Re: [PATCH v3 09/16] mm/rmap: use page_move_anon_rmap() when reusing a mapped PageAnon() page exclusively

[Vlastimil Babka <vbabka@suse.cz>]

On 3/29/22 18:04, David Hildenbrand wrote:
> We want to mark anonymous pages exclusive, and when using
> page_move_anon_rmap() we know that we are the exclusive user, as
> properly documented. This is a preparation for marking anonymous pages
> exclusive in page_move_anon_rmap().
> 
> In both instances, we're holding page lock and are sure that we're the
> exclusive owner (page_count() == 1). hugetlb already properly uses
> page_move_anon_rmap() in the write fault handler.

Yeah, note that do_wp_page() used to call page_move_anon_rmap() always since
the latter was introduced, until commit 09854ba94c6a ("mm: do_wp_page()
simplification"). Probably not intended.

> Note that in case of a PTE-mapped THP, we'll only end up calling this
> function if the whole THP is only referenced by the single PTE mapping
> a single subpage (page_count() == 1); consequently, it's fine to modify
> the compound page mapping inside page_move_anon_rmap().
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

> ---
>  mm/huge_memory.c | 2 ++
>  mm/memory.c      | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c4526343565a..dd16819c5edc 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1317,6 +1317,8 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>  		try_to_free_swap(page);
>  	if (page_count(page) == 1) {
>  		pmd_t entry;
> +
> +		page_move_anon_rmap(page, vma);
>  		entry = pmd_mkyoung(orig_pmd);
>  		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>  		if (pmdp_set_access_flags(vma, haddr, vmf->pmd, entry, 1))
> diff --git a/mm/memory.c b/mm/memory.c
> index 03e29c9614e0..4303c0fdcf17 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3303,6 +3303,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  		 * and the page is locked, it's dark out, and we're wearing
>  		 * sunglasses. Hit it.
>  		 */
> +		page_move_anon_rmap(page, vma);
>  		unlock_page(page);
>  		wp_page_reuse(vmf);
>  		return VM_FAULT_WRITE;