Messages in this thread |  | | Date | Wed, 14 Dec 2022 11:59:35 +0100 | Subject | Re: [PATCH] mm/uffd: Always wr-protect pte in pte|pmd_mkuffd_wp() | From | David Hildenbrand <> |
| |
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 pte_modify().
> > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > > 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).
-- Thanks,
David / dhildenb
|  |