Re: [PATCH 2/3] mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped
From: Kirill A. Shutemov
Date: Mon Nov 21 2022 - 08:09:12 EST
On Fri, Nov 18, 2022 at 01:14:17AM -0800, Hugh Dickins wrote:
> 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.
Yay! New home for PageDoubleMap()! :P
> 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.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Jokes aside, looks neat.
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
As always few minor nits below.
> ---
> Documentation/mm/transhuge.rst | 7 +-
> include/linux/mm.h | 19 ++++-
> include/linux/rmap.h | 12 ++--
> mm/debug.c | 2 +-
> mm/rmap.c | 124 +++++++--------------------------
> 5 files changed, 52 insertions(+), 112 deletions(-)
>
> diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
> index af4c9d70321d..ec3dc5b04226 100644
> --- a/Documentation/mm/transhuge.rst
> +++ b/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
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c9e46d4d46f2..a2bfb5e4be62 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -828,7 +828,16 @@ static inline int head_compound_mapcount(struct page *head)
> }
>
> /*
> - * Number of sub-pages mapped by PTE, does not include 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, plus COMPOUND_MAPPED if compound mapped.
> * Must be called only on head of compound page.
> */
> static inline int head_subpages_mapcount(struct page *head)
> @@ -893,8 +902,12 @@ 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_mapcount_ptr(folio)) >= 0 ||
> + atomic_read(folio_subpages_mapcount_ptr(folio)) > 0;
Maybe check folio_subpages_mapcount_ptr() first? It would avoid
folio_mapcount_ptr() read for everything, but hugetlb.
> }
>
> /**
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 860f558126ac..bd3504d11b15 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -204,14 +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)
> {
> - if (likely(!compound /* page is mapped by PTE */))
> - atomic_inc(&page->_mapcount);
> - else
> - page_dup_compound_rmap(page);
> + __page_dup_rmap(page, compound);
> }
>
> /**
> @@ -260,7 +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;
> }
>
> diff --git a/mm/debug.c b/mm/debug.c
> index 7f8e5f744e42..1ef2ff6a05cb 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -97,7 +97,7 @@ static void __dump_page(struct page *page)
> pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n",
> head, compound_order(head),
> head_compound_mapcount(head),
> - head_subpages_mapcount(head),
> + head_subpages_mapcount(head) & SUBPAGES_MAPPED,
Looks like applying the SUBPAGES_MAPPED mask belong to the
head_subpages_mapcount() helper, not to the caller.
> head_compound_pincount(head));
> }
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 66be8cae640f..5e4ce0a6d6f1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -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);
> @@ -1124,7 +1092,7 @@ int total_compound_mapcount(struct page *head)
> int i;
>
> /* In the common case, avoid the loop when no subpages mapped by PTE */
> - if (head_subpages_mapcount(head) == 0)
> + if ((head_subpages_mapcount(head) & SUBPAGES_MAPPED) == 0)
> return mapcount;
> /*
> * Add all the PTE mappings of those subpages mapped by PTE.
> @@ -1140,35 +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
> @@ -1278,7 +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;
> @@ -1290,24 +1229,20 @@ 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 +1295,7 @@ 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 +1315,7 @@ 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;
>
> @@ -1390,24 +1326,20 @@ 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)
> @@ -1431,7 +1363,7 @@ 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;
>
> @@ -1451,24 +1383,20 @@ 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) {
> --
> 2.35.3
>
--
Kiryl Shutsemau / Kirill A. Shutemov