linux-kernel - Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

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Message-ID: <CAHk-=wifcVaxaTn_RbZ=idfYFazTPwm8t5cB1rY6xEBjbcfO5g@mail.gmail.com>
Date:   Sat, 19 Dec 2020 12:34:17 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

On Sat, Dec 19, 2020 at 4:41 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> @@ -2884,19 +2966,18 @@ void filemap_map_pages(struct vm_fault *vmf,
>                 if (vmf->pte)
>                         vmf->pte += xas.xa_index - last_pgoff;
>                 last_pgoff = xas.xa_index;
> -               if (alloc_set_pte(vmf, page))
> -                       goto unlock;
> +               if (pte_none(*vmf->pte))
> +                       do_set_pte(vmf, page);
> +               /* no need to invalidate: a not-present page won't be cached */
> +               update_mmu_cache(vma, vmf->address, vmf->pte);
>                 unlock_page(head);
> -               goto next;
> +               continue;

This can't be right.

Look at what happens if "pte_none()" is not true.. It won't install
the new pte, but it also won't drop the ref to the page.

So I think it needs to be

-               if (alloc_set_pte(vmf, page))
+               if (!pte_none(*vmf->pte))
                        goto unlock;
+               do_set_pte(vmf, page);

instead, so that the "if somebody else already filled the page table"
case gets handled right.

Hmm?

              Linus