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

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Message-ID: <7225391a-5798-94ae-7a01-9a9dd344e13a@suse.cz>
Date:   Tue, 12 Apr 2022 11:26:44 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 09/16] mm/rmap: use page_move_anon_rmap() when reusing
 a mapped PageAnon() page exclusively

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@...hat.com>

Acked-by: Vlastimil Babka <vbabka@...e.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;