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    SubjectRe: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd_wp()
    On Wed, Dec 14, 2022 at 11:59:35AM +0100, David Hildenbrand wrote:
    > On 08.12.22 20:46, Peter Xu wrote:
    > > This patch is a cleanup to always wr-protect pte/pmd in mkuffd_wp paths.
    > >
    > > The reasons I still think this patch is worthwhile, are:
    > >
    > > (1) It is a cleanup already; diffstat tells.
    > >
    > > (2) It just feels natural after I thought about this, if the pte is uffd
    > > protected, let's remove the write bit no matter what it was.
    > >
    > > (2) Since x86 is the only arch that supports uffd-wp, it also redefines
    > > pte|pmd_mkuffd_wp() in that it should always contain removals of
    > > write bits. It means any future arch that want to implement uffd-wp
    > > should naturally follow this rule too. It's good to make it a
    > > default, even if with vm_page_prot changes on VM_UFFD_WP.
    > >
    > > (3) It covers more than vm_page_prot. So no chance of any potential
    > > future "accident" (like pte_mkdirty() sparc64 or loongarch, even
    > > though it just got its pte_mkdirty fixed <1 month ago). It'll be
    > > fairly clear when reading the code too that we don't worry anything
    > > before a pte_mkuffd_wp() on uncertainty of the write bit.
    > Don't necessarily agree with (3). If you'd have a broken pte_mkdirty() and
    > do the pte_mkdirty() after pte_mkuffd_wp() it would still be broken. Because
    > sparc64 and loongarch are simply broken.

    That's why I mentioned on the order of operations matters.

    > >
    > > We may call pte_wrprotect() one more time in some paths (e.g. thp split),
    > > but that should be fully local bitop instruction so the overhead should be
    > > negligible.
    > >
    > > Although this patch should logically also fix all the known issues on
    > > uffd-wp too recently on either page migration or numa balancing, but this
    > > is not the plan for that fix. So no fixes, and stable doesn't need this.
    > I don't see how this would fix do_numa_page(), where we only do a
    > pte_modify().

    Yes, this patch won't, because it's a pure cleanup. Otherwise we need
    another line of wr-protect in numa recover path.

    I can remove that sentence in v2 commit log.

    Peter Xu

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