From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: David Stevens <stevensd@chromium.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/khugepaged: skip shmem with armed userfaultfd
Date: Thu, 2 Feb 2023 02:09:43 +0300 [thread overview]
Message-ID: <20230201230943.fg2q6fmvu7gggxar@box.shutemov.name> (raw)
In-Reply-To: <20230201034137.2463113-1-stevensd@google.com>
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
next prev parent reply other threads:[~2023-02-01 23:09 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-01 3:41 [PATCH] mm/khugepaged: skip shmem with armed userfaultfd David Stevens
2023-02-01 17:36 ` Yang Shi
2023-02-01 20:52 ` Peter Xu
2023-02-01 23:57 ` Yang Shi
2023-02-02 20:04 ` Peter Xu
2023-02-02 21:11 ` Yang Shi
2023-02-02 9:56 ` David Stevens
2023-02-02 17:40 ` Yang Shi
2023-02-02 20:22 ` Peter Xu
2023-02-03 6:09 ` David Stevens
2023-02-03 14:56 ` Peter Xu
2023-02-01 23:09 ` Kirill A. Shutemov [this message]
2023-02-02 9:30 ` David Stevens
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