Re: [PATCH v1 10/15] mm/page-flags: reuse PG_slab as PG_anon_exclusive for PageAnon() pages

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 11, 2022 at 07:46:39PM +0100, David Hildenbrand wrote:
> I'm currently testing with the following. My tests so far with a swapfile on
> all different kinds of weird filesystems (excluding networking fs, though)
> revealed no surprises so far:

I like this a lot better than reusing PG_swap.  Thanks!

I'm somewhat reluctant to introduce a new flag that can be set on tail
pages.  Do we lose much if it's always set only on the head page?

> +++ b/include/linux/page-flags.h
> @@ -142,6 +142,60 @@ enum pageflags {
>  
>  	PG_readahead = PG_reclaim,
>  
> +	/*
> +	 * Depending on the way an anonymous folio can be mapped into a page
> +	 * table (e.g., single PMD/PUD/CONT of the head page vs. PTE-mapped
> +	 * THP), PG_anon_exclusive may be set only for the head page or for
> +	 * subpages of an anonymous folio.
> +	 *
> +	 * PG_anon_exclusive is *usually* only expressive in combination with a
> +	 * page table entry. Depending on the page table entry type it might
> +	 * store the following information:
> +	 *
> +	 *	Is what's mapped via this page table entry exclusive to the
> +	 *	single process and can be mapped writable without further
> +	 *	checks? If not, it might be shared and we might have to COW.
> +	 *
> +	 * For now, we only expect PTE-mapped THPs to make use of
> +	 * PG_anon_exclusive in subpages. For other anonymous compound
> +	 * folios (i.e., hugetlb), only the head page is logically mapped and
> +	 * holds this information.
> +	 *
> +	 * For example, an exclusive, PMD-mapped THP only has PG_anon_exclusive
> +	 * set on the head page. When replacing the PMD by a page table full
> +	 * of PTEs, PG_anon_exclusive, if set on the head page, will be set on
> +	 * all tail pages accordingly. Note that converting from a PTE-mapping
> +	 * to a PMD mapping using the same compound page is currently not
> +	 * possible and consequently doesn't require care.
> +	 *
> +	 * If GUP wants to take a reliable pin (FOLL_PIN) on an anonymous page,
> +	 * it should only pin if the relevant PG_anon_bit is set. In that case,
> +	 * the pin will be fully reliable and stay consistent with the pages
> +	 * mapped into the page table, as the bit cannot get cleared (e.g., by
> +	 * fork(), KSM) while the page is pinned. For anonymous pages that
> +	 * are mapped R/W, PG_anon_exclusive can be assumed to always be set
> +	 * because such pages cannot possibly be shared.
> +	 *
> +	 * The page table lock protecting the page table entry is the primary
> +	 * synchronization mechanism for PG_anon_exclusive; GUP-fast that does
> +	 * not take the PT lock needs special care when trying to clear the
> +	 * flag.
> +	 *
> +	 * Page table entry types and PG_anon_exclusive:
> +	 * * Present: PG_anon_exclusive applies.
> +	 * * Swap: the information is lost. PG_anon_exclusive was cleared.
> +	 * * Migration: the entry holds this information instead.
> +	 *		PG_anon_exclusive was cleared.
> +	 * * Device private: PG_anon_exclusive applies.
> +	 * * Device exclusive: PG_anon_exclusive applies.
> +	 * * HW Poison: PG_anon_exclusive is stale and not changed.
> +	 *
> +	 * If the page may be pinned (FOLL_PIN), clearing PG_anon_exclusive is
> +	 * not allowed and the flag will stick around until the page is freed
> +	 * and folio->mapping is cleared.
> +	 */

... I also don't think this is the right place for this comment.  Not
sure where it should go.

> +static __always_inline void SetPageAnonExclusive(struct page *page)
> +{
> +	VM_BUG_ON_PGFLAGS(!PageAnon(page) || PageKsm(page), page);

hm.  seems to me like we should have a PageAnonNotKsm which just
does
	return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) ==
			PAGE_MAPPING_ANON;
because that's "a bit" inefficient.  OK, that's just a VM_BUG_ON,
but we have other users in real code:

mm/migrate.c:   if (PageAnon(page) && !PageKsm(page))
mm/page_idle.c: need_lock = !PageAnon(page) || PageKsm(page);
mm/rmap.c:      if (!is_locked && (!PageAnon(page) || PageKsm(page))) {