page_try_dup_anon_rmap(struct page *page, bool compound, > * the page R/O into both processes. > */ > dup: > - page_dup_file_rmap(page, compound); > + __page_dup_rmap(page, compound); > return 0; > } > > diff --git a/mm/debug.c b/mm/debug.c > index 7f8e5f744e42..1ef2ff6a05cb 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -97,7 +97,7 @@ static void __dump_page(struct page *page) > pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n", > head, compound_order(head), > head_compound_mapcount(head), > - head_subpages_mapcount(head), > + head_subpages_mapcount(head) & SUBPAGES_MAPPED, Looks like applying the SUBPAGES_MAPPED mask belong to the head_subpages_mapcount() helper, not to the caller. > head_compound_pincount(head)); > } > > diff --git a/mm/rmap.c b/mm/rmap.c > index 66be8cae640f..5e4ce0a6d6f1 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1085,38 +1085,6 @@ int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff, > return page_vma_mkclean_one(&pvmw); > } > > -struct compound_mapcounts { > - unsigned int compound_mapcount; > - unsigned int subpages_mapcount; > -}; > - > -/* > - * lock_compound_mapcounts() first locks, then copies subpages_mapcount and > - * compound_mapcount from head[1].compound_mapcount and subpages_mapcount, > - * converting from struct page's internal representation to logical count > - * (that is, adding 1 to compound_mapcount to hide its offset by -1). > - */ > -static void lock_compound_mapcounts(struct page *head, > - struct compound_mapcounts *local) > -{ > - bit_spin_lock(PG_locked, &head[1].flags); > - local->compound_mapcount = atomic_read(compound_mapcount_ptr(head)) + 1; > - local->subpages_mapcount = atomic_read(subpages_mapcount_ptr(head)); > -} > - > -/* > - * After caller has updated subpage._mapcount, local subpages_mapcount and > - * local compound_mapcount, as necessary, unlock_compound_mapcounts() converts > - * and copies them back to the compound head[1] fields, and then unlocks. > - */ > -static void unlock_compound_mapcounts(struct page *head, > - struct compound_mapcounts *local) > -{ > - atomic_set(compound_mapcount_ptr(head), local->compound_mapcount - 1); > - atomic_set(subpages_mapcount_ptr(head), local->subpages_mapcount); > - bit_spin_unlock(PG_locked, &head[1].flags); > -} > - > int total_compound_mapcount(struct page *head) > { > int mapcount = head_compound_mapcount(head); > @@ -1124,7 +1092,7 @@ int total_compound_mapcount(struct page *head) > int i; > > /* In the common case, avoid the loop when no subpages mapped by PTE */ > - if (head_subpages_mapcount(head) == 0) > + if ((head_subpages_mapcount(head) & SUBPAGES_MAPPED) == 0) > return mapcount; > /* > * Add all the PTE mappings of those subpages mapped by PTE. > @@ -1140,35 +1108,6 @@ int total_compound_mapcount(struct page *head) > return mapcount; > } > > -/* > - * page_dup_compound_rmap(), used when copying mm, > - * provides a simple example of using lock_ and unlock_compound_mapcounts(). > - */ > -void page_dup_compound_rmap(struct page *head) > -{ > - struct compound_mapcounts mapcounts; > - > - /* > - * Hugetlb pages could use lock_compound_mapcounts(), like THPs do; > - * but at present they are still being managed by atomic operations: > - * which are likely to be somewhat faster, so don't rush to convert > - * them over without evaluating the effect. > - * > - * Note that hugetlb does not call page_add_file_rmap(): > - * here is where hugetlb shared page mapcount is raised. > - */ > - if (PageHuge(head)) { > - atomic_inc(compound_mapcount_ptr(head)); > - > - } else if (PageTransHuge(head)) { > - /* That test is redundant: it's for safety or to optimize out */ > - > - lock_compound_mapcounts(head, &mapcounts); > - mapcounts.compound_mapcount++; > - unlock_compound_mapcounts(head, &mapcounts); > - } > -} > - > /** > * page_move_anon_rmap - move a page to our anon_vma > * @page: the page to move to our anon_vma > @@ -1278,7 +1217,7 @@ static void __page_check_anon_rmap(struct page *page, > void page_add_anon_rmap(struct page *page, > struct vm_area_struct *vma, unsigned long address, rmap_t flags) > { > - struct compound_mapcounts mapcounts; > + atomic_t *mapped; > int nr = 0, nr_pmdmapped = 0; > bool compound = flags & RMAP_COMPOUND; > bool first; > @@ -1290,24 +1229,20 @@ void page_add_anon_rmap(struct page *page, > first = atomic_inc_and_test(&page->_mapcount); > nr = first; > if (first && PageCompound(page)) { > - struct page *head = compound_head(page); > - > - lock_compound_mapcounts(head, &mapcounts); > - mapcounts.subpages_mapcount++; > - nr = !mapcounts.compound_mapcount; > - unlock_compound_mapcounts(head, &mapcounts); > + mapped = subpages_mapcount_ptr(compound_head(page)); > + nr = atomic_inc_return_relaxed(mapped); > + nr = !(nr & COMPOUND_MAPPED); > } > } else if (PageTransHuge(page)) { > /* That test is redundant: it's for safety or to optimize out */ > > - lock_compound_mapcounts(page, &mapcounts); > - first = !mapcounts.compound_mapcount; > - mapcounts.compound_mapcount++; > + first = atomic_inc_and_test(compound_mapcount_ptr(page)); > if (first) { > + mapped = subpages_mapcount_ptr(page); > + nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped); > nr_pmdmapped = thp_nr_pages(page); > - nr = nr_pmdmapped - mapcounts.subpages_mapcount; > + nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED); > } > - unlock_compound_mapcounts(page, &mapcounts); > } > > VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page); > @@ -1360,6 +1295,7 @@ void page_add_new_anon_rmap(struct page *page, > VM_BUG_ON_PAGE(!PageTransHuge(page), page); > /* increment count (starts at -1) */ > atomic_set(compound_mapcount_ptr(page), 0); > + atomic_set(subpages_mapcount_ptr(page), COMPOUND_MAPPED); > nr = thp_nr_pages(page); > __mod_lruvec_page_state(page, NR_ANON_THPS, nr); > } > @@ -1379,7 +1315,7 @@ void page_add_new_anon_rmap(struct page *page, > void page_add_file_rmap(struct page *page, > struct vm_area_struct *vma, bool compound) > { > - struct compound_mapcounts mapcounts; > + atomic_t *mapped; > int nr = 0, nr_pmdmapped = 0; > bool first; > > @@ -1390,24 +1326,20 @@ void page_add_file_rmap(struct page *page, > first = atomic_inc_and_test(&page->_mapcount); > nr = first; > if (first && PageCompound(page)) { > - struct page *head = compound_head(page); > - > - lock_compound_mapcounts(head, &mapcounts); > - mapcounts.subpages_mapcount++; > - nr = !mapcounts.compound_mapcount; > - unlock_compound_mapcounts(head, &mapcounts); > + mapped = subpages_mapcount_ptr(compound_head(page)); > + nr = atomic_inc_return_relaxed(mapped); > + nr = !(nr & COMPOUND_MAPPED); > } > } else if (PageTransHuge(page)) { > /* That test is redundant: it's for safety or to optimize out */ > > - lock_compound_mapcounts(page, &mapcounts); > - first = !mapcounts.compound_mapcount; > - mapcounts.compound_mapcount++; > + first = atomic_inc_and_test(compound_mapcount_ptr(page)); > if (first) { > + mapped = subpages_mapcount_ptr(page); > + nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped); > nr_pmdmapped = thp_nr_pages(page); > - nr = nr_pmdmapped - mapcounts.subpages_mapcount; > + nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED); > } > - unlock_compound_mapcounts(page, &mapcounts); > } > > if (nr_pmdmapped) > @@ -1431,7 +1363,7 @@ void page_add_file_rmap(struct page *page, > void page_remove_rmap(struct page *page, > struct vm_area_struct *vma, bool compound) > { > - struct compound_mapcounts mapcounts; > + atomic_t *mapped; > int nr = 0, nr_pmdmapped = 0; > bool last; > > @@ -1451,24 +1383,20 @@ void page_remove_rmap(struct page *page, > last = atomic_add_negative(-1, &page->_mapcount); > nr = last; > if (last && PageCompound(page)) { > - struct page *head = compound_head(page); > - > - lock_compound_mapcounts(head, &mapcounts); > - mapcounts.subpages_mapcount--; > - nr = !mapcounts.compound_mapcount; > - unlock_compound_mapcounts(head, &mapcounts); > + mapped = subpages_mapcount_ptr(compound_head(page)); > + nr = atomic_dec_return_relaxed(mapped); > + nr = !(nr & COMPOUND_MAPPED); > } > } else if (PageTransHuge(page)) { > /* That test is redundant: it's for safety or to optimize out */ > > - lock_compound_mapcounts(page, &mapcounts); > - mapcounts.compound_mapcount--; > - last = !mapcounts.compound_mapcount; > + last = atomic_add_negative(-1, compound_mapcount_ptr(page)); > if (last) { > + mapped = subpages_mapcount_ptr(page); > + nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped); > nr_pmdmapped = thp_nr_pages(page); > - nr = nr_pmdmapped - mapcounts.subpages_mapcount; > + nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED); > } > - unlock_compound_mapcounts(page, &mapcounts); > } > > if (nr_pmdmapped) { > -- > 2.35.3 >
On Mon, 21 Nov 2022, Kirill A. Shutemov wrote: > On Fri, Nov 18, 2022 at 01:14:17AM -0800, Hugh Dickins wrote: > > Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? > > Yes. Not by atomic64_t or cmpxchg games, those get difficult on 32-bit; > > but if we slightly abuse subpages_mapcount by additionally demanding that > > one bit be set there when the compound page is PMD-mapped, then a cascade > > of two atomic ops is able to maintain the stats without bit_spin_lock. > > Yay! New home for PageDoubleMap()! :P :) You only asked for one bit for PageDoubleMap, I've been greedier; so it's not surprising if it has worked out better now. ... > Jokes aside, looks neat. > > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Thanks; but I'm very glad that Linus expressed his dissatisfaction with the first implementation, this one does feel much better. > > As always few minor nits below. ... > > @@ -893,8 +902,12 @@ static inline int total_mapcount(struct page *page) > > > > static inline bool folio_large_is_mapped(struct folio *folio) > > { > > - return atomic_read(folio_mapcount_ptr(folio)) + > > - atomic_read(folio_subpages_mapcount_ptr(folio)) >= 0; > > + /* > > + * Reading folio_mapcount_ptr() below could be omitted if hugetlb > > + * participated in incrementing subpages_mapcount when compound mapped. > > + */ > > + return atomic_read(folio_mapcount_ptr(folio)) >= 0 || > > + atomic_read(folio_subpages_mapcount_ptr(folio)) > 0; > > Maybe check folio_subpages_mapcount_ptr() first? It would avoid > folio_mapcount_ptr() read for everything, but hugetlb. Okay: I'm not convinced, but don't mind switching those around: done. > > --- a/mm/debug.c > > +++ b/mm/debug.c > > @@ -97,7 +97,7 @@ static void __dump_page(struct page *page) > > pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n", > > head, compound_order(head), > > head_compound_mapcount(head), > > - head_subpages_mapcount(head), > > + head_subpages_mapcount(head) & SUBPAGES_MAPPED, > > Looks like applying the SUBPAGES_MAPPED mask belong to the > head_subpages_mapcount() helper, not to the caller. Yes, that would be more consistent, helper function doing the massage. Done. __dump_page() then remains unchanged, but free_tail_pages_check() uses subpages_mapcount_ptr(head_page) to check the whole field is zero. v2 coming up - thanks. Hugh
diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst index af4c9d70321d..ec3dc5b04226 100644 --- a/Documentation/mm/transhuge.rst +++ b/Documentation/mm/transhuge.rst @@ -118,15 +118,14 @@ pages: succeeds on tail pages. - map/unmap of PMD entry for the whole compound page increment/decrement - ->compound_mapcount, stored in the first tail page of the compound page. + ->compound_mapcount, stored in the first tail page of the compound page; + and also increment/decrement ->subpages_mapcount (also in the first tail) + by COMPOUND_MAPPED when compound_mapcount goes from -1 to 0 or 0 to -1. - map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount on relevant sub-page of the compound page, and also increment/decrement ->subpages_mapcount, stored in first tail page of the compound page, when _mapcount goes from -1 to 0 or 0 to -1: counting sub-pages mapped by PTE. - In order to have race-free accounting of sub-pages mapped, changes to - sub-page ->_mapcount, ->subpages_mapcount and ->compound_mapcount are - are all locked by bit_spin_lock of PG_locked in the first tail ->flags. split_huge_page internally has to distribute the refcounts in the head page to the tail pages before clearing all PG_head/tail bits from the page diff --git a/include/linux/mm.h b/include/linux/mm.h index c9e46d4d46f2..a2bfb5e4be62 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -828,7 +828,16 @@ static inline int head_compound_mapcount(struct page *head) } /* - * Number of sub-pages mapped by PTE, does not include compound mapcount. + * If a 16GB hugetlb page were mapped by PTEs of all of its 4kB sub-pages, + * its subpages_mapcount would be 0x400000: choose the COMPOUND_MAPPED bit + * above that range, instead of 2*(PMD_SIZE/PAGE_SIZE). Hugetlb currently + * leaves subpages_mapcount at 0, but avoid surprise if it participates later. + */ +#define COMPOUND_MAPPED 0x800000 +#define SUBPAGES_MAPPED (COMPOUND_MAPPED - 1) + +/* + * Number of sub-pages mapped by PTE, plus COMPOUND_MAPPED if compound mapped. * Must be called only on head of compound page. */ static inline int head_subpages_mapcount(struct page *head) @@ -893,8 +902,12 @@ static inline int total_mapcount(struct page *page) static inline bool folio_large_is_mapped(struct folio *folio) { - return atomic_read(folio_mapcount_ptr(folio)) + - atomic_read(folio_subpages_mapcount_ptr(folio)) >= 0; + /* + * Reading folio_mapcount_ptr() below could be omitted if hugetlb + * participated in incrementing subpages_mapcount when compound mapped. + */ + return atomic_read(folio_mapcount_ptr(folio)) >= 0 || + atomic_read(folio_subpages_mapcount_ptr(folio)) > 0; } /** diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 860f558126ac..bd3504d11b15 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -204,14 +204,14 @@ void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *, void hugepage_add_new_anon_rmap(struct page *, struct vm_area_struct *, unsigned long address); -void page_dup_compound_rmap(struct page *page); +static inline void __page_dup_rmap(struct page *page, bool compound) +{ + atomic_inc(compound ? compound_mapcount_ptr(page) : &page->_mapcount); +} static inline void page_dup_file_rmap(struct page *page, bool compound) { - if (likely(!compound /* page is mapped by PTE */)) - atomic_inc(&page->_mapcount); - else - page_dup_compound_rmap(page); + __page_dup_rmap(page, compound); } /** @@ -260,7 +260,7 @@ static inline int page_try_dup_anon_rmap(struct page *page, bool compound, * the page R/O into both processes. */ dup: - page_dup_file_rmap(page, compound); + __page_dup_rmap(page, compound); return 0; } diff --git a/mm/debug.c b/mm/debug.c index 7f8e5f744e42..1ef2ff6a05cb 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -97,7 +97,7 @@ static void __dump_page(struct page *page) pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n", head, compound_order(head), head_compound_mapcount(head), - head_subpages_mapcount(head), + head_subpages_mapcount(head) & SUBPAGES_MAPPED, head_compound_pincount(head)); } diff --git a/mm/rmap.c b/mm/rmap.c index 66be8cae640f..5e4ce0a6d6f1 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1085,38 +1085,6 @@ int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff, return page_vma_mkclean_one(&pvmw); } -struct compound_mapcounts { - unsigned int compound_mapcount; - unsigned int subpages_mapcount; -}; - -/* - * lock_compound_mapcounts() first locks, then copies subpages_mapcount and - * compound_mapcount from head[1].compound_mapcount and subpages_mapcount, - * converting from struct page's internal representation to logical count - * (that is, adding 1 to compound_mapcount to hide its offset by -1). - */ -static void lock_compound_mapcounts(struct page *head, - struct compound_mapcounts *local) -{ - bit_spin_lock(PG_locked, &head[1].flags); - local->compound_mapcount = atomic_read(compound_mapcount_ptr(head)) + 1; - local->subpages_mapcount = atomic_read(subpages_mapcount_ptr(head)); -} - -/* - * After caller has updated subpage._mapcount, local subpages_mapcount and - * local compound_mapcount, as necessary, unlock_compound_mapcounts() converts - * and copies them back to the compound head[1] fields, and then unlocks. - */ -static void unlock_compound_mapcounts(struct page *head, - struct compound_mapcounts *local) -{ - atomic_set(compound_mapcount_ptr(head), local->compound_mapcount - 1); - atomic_set(subpages_mapcount_ptr(head), local->subpages_mapcount); - bit_spin_unlock(PG_locked, &head[1].flags); -} - int total_compound_mapcount(struct page *head) { int mapcount = head_compound_mapcount(head); @@ -1124,7 +1092,7 @@ int total_compound_mapcount(struct page *head) int i; /* In the common case, avoid the loop when no subpages mapped by PTE */ - if (head_subpages_mapcount(head) == 0) + if ((head_subpages_mapcount(head) & SUBPAGES_MAPPED) == 0) return mapcount; /* * Add all the PTE mappings of those subpages mapped by PTE. @@ -1140,35 +1108,6 @@ int total_compound_mapcount(struct page *head) return mapcount; } -/* - * page_dup_compound_rmap(), used when copying mm, - * provides a simple example of using lock_ and unlock_compound_mapcounts(). - */ -void page_dup_compound_rmap(struct page *head) -{ - struct compound_mapcounts mapcounts; - - /* - * Hugetlb pages could use lock_compound_mapcounts(), like THPs do; - * but at present they are still being managed by atomic operations: - * which are likely to be somewhat faster, so don't rush to convert - * them over without evaluating the effect. - * - * Note that hugetlb does not call page_add_file_rmap(): - * here is where hugetlb shared page mapcount is raised. - */ - if (PageHuge(head)) { - atomic_inc(compound_mapcount_ptr(head)); - - } else if (PageTransHuge(head)) { - /* That test is redundant: it's for safety or to optimize out */ - - lock_compound_mapcounts(head, &mapcounts); - mapcounts.compound_mapcount++; - unlock_compound_mapcounts(head, &mapcounts); - } -} - /** * page_move_anon_rmap - move a page to our anon_vma * @page: the page to move to our anon_vma @@ -1278,7 +1217,7 @@ static void __page_check_anon_rmap(struct page *page, void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma, unsigned long address, rmap_t flags) { - struct compound_mapcounts mapcounts; + atomic_t *mapped; int nr = 0, nr_pmdmapped = 0; bool compound = flags & RMAP_COMPOUND; bool first; @@ -1290,24 +1229,20 @@ void page_add_anon_rmap(struct page *page, first = atomic_inc_and_test(&page->_mapcount); nr = first; if (first && PageCompound(page)) { - struct page *head = compound_head(page); - - lock_compound_mapcounts(head, &mapcounts); - mapcounts.subpages_mapcount++; - nr = !mapcounts.compound_mapcount; - unlock_compound_mapcounts(head, &mapcounts); + mapped = subpages_mapcount_ptr(compound_head(page)); + nr = atomic_inc_return_relaxed(mapped); + nr = !(nr & COMPOUND_MAPPED); } } else if (PageTransHuge(page)) { /* That test is redundant: it's for safety or to optimize out */ - lock_compound_mapcounts(page, &mapcounts); - first = !mapcounts.compound_mapcount; - mapcounts.compound_mapcount++; + first = atomic_inc_and_test(compound_mapcount_ptr(page)); if (first) { + mapped = subpages_mapcount_ptr(page); + nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped); nr_pmdmapped = thp_nr_pages(page); - nr = nr_pmdmapped - mapcounts.subpages_mapcount; + nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED); } - unlock_compound_mapcounts(page, &mapcounts); } VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page); @@ -1360,6 +1295,7 @@ void page_add_new_anon_rmap(struct page *page, VM_BUG_ON_PAGE(!PageTransHuge(page), page); /* increment count (starts at -1) */ atomic_set(compound_mapcount_ptr(page), 0); + atomic_set(subpages_mapcount_ptr(page), COMPOUND_MAPPED); nr = thp_nr_pages(page); __mod_lruvec_page_state(page, NR_ANON_THPS, nr); } @@ -1379,7 +1315,7 @@ void page_add_new_anon_rmap(struct page *page, void page_add_file_rmap(struct page *page, struct vm_area_struct *vma, bool compound) { - struct compound_mapcounts mapcounts; + atomic_t *mapped; int nr = 0, nr_pmdmapped = 0; bool first; @@ -1390,24 +1326,20 @@ void page_add_file_rmap(struct page *page, first = atomic_inc_and_test(&page->_mapcount); nr = first; if (first && PageCompound(page)) { - struct page *head = compound_head(page); - - lock_compound_mapcounts(head, &mapcounts); - mapcounts.subpages_mapcount++; - nr = !mapcounts.compound_mapcount; - unlock_compound_mapcounts(head, &mapcounts); + mapped = subpages_mapcount_ptr(compound_head(page)); + nr = atomic_inc_return_relaxed(mapped); + nr = !(nr & COMPOUND_MAPPED); } } else if (PageTransHuge(page)) { /* That test is redundant: it's for safety or to optimize out */ - lock_compound_mapcounts(page, &mapcounts); - first = !mapcounts.compound_mapcount; - mapcounts.compound_mapcount++; + first = atomic_inc_and_test(compound_mapcount_ptr(page)); if (first) { + mapped = subpages_mapcount_ptr(page); + nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped); nr_pmdmapped = thp_nr_pages(page); - nr = nr_pmdmapped - mapcounts.subpages_mapcount; + nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED); } - unlock_compound_mapcounts(page, &mapcounts); } if (nr_pmdmapped) @@ -1431,7 +1363,7 @@ void page_add_file_rmap(struct page *page, void page_remove_rmap(struct page *page, struct vm_area_struct *vma, bool compound) { - struct compound_mapcounts mapcounts; + atomic_t *mapped; int nr = 0, nr_pmdmapped = 0; bool last; @@ -1451,24 +1383,20 @@ void page_remove_rmap(struct page *page, last = atomic_add_negative(-1, &page->_mapcount); nr = last; if (last && PageCompound(page)) { - struct page *head = compound_head(page); - - lock_compound_mapcounts(head, &mapcounts); - mapcounts.subpages_mapcount--; - nr = !mapcounts.compound_mapcount; - unlock_compound_mapcounts(head, &mapcounts); + mapped = subpages_mapcount_ptr(compound_head(page)); + nr = atomic_dec_return_relaxed(mapped); + nr = !(nr & COMPOUND_MAPPED); } } else if (PageTransHuge(page)) { /* That test is redundant: it's for safety or to optimize out */ - lock_compound_mapcounts(page, &mapcounts); - mapcounts.compound_mapcount--; - last = !mapcounts.compound_mapcount; + last = atomic_add_negative(-1, compound_mapcount_ptr(page)); if (last) { + mapped = subpages_mapcount_ptr(page); + nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped); nr_pmdmapped = thp_nr_pages(page); - nr = nr_pmdmapped - mapcounts.subpages_mapcount; + nr = nr_pmdmapped - (nr & SUBPAGES_MAPPED); } - unlock_compound_mapcounts(page, &mapcounts); } if (nr_pmdmapped) {
Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? Yes. Not by atomic64_t or cmpxchg games, those get difficult on 32-bit; but if we slightly abuse subpages_mapcount by additionally demanding that one bit be set there when the compound page is PMD-mapped, then a cascade of two atomic ops is able to maintain the stats without bit_spin_lock. This is harder to reason about than when bit_spin_locked, but I believe safe; and no drift in stats detected when testing. When there are racing removes and adds, of course the sequence of operations is less well- defined; but each operation on subpages_mapcount is atomically good. What might be disastrous, is if subpages_mapcount could ever fleetingly appear negative: but the pte lock (or pmd lock) these rmap functions are called under, ensures that a last remove cannot race ahead of a first add. Continue to make an exception for hugetlb (PageHuge) pages, though that exception can be easily removed by a further commit if necessary: leave subpages_mapcount 0, don't bother with COMPOUND_MAPPED in its case, just carry on checking compound_mapcount too in folio_mapped(), page_mapped(). Evidence is that this way goes slightly faster than the previous implementation in all cases (pmds after ptes now taking around 103ms); and relieves us of worrying about contention on the bit_spin_lock. Signed-off-by: Hugh Dickins <hughd@google.com> --- Documentation/mm/transhuge.rst | 7 +- include/linux/mm.h | 19 ++++- include/linux/rmap.h | 12 ++-- mm/debug.c | 2 +- mm/rmap.c | 124 +++++++-------------------------- 5 files changed, 52 insertions(+), 112 deletions(-)