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Subject[PATCH 3/3] 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.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
Documentation/mm/transhuge.rst | 16 +-
include/linux/rmap.h | 14 +-
mm/huge_memory.c | 3 +-
mm/rmap.c | 333 +++++++++++++++++++--------------
4 files changed, 204 insertions(+), 162 deletions(-)

diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
index a560e0c01b16..1e2a637cc607 100644
--- 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 1973649e8f93..011a7530dc76 100644
--- 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 23ff175768c3..2c4c668eee6c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2093,7 +2093,6 @@ 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
@@ -2170,7 +2169,7 @@ 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 f43339ea4970..512e53cae2ca 100644
--- 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 @@ void page_add_file_rmap(struct page *page,
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);
--
2.35.3
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