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

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Date:   Sat, 26 Dec 2020 13:07:14 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Hugh Dickins <hughd@...gle.com>,
        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

I was going to just apply this patch, because I like it so much, but
then I decided to take one last look, and:

On Sat, Dec 26, 2020 at 12:43 PM Kirill A. Shutemov
<kirill@...temov.name> wrote:
>
> +static bool filemap_map_pmd(struct vm_fault *vmf, struct page *page)
> +{
> +       struct mm_struct *mm = vmf->vma->vm_mm;
> +
> +       /* Huge page is mapped? No need to proceed. */
> +       if (pmd_trans_huge(*vmf->pmd))
> +               return true;

doesn't this cause us to leak a locked page?

I get the feeling that every single "return true" case here should
always unlock the page and - with the exception of a successful
do_set_pmd() - do a "put_page()".

Which kind of argues that we should just do it in the caller (and get
an extra ref in the do_set_pmd() case, so that the caller can always
do

        if (filemap_map_pmd(..)) {
                unlock_page(page);
                put_page(page);
                rcu_read_unlock();
                return;
        }

andf then there are no odd cases inside that filemap_map_pmd() function. Hmm?

Other than that, I really find it all much more legible.

Of course, if I'm wrong about the above, that just proves that I'm
missing something and it wasn't so legible after all..

              Linus