linux-kernel - Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

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Date:   Wed, 22 Sep 2021 16:04:44 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

On Wed, Sep 22, 2021 at 12:29:35PM -0700, Yang Shi wrote:
> khugepqged does remove the pgtables. Please check out
> retract_page_tables(). The pmd will be cleared and the ptes will be
> freed otherwise the collapsed THP won't get PMD mapped by later
> access.

Indeed.

I should probably still properly disable khugepaged for at least VM_SHARED &&
VM_UFFD_WP, then I'd keep the anonymous && minor mode behavior untouched.

The other problem is even if current mm/vma doesn't have UFFD_WP registered,
some other mm/vma could have UFFD_WP enabled there that mapped the same file.
Checking that up within retract_page_tables() on all VMAs seems to be a bit too
late.

Checking it early may not trivially work too - I can walk the vma interval tree
at the entry of khugepaged_scan_file(), making sure no vma has UFFD_WP set.
However I don't see how it'll stop some of the vma from having UFFD_WP
registered later after that point but before retract_page_tables().

I'll need to think about it, but thanks for the input, Yang.  That's a very
important point.

-- 
Peter Xu