mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts

Fix the races in maintaining compound_mapcount, subpages_mapcount and
subpage _mapcount by using PG_locked in the first tail of any compound
page for a bit_spin_lock() on such modifications; skipping the usual
atomic operations on those fields in this case.

Bring page_remove_file_rmap() and page_remove_anon_compound_rmap() back
into page_remove_rmap() itself.  Rearrange page_add_anon_rmap() and
page_add_file_rmap() and page_remove_rmap() to follow the same "if
(compound) {lock} else if (PageCompound) {lock} else {atomic}" pattern
(with a PageTransHuge in the compound test, like before, to avoid BUG_ONs
and optimize away that block when THP is not configured).  Move all the
stats updates outside, after the bit_spin_locked section, so that it is
sure to be a leaf lock.

Add page_dup_compound_rmap() to manage compound locking versus atomics in
sync with the rest.  In particular, hugetlb pages are still using the
atomics: to avoid unnecessary interference there, and because they never
have subpage mappings; but this exception can easily be changed.
Conveniently, page_dup_compound_rmap() turns out to suit an anon THP's
__split_huge_pmd_locked() too.

bit_spin_lock() is not popular with PREEMPT_RT folks: but PREEMPT_RT
sensibly excludes TRANSPARENT_HUGEPAGE already, so its only exposure is to
the non-hugetlb non-THP pte-mapped compound pages (with large folios being
currently dependent on TRANSPARENT_HUGEPAGE).  There is never any scan of
subpages in this case; but we have chosen to use PageCompound tests rather
than PageTransCompound tests to gate the use of lock_compound_mapcounts(),
so that page_mapped() is correct on all compound pages, whether or not
TRANSPARENT_HUGEPAGE is enabled: could that be a problem for PREEMPT_RT,
when there is contention on the lock - under heavy concurrent forking for
example?  If so, then it can be turned into a sleeping lock (like
folio_lock()) when PREEMPT_RT.

A simple 100 X munmap(mmap(2GB, MAP_SHARED|MAP_POPULATE, tmpfs), 2GB) took
18 seconds on small pages, and used to take 1 second on huge pages, but
now takes 115 milliseconds on huge pages.  Mapping by pmds a second time
used to take 860ms and now takes 86ms; mapping by pmds after mapping by
ptes (when the scan is needed) used to take 870ms and now takes 495ms.
Mapping huge pages by ptes is largely unaffected but variable: between 5%
faster and 5% slower in what I've recorded.  Contention on the lock is
likely to behave worse than contention on the atomics behaved.

Link: https://lkml.kernel.org/r/1b42bd1a-8223-e827-602f-d466c2db7d3c@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
index a560e0c01b16e1a5db2361c2d7b5768f2ff368c9..1e2a637cc607933349baa035bbe2411b8e54be6c 100644 (file)
--- a/Documentation/mm/transhuge.rst
+++ b/Documentation/mm/transhuge.rst
@@ -117,13 +117,15 @@ pages:
   - ->_refcount in tail pages is always zero: get_page_unless_zero() never
     succeeds on tail pages.
 
-  - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
-    on relevant sub-page of the compound page.
-
-  - map/unmap of the whole compound page is accounted for in compound_mapcount
-    (stored in first tail page). For file huge pages, we also increment
-    ->_mapcount of all sub-pages in order to have race-free detection of
-    last unmap of subpages.
+  - map/unmap of PMD entry for the whole compound page increment/decrement
+    ->compound_mapcount, stored in the first tail page of the compound page.
+
+  - 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.
+    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/rmap.h b/include/linux/rmap.h
index 1973649e8f93e1b307a42561e0c88d4d4bbce378..011a7530dc76dd7b02f45d07926b8b996032db10 100644 (file)
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -204,16 +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);
 
-static inline void __page_dup_rmap(struct page *page, bool compound)
-{
-       if (!compound && PageCompound(page))
-               atomic_inc(subpages_mapcount_ptr(compound_head(page)));
-       atomic_inc(compound ? compound_mapcount_ptr(page) : &page->_mapcount);
-}
+void page_dup_compound_rmap(struct page *page, bool compound);
 
 static inline void page_dup_file_rmap(struct page *page, bool compound)
 {
-       __page_dup_rmap(page, compound);
+       if (PageCompound(page))
+               page_dup_compound_rmap(page, compound);
+       else
+               atomic_inc(&page->_mapcount);
 }
 
 /**
@@ -262,7 +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_rmap(page, compound);
+       page_dup_file_rmap(page, compound);
        return 0;
 }
 

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 7703169107c6ca39b274f8b94799e0b2cd7ad039..114517e8cbfc063a78b6738cc622eba08a0e3b0f 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2142,7 +2142,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 
                VM_BUG_ON_PAGE(!page_count(page), page);
                page_ref_add(page, HPAGE_PMD_NR - 1);
-               atomic_add(HPAGE_PMD_NR, subpages_mapcount_ptr(page));
 
                /*
                 * Without "freeze", we'll simply split the PMD, propagating the
@@ -2222,7 +2221,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                BUG_ON(!pte_none(*pte));
                set_pte_at(mm, addr, pte, entry);
                if (!pmd_migration)
-                       atomic_inc(&page[i]._mapcount);
+                       page_dup_compound_rmap(page + i, false);
                pte_unmap(pte);
        }
 

diff --git a/mm/rmap.c b/mm/rmap.c
index f43339ea497013250fad254b4c043e9b2c3126ed..512e53cae2ca12966dcc30022509c219601907cb 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1085,11 +1085,66 @@ 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);
+}
+
+/*
+ * When acting on a compound page under lock_compound_mapcounts(), avoid the
+ * unnecessary overhead of an actual atomic operation on its subpage mapcount.
+ * Return true if this is the first increment or the last decrement
+ * (remembering that page->_mapcount -1 represents logical mapcount 0).
+ */
+static bool subpage_mapcount_inc(struct page *page)
+{
+       int orig_mapcount = atomic_read(&page->_mapcount);
+
+       atomic_set(&page->_mapcount, orig_mapcount + 1);
+       return orig_mapcount < 0;
+}
+
+static bool subpage_mapcount_dec(struct page *page)
+{
+       int orig_mapcount = atomic_read(&page->_mapcount);
+
+       atomic_set(&page->_mapcount, orig_mapcount - 1);
+       return orig_mapcount == 0;
+}
+
 /*
  * When mapping a THP's first pmd, or unmapping its last pmd, if that THP
  * also has pte mappings, then those must be discounted: in order to maintain
  * NR_ANON_MAPPED and NR_FILE_MAPPED statistics exactly, without any drift,
  * and to decide when an anon THP should be put on the deferred split queue.
+ * This function must be called between lock_ and unlock_compound_mapcounts().
  */
 static int nr_subpages_unmapped(struct page *head, int nr_subpages)
 {
@@ -1103,6 +1158,40 @@ static int nr_subpages_unmapped(struct page *head, int nr_subpages)
        return nr;
 }
 
+/*
+ * page_dup_compound_rmap(), used when copying mm, or when splitting pmd,
+ * provides a simple example of using lock_ and unlock_compound_mapcounts().
+ */
+void page_dup_compound_rmap(struct page *page, bool compound)
+{
+       struct compound_mapcounts mapcounts;
+       struct page *head;
+
+       /*
+        * 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(page)) {
+               atomic_inc(compound_mapcount_ptr(page));
+               return;
+       }
+
+       head = compound_head(page);
+       lock_compound_mapcounts(head, &mapcounts);
+       if (compound) {
+               mapcounts.compound_mapcount++;
+       } else {
+               mapcounts.subpages_mapcount++;
+               subpage_mapcount_inc(page);
+       }
+       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
@@ -1212,7 +1301,8 @@ 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)
 {
-       int nr, nr_pages;
+       struct compound_mapcounts mapcounts;
+       int nr = 0, nr_pmdmapped = 0;
        bool compound = flags & RMAP_COMPOUND;
        bool first;
 
@@ -1222,33 +1312,37 @@ void page_add_anon_rmap(struct page *page,
                VM_BUG_ON_PAGE(!PageLocked(page), page);
 
        if (compound && PageTransHuge(page)) {
-               atomic_t *mapcount;
-               VM_BUG_ON_PAGE(!PageLocked(page), page);
-               mapcount = compound_mapcount_ptr(page);
-               first = atomic_inc_and_test(mapcount);
+               lock_compound_mapcounts(page, &mapcounts);
+               first = !mapcounts.compound_mapcount;
+               mapcounts.compound_mapcount++;
+               if (first) {
+                       nr = nr_pmdmapped = thp_nr_pages(page);
+                       if (mapcounts.subpages_mapcount)
+                               nr = nr_subpages_unmapped(page, nr_pmdmapped);
+               }
+               unlock_compound_mapcounts(page, &mapcounts);
 
-               nr = nr_pages = thp_nr_pages(page);
-               if (first && head_subpages_mapcount(page))
-                       nr = nr_subpages_unmapped(page, nr_pages);
-       } else {
-               nr = 1;
-               if (PageTransCompound(page)) {
-                       struct page *head = compound_head(page);
+       } else if (PageCompound(page)) {
+               struct page *head = compound_head(page);
 
-                       atomic_inc(subpages_mapcount_ptr(head));
-                       nr = !head_compound_mapcount(head);
-               }
+               lock_compound_mapcounts(head, &mapcounts);
+               mapcounts.subpages_mapcount++;
+               first = subpage_mapcount_inc(page);
+               nr = first && !mapcounts.compound_mapcount;
+               unlock_compound_mapcounts(head, &mapcounts);
+
+       } else {
                first = atomic_inc_and_test(&page->_mapcount);
+               nr = first;
        }
 
        VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
        VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
 
-       if (first) {
-               if (compound)
-                       __mod_lruvec_page_state(page, NR_ANON_THPS, nr_pages);
+       if (nr_pmdmapped)
+               __mod_lruvec_page_state(page, NR_ANON_THPS, nr_pmdmapped);
+       if (nr)
                __mod_lruvec_page_state(page, NR_ANON_MAPPED, nr);
-       }
 
        if (unlikely(PageKsm(page)))
                unlock_page_memcg(page);
@@ -1308,39 +1402,41 @@ void page_add_new_anon_rmap(struct page *page,
 void page_add_file_rmap(struct page *page,
        struct vm_area_struct *vma, bool compound)
 {
-       int nr = 0;
+       struct compound_mapcounts mapcounts;
+       int nr = 0, nr_pmdmapped = 0;
+       bool first;
 
        VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
        lock_page_memcg(page);
+
        if (compound && PageTransHuge(page)) {
-               int nr_pages;
+               lock_compound_mapcounts(page, &mapcounts);
+               first = !mapcounts.compound_mapcount;
+               mapcounts.compound_mapcount++;
+               if (first) {
+                       nr = nr_pmdmapped = thp_nr_pages(page);
+                       if (mapcounts.subpages_mapcount)
+                               nr = nr_subpages_unmapped(page, nr_pmdmapped);
+               }
+               unlock_compound_mapcounts(page, &mapcounts);
 
-               if (!atomic_inc_and_test(compound_mapcount_ptr(page)))
-                       goto out;
+       } else if (PageCompound(page)) {
+               struct page *head = compound_head(page);
 
-               nr = nr_pages = thp_nr_pages(page);
-               if (head_subpages_mapcount(page))
-                       nr = nr_subpages_unmapped(page, nr_pages);
+               lock_compound_mapcounts(head, &mapcounts);
+               mapcounts.subpages_mapcount++;
+               first = subpage_mapcount_inc(page);
+               nr = first && !mapcounts.compound_mapcount;
+               unlock_compound_mapcounts(head, &mapcounts);
 
-               if (PageSwapBacked(page))
-                       __mod_lruvec_page_state(page, NR_SHMEM_PMDMAPPED,
-                                               nr_pages);
-               else
-                       __mod_lruvec_page_state(page, NR_FILE_PMDMAPPED,
-                                               nr_pages);
        } else {
-               bool pmd_mapped = false;
-
-               if (PageTransCompound(page)) {
-                       struct page *head = compound_head(page);
-
-                       atomic_inc(subpages_mapcount_ptr(head));
-                       pmd_mapped = head_compound_mapcount(head);
-               }
-               if (atomic_inc_and_test(&page->_mapcount) && !pmd_mapped)
-                       nr++;
+               first = atomic_inc_and_test(&page->_mapcount);
+               nr = first;
        }
-out:
+
+       if (nr_pmdmapped)
+               __mod_lruvec_page_state(page, PageSwapBacked(page) ?
+                       NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
        if (nr)
                __mod_lruvec_page_state(page, NR_FILE_MAPPED, nr);
        unlock_page_memcg(page);
@@ -1348,137 +1444,84 @@ out:
        mlock_vma_page(page, vma, compound);
 }
 
-static void page_remove_file_rmap(struct page *page, bool 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)
 {
-       int nr = 0;
+       struct compound_mapcounts mapcounts;
+       int nr = 0, nr_pmdmapped = 0;
+       bool last;
 
        VM_BUG_ON_PAGE(compound && !PageHead(page), page);
 
-       /* Hugepages are not counted in NR_FILE_MAPPED for now. */
+       /* Hugetlb pages are not counted in NR_*MAPPED */
        if (unlikely(PageHuge(page))) {
                /* hugetlb pages are always mapped with pmds */
                atomic_dec(compound_mapcount_ptr(page));
                return;
        }
 
+       lock_page_memcg(page);
+
        /* page still mapped by someone else? */
        if (compound && PageTransHuge(page)) {
-               int nr_pages;
+               lock_compound_mapcounts(page, &mapcounts);
+               mapcounts.compound_mapcount--;
+               last = !mapcounts.compound_mapcount;
+               if (last) {
+                       nr = nr_pmdmapped = thp_nr_pages(page);
+                       if (mapcounts.subpages_mapcount)
+                               nr = nr_subpages_unmapped(page, nr_pmdmapped);
+               }
+               unlock_compound_mapcounts(page, &mapcounts);
 
-               if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
-                       return;
+       } else if (PageCompound(page)) {
+               struct page *head = compound_head(page);
 
-               nr = nr_pages = thp_nr_pages(page);
-               if (head_subpages_mapcount(page))
-                       nr = nr_subpages_unmapped(page, nr_pages);
+               lock_compound_mapcounts(head, &mapcounts);
+               mapcounts.subpages_mapcount--;
+               last = subpage_mapcount_dec(page);
+               nr = last && !mapcounts.compound_mapcount;
+               unlock_compound_mapcounts(head, &mapcounts);
 
-               if (PageSwapBacked(page))
-                       __mod_lruvec_page_state(page, NR_SHMEM_PMDMAPPED,
-                                               -nr_pages);
-               else
-                       __mod_lruvec_page_state(page, NR_FILE_PMDMAPPED,
-                                               -nr_pages);
        } else {
-               bool pmd_mapped = false;
-
-               if (PageTransCompound(page)) {
-                       struct page *head = compound_head(page);
-
-                       atomic_dec(subpages_mapcount_ptr(head));
-                       pmd_mapped = head_compound_mapcount(head);
-               }
-               if (atomic_add_negative(-1, &page->_mapcount) && !pmd_mapped)
-                       nr++;
+               last = atomic_add_negative(-1, &page->_mapcount);
+               nr = last;
        }
 
-       if (nr)
-               __mod_lruvec_page_state(page, NR_FILE_MAPPED, -nr);
-}
-
-static void page_remove_anon_compound_rmap(struct page *page)
-{
-       int nr, nr_pages;
-
-       if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
-               return;
-
-       /* Hugepages are not counted in NR_ANON_PAGES for now. */
-       if (unlikely(PageHuge(page)))
-               return;
-
-       if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-               return;
-
-       nr = nr_pages = thp_nr_pages(page);
-       __mod_lruvec_page_state(page, NR_ANON_THPS, -nr);
-
-       if (head_subpages_mapcount(page)) {
-               nr = nr_subpages_unmapped(page, nr_pages);
-
+       if (nr_pmdmapped) {
+               __mod_lruvec_page_state(page, PageAnon(page) ? NR_ANON_THPS :
+                               (PageSwapBacked(page) ? NR_SHMEM_PMDMAPPED :
+                               NR_FILE_PMDMAPPED), -nr_pmdmapped);
+       }
+       if (nr) {
+               __mod_lruvec_page_state(page, PageAnon(page) ? NR_ANON_MAPPED :
+                               NR_FILE_MAPPED, -nr);
                /*
-                * Queue the page for deferred split if at least one small
+                * Queue anon THP for deferred split if at least one small
                 * page of the compound page is unmapped, but at least one
                 * small page is still mapped.
                 */
-               if (nr && nr < nr_pages)
-                       deferred_split_huge_page(page);
-       }
-
-       if (nr)
-               __mod_lruvec_page_state(page, NR_ANON_MAPPED, -nr);
-}
-
-/**
- * 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)
-{
-       bool pmd_mapped = false;
-
-       lock_page_memcg(page);
-
-       if (!PageAnon(page)) {
-               page_remove_file_rmap(page, compound);
-               goto out;
+               if (PageTransCompound(page) && PageAnon(page))
+                       if (!compound || nr < nr_pmdmapped)
+                               deferred_split_huge_page(compound_head(page));
        }
 
-       if (compound) {
-               page_remove_anon_compound_rmap(page);
-               goto out;
-       }
-
-       if (PageTransCompound(page)) {
-               struct page *head = compound_head(page);
-
-               atomic_dec(subpages_mapcount_ptr(head));
-               pmd_mapped = head_compound_mapcount(head);
-       }
-
-       /* page still mapped by someone else? */
-       if (!atomic_add_negative(-1, &page->_mapcount) || pmd_mapped)
-               goto out;
-
-       __dec_lruvec_page_state(page, NR_ANON_MAPPED);
-
-       if (PageTransCompound(page))
-               deferred_split_huge_page(compound_head(page));
-
        /*
-        * It would be tidy to reset the PageAnon mapping here,
+        * It would be tidy to reset PageAnon 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_unref_page,
+        * before us: so leave the reset to free_pages_prepare,
         * and remember that it's only reliable while mapped.
-        * Leaving it set also helps swapoff to reinstate ptes
-        * faster for those pages still in swapcache.
         */
-out:
+
        unlock_page_memcg(page);
 
        munlock_vma_page(page, vma, compound);