Messages in this thread | | | Date | Thu, 2 Feb 2023 02:09:43 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH] mm/khugepaged: skip shmem with armed userfaultfd |
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On Wed, Feb 01, 2023 at 12:41:37PM +0900, David Stevens wrote: > From: David Stevens <stevensd@chromium.org> > > Collapsing memory in a vma that has an armed userfaultfd results in > zero-filling any missing pages, which breaks user-space paging for those > filled pages. Avoid khugepage bypassing userfaultfd by not collapsing > pages in shmem reached via scanning a vma with an armed userfaultfd if > doing so would zero-fill any pages.
Could you elaborate on the failure? Will zero-filling the page prevent userfaultfd from catching future access?
A test-case would help a lot.
And what prevents the same pages be filled (with zeros or otherwise) via write(2) bypassing VMA checks? I cannot immediately see it.
BTW, there's already a check that prevent establishing PMD in the place if VM_UFFD_WP is set.
Maybe just an update of the check in retract_page_tables() from userfaultfd_wp() to userfaultfd_armed() would be enough?
I have very limited understanding of userfaultfd(). Sorry in advance for stupid questions.
-- Kiryl Shutsemau / Kirill A. Shutemov
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