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From:   Yang Shi <shy828301@gmail.com>
Date:   Thu, 2 Feb 2023 13:11:22 -0800
Message-ID: <CAHbLzkrYEOD6-35kjGeS6ftXJ_mfzugwLt_28n+hH3Vy-uBs3g@mail.gmail.com>
Subject: Re: [PATCH] mm/khugepaged: skip shmem with armed userfaultfd
To:     Peter Xu <peterx@redhat.com>
Cc:     David Stevens <stevensd@chromium.org>,
        David Hildenbrand <david@redhat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>
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On Thu, Feb 2, 2023 at 12:04 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Feb 01, 2023 at 03:57:33PM -0800, Yang Shi wrote:
> > > There's another problem where the current vma may not have uffd armed,
> > > khugepaged may think it has nothing to do with uffd and moved on with
> > > collapsing, but actually it's armed in another vma of either the current mm
> > > or just another mm's.
> >
> > Out of curiosity, could you please elaborate how another vma armed
> > with userfaultfd could have an impact on the vmas that are not armed?
>
> It's e.g. when >1 vmas mapped to the same shmem file on the same range, one
> registered with uffd missing, others not.  Then others can cause page cache
> populated without generating message to the vma that got uffd missing mode
> registered, so there'll be the same issue as when khugepaged accidentally
> does thp collapsings.  Thanks,

Got it, thank you.

>
> --
> Peter Xu
>