Re: [PATCH mm-unstable v18 11/14] mm/khugepaged: Introduce mTHP collapse support

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, May 26, 2026 at 06:07:38AM -0600, Nico Pache wrote:
>On Tue, May 26, 2026 at 12:57 AM Wei Yang <richard.weiyang@gmail.com> wrote:
>>
>> On Mon, May 25, 2026 at 12:10:41PM -0700, Andrew Morton wrote:
>> >On Mon, 25 May 2026 08:15:53 -0600 Nico Pache <npache@redhat.com> wrote:
>> >
>> >> Can you please append the following fixup that reverts one of the
>> >> changes requested in V17. The issue with the change is described
>> >> below.
>> >
>> >OK.  fyi, what I received was badly mangled: wordwrapping, tabs messed
>> >up, etc.
>> >
>> >Here's my reconstruction:
>> >
>>
>> Hi, Nico
>>
>> I tried to reply your mail, but found it has some encoding problem, so reply
>> here.
>
>Yeah sorry I didnt properly configure my email client after getting a
>new laptop.
>
>>
>> >
>> >Author: Nico Pache <npache@redhat.com>
>> >Subject: fix potential use-after-free of vma in mthp_collapse()
>> >Date: Mon May 25 07:38:59 2026 -0600
>> >
>> >Between V17 and v18, one reviewer (Wei) brought up that we are not doing
>> >the uffd-armed check until deep in the collapse operation.  While not
>> >functionally incorrect, it can lead to unnecessary work.
>>
>> So we decide to tolerate the behavioral change?
>
>Yes, I believe it is ok for now. Either way we needed to remove the
>potential UAF. It only affects the behavior if mTHP is enabled, so the
>legacy behavior is kept. And the uffd case is limited.
>
>My future work involves further optimizing and cleaning up khugepaged.
>I'll make this part of the goal too. My first thought is to do the
>revalidation at every order (between the locks dropping); but that
>essentially pays the same penalty... I can't think of a clean solution
>at the moment.

One way come into my mind is add a @was_uffd_armed field in collapse_control
and updates it in hugepage_vma_revalidate() when latest vma is retrieved.

Still not elegant enough.

>
>Does that sound ok?
>

Not sure. I can't imagine the impact it would have.

>Cheers,
>-- Nico


-- 
Wei Yang
Help you, Help me