[lkml]   [2022]   [Dec]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd_wp()
On 14.12.22 15:26, Peter Xu wrote:
> 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.

Feel free to add my

Acked-by: David Hildenbrand <>

Nothing jumped at me.


David / dhildenb

 \ /
  Last update: 2022-12-14 15:31    [W:0.045 / U:3.604 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site