[PATCH mm-unstable v1] mm: add a total mapcount for large folios

From: David Hildenbrand
Date: Wed Aug 09 2023 - 04:33:59 EST

Let's track the total mapcount for all large folios in the first subpage.

The total mapcount is what we actually want to know in folio_mapcount()
and it is also sufficient for implementing folio_mapped(). This also
gets rid of any "raceiness" concerns as expressed in
folio_total_mapcount().

With sub-PMD THP becoming more important and things looking promising
that we will soon get support for such anon THP, we want to avoid looping
over all pages of a folio just to calculate the total mapcount. Further,
we may soon want to use the total mapcount in other context more
frequently, so prepare for reading it efficiently and atomically.

Make room for the total mapcount in page[1] of the folio by moving
_nr_pages_mapped to page[2] of the folio: it is not applicable to hugetlb
-- and with the total mapcount in place probably also not desirable even
if PMD-mappable hugetlb pages could get PTE-mapped at some point -- so we
can overlay the hugetlb fields.

Note that we currently don't expect any order-1 compound pages / THP in
rmap code, and that such support is not planned. If ever desired, we could
move the compound mapcount to another page, because it only applies to
PMD-mappable folios that are definitely larger. Let's avoid consuming
more space elsewhere for now -- we might need more space for a different
purpose in some subpages soon.

Maintain the total mapcount also for hugetlb pages. Use the total mapcount
to implement folio_mapcount(), total_mapcount(), folio_mapped() and
page_mapped().

We can now get rid of folio_total_mapcount() and
folio_large_is_mapped(), by just inlining reading of the total mapcount.

_nr_pages_mapped is now only used in rmap code, so not accidentially
externally where it might be used on arbitrary order-1 pages. The remaining
usage is:

(1) Detect how to adjust stats: NR_ANON_MAPPED and NR_FILE_MAPPED
-> If we would account the total folio as mapped when mapping a
page (based on the total mapcount), we could remove that usage.

(2) Detect when to add a folio to the deferred split queue
-> If we would apply a different heuristic, or scan using the rmap on
the memory reclaim path for partially mapped anon folios to
split them, we could remove that usage as well.

So maybe, we can simplify things in the future and remove
_nr_pages_mapped. For now, leave these things as they are, they need more
thought. Hugh really did a nice job implementing that precise tracking
after all.

Note: Not adding order-1 sanity checks to the file_rmap functions for
now. For anon pages, they are certainly not required right now.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
Documentation/mm/transhuge.rst | 12 +++++-----
include/linux/mm.h | 31 ++++++------------------
include/linux/mm_types.h | 6 +++--
include/linux/rmap.h | 7 ++++--
mm/debug.c | 4 ++--
mm/huge_memory.c | 2 ++
mm/hugetlb.c | 4 ++--
mm/internal.h | 17 ++++---------
mm/page_alloc.c | 4 ++--
mm/rmap.c | 44 ++++++++++++++--------------------
10 files changed, 52 insertions(+), 79 deletions(-)

diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
index 9a607059ea11..b0d3b1d3e8ea 100644
--- a/Documentation/mm/transhuge.rst
+++ b/Documentation/mm/transhuge.rst
@@ -116,14 +116,14 @@ pages:
succeeds on tail pages.

- map/unmap of a PMD entry for the whole THP increment/decrement
- folio->_entire_mapcount and also increment/decrement
- folio->_nr_pages_mapped by COMPOUND_MAPPED when _entire_mapcount
- goes from -1 to 0 or 0 to -1.
+ folio->_entire_mapcount, increment/decrement folio->_total_mapcount
+ and also increment/decrement folio->_nr_pages_mapped by COMPOUND_MAPPED
+ when _entire_mapcount goes from -1 to 0 or 0 to -1.

- map/unmap of individual pages with PTE entry increment/decrement
- page->_mapcount and also increment/decrement folio->_nr_pages_mapped
- when page->_mapcount goes from -1 to 0 or 0 to -1 as this counts
- the number of pages mapped by PTE.
+ page->_mapcount, increment/decrement folio->_total_mapcount and also
+ increment/decrement folio->_nr_pages_mapped when page->_mapcount goes
+ from -1 to 0 or 0 to -1 as this counts the number of pages mapped by PTE.

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/mm.h b/include/linux/mm.h
index 6a95dfed4957..30ac004fa0ef 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1187,41 +1187,24 @@ static inline int page_mapcount(struct page *page)
return mapcount;
}

-int folio_total_mapcount(struct folio *folio);
-
/**
- * folio_mapcount() - Calculate the number of mappings of this folio.
+ * folio_mapcount() - Return the total number of mappings of this folio.
* @folio: The folio.
*
- * A large folio tracks both how many times the entire folio is mapped,
- * and how many times each individual page in the folio is mapped.
- * This function calculates the total number of times the folio is
- * mapped.
- *
- * Return: The number of times this folio is mapped.
+ * Return: The total number of times this folio is mapped.
*/
static inline int folio_mapcount(struct folio *folio)
{
if (likely(!folio_test_large(folio)))
return atomic_read(&folio->_mapcount) + 1;
- return folio_total_mapcount(folio);
+ return atomic_read(&folio->_total_mapcount) + 1;
}

static inline int total_mapcount(struct page *page)
{
if (likely(!PageCompound(page)))
return atomic_read(&page->_mapcount) + 1;
- return folio_total_mapcount(page_folio(page));
-}
-
-static inline bool folio_large_is_mapped(struct folio *folio)
-{
- /*
- * Reading _entire_mapcount below could be omitted if hugetlb
- * participated in incrementing nr_pages_mapped when compound mapped.
- */
- return atomic_read(&folio->_nr_pages_mapped) > 0 ||
- atomic_read(&folio->_entire_mapcount) >= 0;
+ return atomic_read(&page_folio(page)->_total_mapcount) + 1;
}

/**
@@ -1234,7 +1217,7 @@ static inline bool folio_mapped(struct folio *folio)
{
if (likely(!folio_test_large(folio)))
return atomic_read(&folio->_mapcount) >= 0;
- return folio_large_is_mapped(folio);
+ return atomic_read(&folio->_total_mapcount) >= 0;
}

/*
@@ -1246,7 +1229,7 @@ static inline bool page_mapped(struct page *page)
{
if (likely(!PageCompound(page)))
return atomic_read(&page->_mapcount) >= 0;
- return folio_large_is_mapped(page_folio(page));
+ return atomic_read(&page_folio(page)->_total_mapcount) >= 0;
}

static inline struct page *virt_to_head_page(const void *x)
@@ -2148,7 +2131,7 @@ static inline size_t folio_size(struct folio *folio)
* looking at the precise mapcount of the first subpage in the folio, and
* assuming the other subpages are the same. This may not be true for large
* folios. If you want exact mapcounts for exact calculations, look at
- * page_mapcount() or folio_total_mapcount().
+ * page_mapcount() or folio_mapcount().
*
* Return: The estimated number of processes sharing a folio.
*/
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 291c05cacd48..16e66b3332c6 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -267,7 +267,8 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
* @_folio_dtor: Which destructor to use for this folio.
* @_folio_order: Do not use directly, call folio_order().
* @_entire_mapcount: Do not use directly, call folio_entire_mapcount().
- * @_nr_pages_mapped: Do not use directly, call folio_mapcount().
+ * @_total_mapcount: Do not use directly, call folio_mapcount().
+ * @_nr_pages_mapped: Do not use outside of rmap code (and not for hugetlb).
* @_pincount: Do not use directly, call folio_maybe_dma_pinned().
* @_folio_nr_pages: Do not use directly, call folio_nr_pages().
* @_hugetlb_subpool: Do not use directly, use accessor in hugetlb.h.
@@ -321,7 +322,7 @@ struct folio {
unsigned char _folio_dtor;
unsigned char _folio_order;
atomic_t _entire_mapcount;
- atomic_t _nr_pages_mapped;
+ atomic_t _total_mapcount;
atomic_t _pincount;
#ifdef CONFIG_64BIT
unsigned int _folio_nr_pages;
@@ -346,6 +347,7 @@ struct folio {
unsigned long _head_2a;
/* public: */
struct list_head _deferred_list;
+ atomic_t _nr_pages_mapped;
/* private: the union with struct page is transitional */
};
struct page __page_2;
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index a3825ce81102..a7b1c005e0c9 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -210,14 +210,17 @@ void hugepage_add_new_anon_rmap(struct folio *, struct vm_area_struct *,

static inline void __page_dup_rmap(struct page *page, bool compound)
{
- if (compound) {
- struct folio *folio = (struct folio *)page;
+ struct folio *folio = page_folio(page);

+ if (compound) {
VM_BUG_ON_PAGE(compound && !PageHead(page), page);
atomic_inc(&folio->_entire_mapcount);
} else {
atomic_inc(&page->_mapcount);
}
+
+ if (folio_test_large(folio))
+ atomic_inc(&folio->_total_mapcount);
}

static inline void page_dup_file_rmap(struct page *page, bool compound)
diff --git a/mm/debug.c b/mm/debug.c
index ee533a5ceb79..97f6f6b32ae7 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -99,10 +99,10 @@ static void __dump_page(struct page *page)
page, page_ref_count(head), mapcount, mapping,
page_to_pgoff(page), page_to_pfn(page));
if (compound) {
- pr_warn("head:%p order:%u entire_mapcount:%d nr_pages_mapped:%d pincount:%d\n",
+ pr_warn("head:%p order:%u entire_mapcount:%d total_mapcount:%d pincount:%d\n",
head, compound_order(head),
folio_entire_mapcount(folio),
- folio_nr_pages_mapped(folio),
+ folio_mapcount(folio),
atomic_read(&folio->_pincount));
}

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 154c210892a1..43a2150e41e3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -583,6 +583,7 @@ void prep_transhuge_page(struct page *page)

VM_BUG_ON_FOLIO(folio_order(folio) < 2, folio);
INIT_LIST_HEAD(&folio->_deferred_list);
+ atomic_set(&folio->_nr_pages_mapped, 0);
folio_set_compound_dtor(folio, TRANSHUGE_PAGE_DTOR);
}

@@ -2795,6 +2796,7 @@ void free_transhuge_page(struct page *page)
}
spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
}
+ VM_WARN_ON_ONCE(atomic_read(&folio->_nr_pages_mapped));
free_compound_page(page);
}

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5f498e8025cc..6a614c559ccf 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1479,7 +1479,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
struct page *p;

atomic_set(&folio->_entire_mapcount, 0);
- atomic_set(&folio->_nr_pages_mapped, 0);
+ atomic_set(&folio->_total_mapcount, 0);
atomic_set(&folio->_pincount, 0);

for (i = 1; i < nr_pages; i++) {
@@ -2027,7 +2027,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
/* we rely on prep_new_hugetlb_folio to set the destructor */
folio_set_order(folio, order);
atomic_set(&folio->_entire_mapcount, -1);
- atomic_set(&folio->_nr_pages_mapped, 0);
+ atomic_set(&folio->_total_mapcount, -1);
atomic_set(&folio->_pincount, 0);
return true;

diff --git a/mm/internal.h b/mm/internal.h
index 02a6fd36f46e..e4646fed44a5 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -53,10 +53,10 @@ struct folio_batch;
void page_writeback_init(void);

/*
- * If a 16GB hugetlb folio were mapped by PTEs of all of its 4kB pages,
+ * If a 16GB folio were mapped by PTEs of all of its 4kB pages,
* its nr_pages_mapped would be 0x400000: choose the COMPOUND_MAPPED bit
- * above that range, instead of 2*(PMD_SIZE/PAGE_SIZE). Hugetlb currently
- * leaves nr_pages_mapped at 0, but avoid surprise if it participates later.
+ * above that range, instead of 2*(PMD_SIZE/PAGE_SIZE). Not applicable to
+ * hugetlb.
*/
#define COMPOUND_MAPPED 0x800000
#define FOLIO_PAGES_MAPPED (COMPOUND_MAPPED - 1)
@@ -67,15 +67,6 @@ void page_writeback_init(void);
*/
#define SHOW_MEM_FILTER_NODES (0x0001u) /* disallowed nodes */

-/*
- * How many individual pages have an elevated _mapcount. Excludes
- * the folio's entire_mapcount.
- */
-static inline int folio_nr_pages_mapped(struct folio *folio)
-{
- return atomic_read(&folio->_nr_pages_mapped) & FOLIO_PAGES_MAPPED;
-}
-
static inline void *folio_raw_mapping(struct folio *folio)
{
unsigned long mapping = (unsigned long)folio->mapping;
@@ -420,7 +411,7 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
folio_set_order(folio, order);
atomic_set(&folio->_entire_mapcount, -1);
- atomic_set(&folio->_nr_pages_mapped, 0);
+ atomic_set(&folio->_total_mapcount, -1);
atomic_set(&folio->_pincount, 0);
}

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 96b7c1a7d1f2..5bbd5782b543 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1009,8 +1009,8 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
bad_page(page, "nonzero entire_mapcount");
goto out;
}
- if (unlikely(atomic_read(&folio->_nr_pages_mapped))) {
- bad_page(page, "nonzero nr_pages_mapped");
+ if (unlikely(atomic_read(&folio->_total_mapcount) + 1)) {
+ bad_page(page, "nonzero total_mapcount");
goto out;
}
if (unlikely(atomic_read(&folio->_pincount))) {
diff --git a/mm/rmap.c b/mm/rmap.c
index 1f04debdc87a..a7dcae48245b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1070,29 +1070,6 @@ int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
return page_vma_mkclean_one(&pvmw);
}

-int folio_total_mapcount(struct folio *folio)
-{
- int mapcount = folio_entire_mapcount(folio);
- int nr_pages;
- int i;
-
- /* In the common case, avoid the loop when no pages mapped by PTE */
- if (folio_nr_pages_mapped(folio) == 0)
- return mapcount;
- /*
- * Add all the PTE mappings of those pages mapped by PTE.
- * Limit the loop to folio_nr_pages_mapped()?
- * Perhaps: given all the raciness, that may be a good or a bad idea.
- */
- nr_pages = folio_nr_pages(folio);
- for (i = 0; i < nr_pages; i++)
- mapcount += atomic_read(&folio_page(folio, i)->_mapcount);
-
- /* But each of those _mapcounts was based on -1 */
- mapcount += nr_pages;
- return mapcount;
-}
-
/**
* page_move_anon_rmap - move a page to our anon_vma
* @page: the page to move to our anon_vma
@@ -1236,6 +1213,9 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
}
}

+ if (folio_test_large(folio))
+ atomic_inc(&folio->_total_mapcount);
+
VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);

@@ -1289,6 +1269,10 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
}

+ if (folio_test_large(folio))
+ /* increment count (starts at -1) */
+ atomic_set(&folio->_total_mapcount, 0);
+
__lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
}
@@ -1310,14 +1294,14 @@ void folio_add_file_rmap_range(struct folio *folio, struct page *page,
bool compound)
{
atomic_t *mapped = &folio->_nr_pages_mapped;
- unsigned int nr_pmdmapped = 0, first;
+ unsigned int nr_pmdmapped = 0, i, first;
int nr = 0;

VM_WARN_ON_FOLIO(compound && !folio_test_pmd_mappable(folio), folio);

/* Is page being mapped by PTE? Is this its first map to be added? */
if (likely(!compound)) {
- do {
+ for (i = 0; i < nr_pages; i++, page++) {
first = atomic_inc_and_test(&page->_mapcount);
if (first && folio_test_large(folio)) {
first = atomic_inc_return_relaxed(mapped);
@@ -1326,7 +1310,7 @@ void folio_add_file_rmap_range(struct folio *folio, struct page *page,

if (first)
nr++;
- } while (page++, --nr_pages > 0);
+ }
} else if (folio_test_pmd_mappable(folio)) {
/* That test is redundant: it's for safety or to optimize out */

@@ -1346,6 +1330,9 @@ void folio_add_file_rmap_range(struct folio *folio, struct page *page,
}
}

+ if (folio_test_large(folio))
+ atomic_add(compound ? 1 : nr_pages, &folio->_total_mapcount);
+
if (nr_pmdmapped)
__lruvec_stat_mod_folio(folio, folio_test_swapbacked(folio) ?
NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
@@ -1398,6 +1385,9 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,

VM_BUG_ON_PAGE(compound && !PageHead(page), page);

+ if (folio_test_large(folio))
+ atomic_dec(&folio->_total_mapcount);
+
/* Hugetlb pages are not counted in NR_*MAPPED */
if (unlikely(folio_test_hugetlb(folio))) {
/* hugetlb pages are always mapped with pmds */
@@ -2538,6 +2528,7 @@ void hugepage_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
BUG_ON(!anon_vma);
/* address might be in next vma when migration races vma_merge */
first = atomic_inc_and_test(&folio->_entire_mapcount);
+ atomic_inc(&folio->_total_mapcount);
VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
if (first)
@@ -2551,6 +2542,7 @@ void hugepage_add_new_anon_rmap(struct folio *folio,
BUG_ON(address < vma->vm_start || address >= vma->vm_end);
/* increment count (starts at -1) */
atomic_set(&folio->_entire_mapcount, 0);
+ atomic_set(&folio->_total_mapcount, 0);
folio_clear_hugetlb_restore_reserve(folio);
__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
}
--
2.41.0