Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

Peter Xu
2021-09-22T20:23:29Z

On Wed, Sep 22, 2021 at 04:04:44PM -0400, Peter Xu wrote:
> On Wed, Sep 22, 2021 at 12:29:35PM -0700, Yang Shi wrote:
> > khugepqged does remove the pgtables. Please check out
> > retract_page_tables(). The pmd will be cleared and the ptes will be
> > freed otherwise the collapsed THP won't get PMD mapped by later
> > access.
>
> Indeed.
>
> I should probably still properly disable khugepaged for at least VM_SHARED &&
> VM_UFFD_WP, then I'd keep the anonymous && minor mode behavior untouched.
>
> The other problem is even if current mm/vma doesn't have UFFD_WP registered,
> some other mm/vma could have UFFD_WP enabled there that mapped the same file.
> Checking that up within retract_page_tables() on all VMAs seems to be a bit too
> late.
>
> Checking it early may not trivially work too - I can walk the vma interval tree
> at the entry of khugepaged_scan_file(), making sure no vma has UFFD_WP set.
> However I don't see how it'll stop some of the vma from having UFFD_WP
> registered later after that point but before retract_page_tables().
>
> I'll need to think about it, but thanks for the input, Yang. That's a very
> important point.

Perhaps I need something like this:

---8<---
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 045cc579f724..c63e957336d1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1583,6 +1583,15 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
pmd = mm_find_pmd(mm, addr);
if (!pmd)
continue;
+ /*
+ * When a vma is registered with uffd-wp, we can't recycle the
+ * pmd pgtable because there can be pte markers installed.
+ * Skip it only, so the rest mm/vma can still have the same
+ * file mapped hugely, however it'll always mapped in small
+ * page size for uffd-wp registered ranges.
+ */
+ if (userfaultfd_wp(vma))
+ continue;
/*
* We need exclusive mmap_lock to retract page table.
*
---8<---

I won't post a v2 because then that patch will be shmem-only and uffd-wp-only.
I'll keep it with the upcoming series I'm going to post to support shmem.

Thanks,

--
Peter Xu