On 13.03.24 22:31, peterx@xxxxxxxxxx wrote:
From: Peter Xu <peterx@xxxxxxxxxx>
Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd
unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().
Good that I added the WARN_ON_ONCE() :)
The WARN_ON_ONCE() asserts that an VMA cannot be registered with
userfaultfd-wp if it contains a !normal page, but it's actually possible.
One example is an anonymous vma, register with uffd-wp, read anything will
install a zero page. Then when zap on it, this should trigger.
Are you sure? zap_install_uffd_wp_if_needed() contains right at the start:
/* Zap on anonymous always means dropping everything */
if (vma_is_anonymous(vma))
return;
So if that's the case the unit test triggers, I'm confused.
What's more, removing that WARN_ON_ONCE may not be enough either, because
we should also not rely on "whether it's a normal page" to decide whether
pte marker is needed. For example, one can register wr-protect over some
DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which
case it can have page==NULL for a devmap but we may want to keep the marker
around.
I thought uffd-wp was limited to specific backends only. But looks like
that changed with UFFD_FEATURE_WP_ASYNC, I guess?
Change itself looks, good. Not sure about the anon_vma example above.
Thanks!
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Fixes: 0cf18e839f64 ("mm/memory: handle !page case in zap_present_pte() separately")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
mm/memory.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/mm/memory.c b/mm/memory.c
index f2bc6dd15eb8..904f70b99498 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1536,7 +1536,9 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
arch_check_zapped_pte(vma, ptent);
tlb_remove_tlb_entry(tlb, pte, addr);
- VM_WARN_ON_ONCE(userfaultfd_wp(vma));
+ if (userfaultfd_pte_wp(vma, ptent))
+ zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
+ details, ptent);
ksm_might_unmap_zero_page(mm, ptent);
return 1;
}
--
Cheers,
David / dhildenb
