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

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Date:   Sat, 26 Dec 2020 13:16:09 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        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 26, 2020 at 1:04 PM Hugh Dickins <hughd@...gle.com> wrote:
>
>
> Hold on. I guess this one will suffer from the same bug as the previous.
> I was about to report back, after satisfactory overnight testing of that
> version - provided that one big little bug is fixed:
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2919,7 +2919,7 @@ static bool filemap_map_pmd(struct vm_fa
>
>         if (pmd_none(*vmf->pmd) &&
>             PageTransHuge(page) &&
> -           do_set_pmd(vmf, page)) {
> +           do_set_pmd(vmf, page) == 0) {
>                 unlock_page(page);
>                 return true;
>         }

I missed that entirely, because when just reading the patch it looks
fine and I didn't look at what do_set_pmd() function returns outside
the patch.

And maybe it would be better to write it as

        if (pmd_none(*vmf->pmd) && PageTransHuge(page)) {
                vm_fault_t ret = do_set_pmd(vmf, page);
                if (!ret) {
                        ...

instead to make it a bit more explicit about how that return value is
a vm_fault_t there...

And see my other email about how I suspect there is still a leak in
that patch for the previous test-case.

            Linus