linux-kernel - Re: [PATCH v2 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing happens

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Message-Id: <20230417164822.d1f5d162115c53aab4c85e85@linux-foundation.org>
Date:   Mon, 17 Apr 2023 16:48:22 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mika Penttilä <mpenttil@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        David Hildenbrand <david@...hat.com>,
        linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/6] mm/hugetlb: Fix uffd-wp bit lost when unsharing
 happens

On Mon, 17 Apr 2023 15:53:13 -0400 Peter Xu <peterx@...hat.com> wrote:

> When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can
> get lost during unsharing.  Fix it by carrying it over.
> 
> This should be very rare, only if an unsharing happened on a private
> hugetlb page with uffd-wp protected (e.g. in a child which shares the same
> page with parent with UFFD_FEATURE_EVENT_FORK enabled).

What are the user-visible consequences of the bug?

> Cc: linux-stable <stable@...r.kernel.org>

When proposing a backport, it's better to present the patch as a
standalone thing, against current -linus.  I'll then queue it in
mm-hotfixes and shall send it upstream during this -rc cycle.

As presented, this patch won't go upstream until after 6.3 is released,
and as it comes later in time, more backporting effort might be needed.

I can rework things if this fix is reasonably urgent (the "user-visible
consequences" info is the guide).  If not urgent, we can leave things
as they are.