linux-kernel - Re: [PATCH 2/2] mm: Fix a huge pud insertion race during faulting

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Message-Id: <20191115115800.45c053abcdb550d70b9baec9@linux-foundation.org>
Date:   Fri, 15 Nov 2019 11:58:00 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Thomas Hellstrom <thellstrom@...are.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 2/2] mm: Fix a huge pud insertion race during faulting

On Fri, 15 Nov 2019 12:58:08 +0100 Thomas Hellström (VMware) <thomas_os@...pmail.org> wrote:

> A huge pud page can theoretically be faulted in racing with pmd_alloc()
> in __handle_mm_fault(). That will lead to pmd_alloc() returning an
> invalid pmd pointer. Fix this by adding a pud_trans_unstable() function
> similar to pmd_trans_unstable() and check whether the pud is really stable
> before using the pmd pointer.
> 
> Race:
> Thread 1:             Thread 2:                 Comment
> create_huge_pud()                               Fallback - not taken.
> 		      create_huge_pud()         Taken.
> pmd_alloc()                                     Returns an invalid pointer.

What are the user-visible runtime effects of this change?

Is a -stable backport warranted?