linux-kernel - Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with folio

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Message-ID: <Y2LDL8zjgxDPCzH9@casper.infradead.org>
Date:   Wed, 2 Nov 2022 19:21:19 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
        Hugh Dickins <hughd@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with
 folio_add functions

On Wed, Nov 02, 2022 at 03:02:35PM -0400, Peter Xu wrote:
> Does the patch attached look reasonable to you?

Mmm, no.  If the page is in the swap cache, this will be "true".

> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 3d0fef3980b3..650ab6cfd5f4 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -64,7 +64,7 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
>  	pte_t _dst_pte, *dst_pte;
>  	bool writable = dst_vma->vm_flags & VM_WRITE;
>  	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> -	bool page_in_cache = page->mapping;
> +	bool page_in_cache = page_mapping(page);

We could do:

	struct page *head = compound_head(page);
	bool page_in_cache = head->mapping && !PageMappingFlags(head);