linux-kernel - Re: [PATCH v1 4/7] mm/ksm: fix KSM COW breaking with userfaultfd-wp via FAULT_FLAG

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Date:   Fri, 30 Sep 2022 15:27:42 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v1 4/7] mm/ksm: fix KSM COW breaking with userfaultfd-wp
 via FAULT_FLAG_UNSHARE

On Fri, 30 Sep 2022 16:19:28 +0200 David Hildenbrand <david@...hat.com> wrote:

> Let's stop breaking COW via a fake write fault and let's use
> FAULT_FLAG_UNSHARE instead. This avoids any wrong side effects of the fake
> write fault, such as mapping the PTE writable and marking the pte
> dirty/softdirty.
> 
> Also, this fixes KSM interaction with userfaultfd-wp: when we have a KSM
> page that's write-protected by userfaultfd, break_ksm()->handle_mm_fault()
> will fail with VM_FAULT_SIGBUS and will simpy return in break_ksm() with 0.
> The warning in dmesg indicates this wrong handling:

We're at -rc7.  I'd prefer to avoid merging larger patchsets at this
time.

Is there some minimal fix for 6.0 and -stable?  Or is the problem
non-serious enough to only fix it in 6.1 and later?