Re: [PATCH v3 11/16] mm/page-flags: reuse PG_mappedtodisk as PG_anon_exclusive for PageAnon() pages

[Vlastimil Babka <vbabka@suse.cz>]

On 3/29/22 18:04, David Hildenbrand wrote:
> The basic question we would like to have a reliable and efficient answer
> to is: is this anonymous page exclusive to a single process or might it
> be shared? We need that information for ordinary/single pages, hugetlb
> pages, and possibly each subpage of a THP.
> 
> Introduce a way to mark an anonymous page as exclusive, with the
> ultimate goal of teaching our COW logic to not do "wrong COWs", whereby
> GUP pins lose consistency with the pages mapped into the page table,
> resulting in reported memory corruptions.
> 
> Most pageflags already have semantics for anonymous pages, however,
> PG_mappedtodisk should never apply to pages in the swapcache, so let's
> reuse that flag.
> 
> As PG_has_hwpoisoned also uses that flag on the second tail page of a
> compound page, convert it to PG_error instead, which is marked as
> PF_NO_TAIL, so never used for tail pages.
> 
> Use custom page flag modification functions such that we can do
> additional sanity checks. The semantics we'll put into some kernel doc
> in the future are:
> 
> "
>   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,

					^ PG_anon_exclusive bit ?

>   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.

Or also if it's unpinned?

> "
> 
> We won't be clearing PG_anon_exclusive on destructive unmapping (i.e.,
> zapping) of page table entries, page freeing code will handle that when
> also invalidate page->mapping to not indicate PageAnon() anymore.
> Letting information about exclusivity stick around will be an important
> property when adding sanity checks to unpinning code.
> 
> Note that we properly clear the flag in free_pages_prepare() via
> PAGE_FLAGS_CHECK_AT_PREP for each individual subpage of a compound page,
> so there is no need to manually clear the flag.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3663,6 +3663,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		goto out_nomap;
>  	}
>  
> +	/*
> +	 * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte
> +	 * must never point at an anonymous page in the swapcache that is
> +	 * PG_anon_exclusive. Sanity check that this holds and especially, that
> +	 * no filesystem set PG_mappedtodisk on a page in the swapcache. Sanity
> +	 * check after taking the PT lock and making sure that nobody
> +	 * concurrently faulted in this page and set PG_anon_exclusive.
> +	 */
> +	BUG_ON(!PageAnon(page) && PageMappedToDisk(page));
> +	BUG_ON(PageAnon(page) && PageAnonExclusive(page));
> +

Hmm, dunno why not VM_BUG_ON?

>  	/*
>  	 * Remove the swap entry and conditionally try to free up the swapcache.
>  	 * We're already holding a reference on the page but haven't mapped it
> diff --git a/mm/memremap.c b/mm/memremap.c
> index af0223605e69..4264f78299a8 100644