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SubjectRe: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd_wp()
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.

> 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

> Signed-off-by: Peter Xu <>
> ---
> Note: this patch should be able to apply cleanly with/without the other
> mm/migrate patch, or David's vm_page_prot changes.
> ---
> arch/x86/include/asm/pgtable.h | 24 ++++++++++++------------
> include/asm-generic/hugetlb.h | 16 ++++++++--------
> mm/huge_memory.c | 8 +++-----
> mm/hugetlb.c | 4 ++--
> mm/memory.c | 8 +++-----
> mm/mprotect.c | 6 ++----
> mm/userfaultfd.c | 18 ++----------------
> 7 files changed, 32 insertions(+), 52 deletions(-)

It's certainly a cleanup, even though we might unnecessarily wrprotect
(I don't think we care).


David / dhildenb

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