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

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Message-ID: <e9d19ba0-2a92-02a1-3f1f-e21f65d2a3c5@redhat.com>
Date:   Sat, 1 Oct 2022 10:13:41 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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 01.10.22 00:27, Andrew Morton wrote:
> 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.

Yes, this is 6.1 material.

> 
> 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?
> 

See the end of this lengthy patch description:

"This is primarily a fix for KSM+userfaultfd-wp, however, the fake write
fault was always questionable. As this fix is not easy to backport and 
it's not very critical, let's not cc stable."

This can wait, thanks!

-- 
Thanks,

David / dhildenb