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Date:   Wed, 5 Jul 2023 22:37:22 +0200
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Subject: Re: [PATCH v3 2/2] mm: disable CONFIG_PER_VMA_LOCK until its fixed
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To:     Peter Xu <peterx@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, jirislaby@kernel.org,
        jacobly.alt@gmail.com, holger@applied-asynchrony.com,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 05.07.23 22:25, Peter Xu wrote:
> On Wed, Jul 05, 2023 at 10:22:27AM -0700, Suren Baghdasaryan wrote:
>> On Wed, Jul 5, 2023 at 10:16 AM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 05.07.23 19:12, Suren Baghdasaryan wrote:
>>>> A memory corruption was reported in [1] with bisection pointing to the
>>>> patch [2] enabling per-VMA locks for x86.
>>>> Disable per-VMA locks config to prevent this issue while the problem is
>>>> being investigated. This is expected to be a temporary measure.
>>>>
>>>> [1] https://bugzilla.kernel.org/show_bug.cgi?id=217624
>>>> [2] https://lore.kernel.org/all/20230227173632.3292573-30-surenb@google.com
>>>>
>>>> Reported-by: Jiri Slaby <jirislaby@kernel.org>
>>>> Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/
>>>> Reported-by: Jacob Young <jacobly.alt@gmail.com>
>>>> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624
>>>> Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
>>>> Cc: stable@vger.kernel.org
>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>>> ---
>>>>    mm/Kconfig | 3 ++-
>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index 09130434e30d..0abc6c71dd89 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -1224,8 +1224,9 @@ config ARCH_SUPPORTS_PER_VMA_LOCK
>>>>           def_bool n
>>>>
>>>>    config PER_VMA_LOCK
>>>> -     def_bool y
>>>> +     bool "Enable per-vma locking during page fault handling."
>>>>        depends on ARCH_SUPPORTS_PER_VMA_LOCK && MMU && SMP
>>>> +     depends on BROKEN
>>>>        help
>>>>          Allow per-vma locking during page fault handling.
>>>>
>>> Do we have any testing results (that don't reveal other issues :) ) for
>>> patch #1? Not sure if we really want to mark it broken if patch #1 fixes
>>> the issue.
>>
>> I tested the fix using the only reproducer provided in the reports
>> plus kernel compilation and my fork stress test. All looked good and
>> stable but I don't know if other reports had the same issue or
>> something different.
> 
> The commit log seems slightly confusing.  It mostly says the bug was still
> not solved, but I assume patch 1 is the current "fix", it's just not clear
> whether there's any other potential issues?
> 
> According to the stable tree rules:
> 
>   - It must fix a problem that causes a build error (but not for things
>     marked CONFIG_BROKEN), an oops, a hang, data corruption, a real
>     security issue, or some "oh, that's not good" issue.  In short, something
>     critical.
> 
> I think it means vma lock will never be fixed in 6.4, and it can't (because
> after this patch it'll be BROKEN, and this patch copies stable, and we
> can't fix BROKEN things in stables).
> 
> Totally no problem I see, just to make sure this is what you wanted..
> 
> There'll still try to be a final fix, am I right?  As IIRC allowing page
> faults during fork() is one of the major goals of vma lock.

At least not that I am aware of (and people who care about that should 
really work on scalable fork() alternatives, like that io_uring fork() 
thingy).

My understanding is that CONFIG_PER_VMA_LOCK wants to speed up page 
concurrent page faults *after* fork() [or rather, after new process 
creation], IOW, when we have a lot of mmap() activity going on while 
some threads of the new process are already active and don't actually 
touch what's getting newly mmaped.

-- 
Cheers,

David / dhildenb