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

Peter Xu
2021-09-23T02:18:40Z

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On Wed, Sep 22, 2021 at 06:22:45PM -0700, Hugh Dickins wrote:
> No, I think I misunderstood you before: thanks for re-explaining.
> (And Axel's !userfaultfd_minor() check before calling do_fault_around()
> plays an important part in making sure that it does reach shmem_fault().)

Still thanks for confirming this, Hugh.

Said that, Axel, I didn't mean I'm against doing something similar like
uffd-wp; it's just a heads-up that maybe you won't find a reproducer with real
issues with minor mode.

Even if I think minor mode should be fine with current code, we could still
choose to disable khugepaged from removing the pmd for VM_UFFD_MINOR vmas, just
like what we'll do with VM_UFFD_WP. At least it can still reduce false
positives.

So far in my local branch I queued the patch which I attached, that's required
for uffd-wp shmem afaict. If you think minor mode would like that too, I can
post it separately with minor mode added in.

Note that it's slightly different from what I pasted in reply to Yang Shi - I
made it slightly more complicated just to make sure there's no race. I
mentioned the possible race (I think) in the commit log.

Let me know your preference.

Thanks,

--
Peter Xu

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From 989d36914ac144177e17f9aacbf2785bb8f21420 Mon Sep 17 00:00:00 2001
From: Peter Xu <peterx@redhat.com>
Date: Wed, 22 Sep 2021 16:23:33 -0400
Subject: [PATCH] mm/khugepaged: Don't recycle vma pgtable if uffd-wp
registered

When we're trying to collapse a 2M huge shmem page, don't retract pgtable pmd
page if it's registered with uffd-wp, because that pgtable could have pte
markers installed. Recycling of that pgtable means we'll lose the pte markers.
That could cause data loss for an uffd-wp enabled application on shmem.

Instead of disabling khugepaged on these files, simply skip retracting these
special VMAs, then the page cache can still be merged into a huge thp, and
other mm/vma can still map the range of file with a huge thp when proper.

Note that checking VM_UFFD_WP needs to be done with mmap_sem held for write,
that avoids race like:

khugepaged user thread
========== ===========
check VM_UFFD_WP, not set
UFFDIO_REGISTER with uffd-wp on shmem
wr-protect some pages (install markers)
take mmap_sem write lock
erase pmd and free pmd page
--> pte markers are dropped unnoticed!

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/khugepaged.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 045cc579f724..23e1d03156b3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1451,6 +1451,10 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
return;

+ /* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
+ if (userfaultfd_wp(vma))
+ return;
+
hpage = find_lock_page(vma->vm_file->f_mapping,
linear_page_index(vma, haddr));
if (!hpage)
@@ -1591,7 +1595,15 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
* reverse order. Trylock is a way to avoid deadlock.
*/
if (mmap_write_trylock(mm)) {
- if (!khugepaged_test_exit(mm)) {
+ /*
+ * 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 (!khugepaged_test_exit(mm) && !userfaultfd_wp(vma)) {
spinlock_t *ptl = pmd_lock(mm, pmd);
/* assume page table is clear */
_pmd = pmdp_collapse_flush(vma, addr, pmd);
--
2.31.1