The patch titled
Subject: mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped
has been added to the -mm mm-unstable branch. Its filename is
mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped.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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------------------------------------------------------
From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped
Date: Tue, 22 Nov 2022 01:49:36 -0800 (PST)
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.
Link: https://lkml.kernel.org/r/3978f3ca-5473-55a7-4e14-efea5968d892@xxxxxxxxxx
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Dan Carpenter <error27@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: James Houghton <jthoughton@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <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: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Zach O'Keefe <zokeefe@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/transhuge.rst | 7 -
include/linux/mm.h | 19 ++++
include/linux/rmap.h | 13 +--
mm/page_alloc.c | 2
mm/rmap.c | 121 ++++++-------------------------
5 files changed, 51 insertions(+), 111 deletions(-)
--- a/Documentation/mm/transhuge.rst~mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped
+++ a/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
--- a/include/linux/mm.h~mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped
+++ a/include/linux/mm.h
@@ -837,12 +837,21 @@ static inline int head_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, does not include compound mapcount.
* Must be called only on head of compound page.
*/
static inline int head_subpages_mapcount(struct page *head)
{
- return atomic_read(subpages_mapcount_ptr(head));
+ return atomic_read(subpages_mapcount_ptr(head)) & SUBPAGES_MAPPED;
}
/*
@@ -902,8 +911,12 @@ static inline int total_mapcount(struct
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_subpages_mapcount_ptr(folio)) > 0 ||
+ atomic_read(folio_mapcount_ptr(folio)) >= 0;
}
/**
--- a/include/linux/rmap.h~mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped
+++ a/include/linux/rmap.h
@@ -204,15 +204,14 @@ void hugepage_add_anon_rmap(struct page
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)
{
- /* Is page being mapped by PTE? */
- if (likely(!compound))
- atomic_inc(&page->_mapcount);
- else
- page_dup_compound_rmap(page);
+ __page_dup_rmap(page, compound);
}
/**
@@ -261,7 +260,7 @@ static inline int page_try_dup_anon_rmap
* the page R/O into both processes.
*/
dup:
- page_dup_file_rmap(page, compound);
+ __page_dup_rmap(page, compound);
return 0;
}
--- a/mm/page_alloc.c~mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped
+++ a/mm/page_alloc.c
@@ -1330,7 +1330,7 @@ static int free_tail_pages_check(struct
bad_page(page, "nonzero compound_mapcount");
goto out;
}
- if (unlikely(head_subpages_mapcount(head_page))) {
+ if (unlikely(atomic_read(subpages_mapcount_ptr(head_page)))) {
bad_page(page, "nonzero subpages_mapcount");
goto out;
}
--- a/mm/rmap.c~mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped
+++ a/mm/rmap.c
@@ -1085,38 +1085,6 @@ 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);
-}
-
int total_compound_mapcount(struct page *head)
{
int mapcount = head_compound_mapcount(head);
@@ -1140,34 +1108,6 @@ int total_compound_mapcount(struct page
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
@@ -1277,7 +1217,7 @@ 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)
{
- struct compound_mapcounts mapcounts;
+ atomic_t *mapped;
int nr = 0, nr_pmdmapped = 0;
bool compound = flags & RMAP_COMPOUND;
bool first = true;
@@ -1290,24 +1230,20 @@ void page_add_anon_rmap(struct page *pag
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 +1296,7 @@ void page_add_new_anon_rmap(struct 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 +1316,7 @@ void page_add_new_anon_rmap(struct 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;
@@ -1391,24 +1328,20 @@ void page_add_file_rmap(struct page *pag
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)
@@ -1432,7 +1365,7 @@ void page_add_file_rmap(struct page *pag
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;
@@ -1452,24 +1385,20 @@ 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) {
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
mmhugetlb-use-folio-fields-in-second-tail-page.patch
mmhugetlb-use-folio-fields-in-second-tail-page-fix.patch
mmthprmap-simplify-compound-page-mapcount-handling.patch
mmthprmap-lock_compound_mapcounts-on-thp-mapcounts.patch
mmthprmap-handle-the-normal-pagecompound-case-first.patch
mmthprmap-subpages_mapcount-of-pte-mapped-subpages.patch
mmthprmap-subpages_mapcount-compound_mapped-if-pmd-mapped.patch
mmthprmap-clean-up-the-end-of-__split_huge_pmd_locked.patch
