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mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped 

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@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Carpenter <error27@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <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: Yu Zhao <yuzhao@google.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2022-11-22 01:49:36 -08:00 committed by Andrew Morton
parent be5ef2d9b0
commit 4b51634cd1
5 changed files with 51 additions and 111 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/mm/transhuge.rst
View File

@ -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

19
include/linux/mm.h
View File

@ -836,13 +836,22 @@ static inline int head_compound_mapcount(struct page *head)
return atomic_read(compound_mapcount_ptr(head)) + 1;
}
/*
* 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 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_subpages_mapcount_ptr(folio)) > 0 ||
atomic_read(folio_mapcount_ptr(folio)) >= 0;
}
/**

13
include/linux/rmap.h
View File

@ -204,15 +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)
{
/* 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(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;
}

2
mm/page_alloc.c
View File

@ -1330,7 +1330,7 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
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;
}

121
mm/rmap.c
View File

@ -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);
@ -1140,34 +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
@ -1277,7 +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 = true;
@ -1290,24 +1230,20 @@ 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 +1296,7 @@ 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 +1316,7 @@ 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;
@ -1391,24 +1328,20 @@ 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)
@ -1432,7 +1365,7 @@ 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;
@ -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) {