The patch titled
Subject: mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts
has been added to the -mm mm-unstable branch. Its filename is
mmthprmap-lock_compound_mapcounts-on-thp-mapcounts.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mmthprmap-lock_compound_mapcounts-on-thp-mapcounts.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: mm,thp,rmap: lock_compound_mapcounts() on THP mapcounts
Date: Wed, 2 Nov 2022 18:53:45 -0700 (PDT)
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@xxxxxxxxxx
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: James Houghton <jthoughton@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill@xxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Mina Almasry <almasrymina@xxxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Zach O'Keefe <zokeefe@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/transhuge.rst | 14 -
include/linux/rmap.h | 14 -
mm/huge_memory.c | 3
mm/rmap.c | 337 +++++++++++++++++--------------
4 files changed, 205 insertions(+), 163 deletions(-)
--- a/Documentation/mm/transhuge.rst~mmthprmap-lock_compound_mapcounts-on-thp-mapcounts
+++ a/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 PMD entry for the whole compound page increment/decrement
+ ->compound_mapcount, stored in the first tail 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 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
--- a/include/linux/rmap.h~mmthprmap-lock_compound_mapcounts-on-thp-mapcounts
+++ a/include/linux/rmap.h
@@ -204,16 +204,14 @@ void hugepage_add_anon_rmap(struct page
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
* the page R/O into both processes.
*/
dup:
- __page_dup_rmap(page, compound);
+ page_dup_file_rmap(page, compound);
return 0;
}
--- a/mm/huge_memory.c~mmthprmap-lock_compound_mapcounts-on-thp-mapcounts
+++ a/mm/huge_memory.c
@@ -2096,7 +2096,6 @@ static void __split_huge_pmd_locked(stru
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
@@ -2176,7 +2175,7 @@ static void __split_huge_pmd_locked(stru
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);
}
--- a/mm/rmap.c~mmthprmap-lock_compound_mapcounts-on-thp-mapcounts
+++ a/mm/rmap.c
@@ -1085,11 +1085,66 @@ int pfn_mkclean_range(unsigned long pfn,
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 p
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(struc
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 *pag
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
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;
- if (!atomic_inc_and_test(compound_mapcount_ptr(page)))
- goto out;
+ if (compound && PageTransHuge(page)) {
+ 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 (head_subpages_mapcount(page))
- nr = nr_subpages_unmapped(page, nr_pages);
-
- 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;
+ } else if (PageCompound(page)) {
+ struct page *head = compound_head(page);
- if (PageTransCompound(page)) {
- struct page *head = compound_head(page);
+ lock_compound_mapcounts(head, &mapcounts);
+ mapcounts.subpages_mapcount++;
+ first = subpage_mapcount_inc(page);
+ nr = first && !mapcounts.compound_mapcount;
+ unlock_compound_mapcounts(head, &mapcounts);
- atomic_inc(subpages_mapcount_ptr(head));
- pmd_mapped = head_compound_mapcount(head);
- }
- if (atomic_inc_and_test(&page->_mapcount) && !pmd_mapped)
- nr++;
+ } else {
+ 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;
-
- if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
- return;
-
- nr = nr_pages = thp_nr_pages(page);
- if (head_subpages_mapcount(page))
- nr = nr_subpages_unmapped(page, nr_pages);
-
- 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);
+ 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);
}
- if (atomic_add_negative(-1, &page->_mapcount) && !pmd_mapped)
- nr++;
- }
+ unlock_compound_mapcounts(page, &mapcounts);
- 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;
+ } else if (PageCompound(page)) {
+ struct page *head = compound_head(page);
- nr = nr_pages = thp_nr_pages(page);
- __mod_lruvec_page_state(page, NR_ANON_THPS, -nr);
+ lock_compound_mapcounts(head, &mapcounts);
+ mapcounts.subpages_mapcount--;
+ last = subpage_mapcount_dec(page);
+ nr = last && !mapcounts.compound_mapcount;
+ unlock_compound_mapcounts(head, &mapcounts);
- if (head_subpages_mapcount(page)) {
- nr = nr_subpages_unmapped(page, nr_pages);
+ } else {
+ last = atomic_add_negative(-1, &page->_mapcount);
+ nr = last;
+ }
+ 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 (PageTransCompound(page) && PageAnon(page))
+ if (!compound || nr < nr_pmdmapped)
+ deferred_split_huge_page(compound_head(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 (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);
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
mmhugetlb-use-folio-fields-in-second-tail-page.patch
mmthprmap-simplify-compound-page-mapcount-handling.patch
mmthprmap-lock_compound_mapcounts-on-thp-mapcounts.patch
