Re: [PATCH v1 1/2] mm/userfaultfd: fix uffd-wp handling for THP migration entries

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On 05.04.23 17:43, Peter Xu wrote:
On Wed, Apr 05, 2023 at 05:17:31PM +0200, David Hildenbrand wrote:
On 05.04.23 17:12, Peter Xu wrote:
On Wed, Apr 05, 2023 at 04:25:34PM +0200, David Hildenbrand wrote:
Looks like what we fixed for hugetlb in commit 44f86392bdd1 ("mm/hugetlb:
fix uffd-wp handling for migration entries in hugetlb_change_protection()")
similarly applies to THP.

Setting/clearing uffd-wp on THP migration entries is not implemented
properly. Further, while removing migration PMDs considers the uffd-wp
bit, inserting migration PMDs does not consider the uffd-wp bit.

We have to set/clear independently of the migration entry type in
change_huge_pmd() and properly copy the uffd-wp bit in
set_pmd_migration_entry().

Verified using a simple reproducer that triggers migration of a THP, that
the set_pmd_migration_entry() no longer loses the uffd-wp bit.

Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

Thanks, one trivial nitpick:

---
   mm/huge_memory.c | 14 ++++++++++++--
   1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 032fb0ef9cd1..bdda4f426d58 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1838,10 +1838,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
   	if (is_swap_pmd(*pmd)) {
   		swp_entry_t entry = pmd_to_swp_entry(*pmd);
   		struct page *page = pfn_swap_entry_to_page(entry);
+		pmd_t newpmd;
   		VM_BUG_ON(!is_pmd_migration_entry(*pmd));
   		if (is_writable_migration_entry(entry)) {
-			pmd_t newpmd;
   			/*
   			 * A protection check is difficult so
   			 * just be safe and disable write
@@ -1855,8 +1855,16 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
   				newpmd = pmd_swp_mksoft_dirty(newpmd);
   			if (pmd_swp_uffd_wp(*pmd))
   				newpmd = pmd_swp_mkuffd_wp(newpmd);
-			set_pmd_at(mm, addr, pmd, newpmd);
+		} else {
+			newpmd = *pmd;
   		}
+
+		if (uffd_wp)
+			newpmd = pmd_swp_mkuffd_wp(newpmd);
+		else if (uffd_wp_resolve)
+			newpmd = pmd_swp_clear_uffd_wp(newpmd);
+		if (!pmd_same(*pmd, newpmd))
+			set_pmd_at(mm, addr, pmd, newpmd);
   		goto unlock;
   	}
   #endif
@@ -3251,6 +3259,8 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
   	pmdswp = swp_entry_to_pmd(entry);
   	if (pmd_soft_dirty(pmdval))
   		pmdswp = pmd_swp_mksoft_dirty(pmdswp);
+	if (pmd_swp_uffd_wp(*pvmw->pmd))
+		pmdswp = pmd_swp_mkuffd_wp(pmdswp);

I think it's fine to use *pmd, but maybe still better to use pmdval?  I
worry pmdp_invalidate()) can be something else in the future that may
affect the bit.

Wondering how I ended up with that, I realized that it's actually
wrong and might have worked by chance for my reproducer on x86.

That should make it work:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f977c965fdad..fffc953fa6ea 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3257,7 +3257,7 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
         pmdswp = swp_entry_to_pmd(entry);
         if (pmd_soft_dirty(pmdval))
                 pmdswp = pmd_swp_mksoft_dirty(pmdswp);
-       if (pmd_swp_uffd_wp(*pvmw->pmd))
+       if (pmd_uffd_wp(pmdval))
                 pmdswp = pmd_swp_mkuffd_wp(pmdswp);
         set_pmd_at(mm, address, pvmw->pmd, pmdswp);
         page_remove_rmap(page, vma, true);

I guess pmd_swp_uffd_wp() just reads the _USER bit 2 which is also set for
a present pte, but then it sets swp uffd-wp always even if it was not set.


Yes. I modified the reproducer to migrate without uffd-wp first and we suddenly gain a uffd-wp bit.

Yes the change must be squashed in to be correct, with that, my R-b keeps.

Thanks, I will resend later.

--
Thanks,

David / dhildenb





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