Re: [PATCH v2] mm/ksm: keep soft-dirty and uffd-wp when merging a page

[Lance Yang <lance.yang@linux.dev>]

On Mon, Oct 19, 2026 at 03:24:11PM +0200, David Hildenbrand (Arm) wrote:
>On 10/19/26 03:09, Samarth Narang wrote:
>> replace_page() maps the KSM page with a PTE built from scratch, so the
>> soft-dirty and uffd bits of the PTE it replaces are lost. Due to this, a
>> write before the merge no longer shows as soft-dirty, the next write to
>> a uffd-wp page raises no write-protect fault, and because the page is
>> mapped with the VMA's normal permissions, on a uffd-rwp VMA the next
>> access raises no RWP fault.
>> 
>> Copy the soft-dirty and uffd bits from the original PTE to the new PTE,
>> and restore PAGE_NONE for RWP VMAs, as try_to_map_unused_to_zeropage()
>> does.
>> 
>> Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")
>> Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
>> Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
>
>I would actually split that up into separate patches.
>
>Over-indicating soft-dirty (as raised by Sashiko) is in general not a problem.

Right.

>Losing WP bits is.
>
>UFFD, the gift that keeps on giving.

Haha, yeah, I stared at the code a bit. Say we arm RWP on an existing KSM
mapping, then do MADV_UNMERGEABLE. We're just replacing the shared page
with a private copy. That shouldn't count as an access to the mapping ...
no?

But the protnone check sends us to do_uffd_rwp() before the
FAULT_FLAG_UNSHARE handling. No FAULT_FLAG_ALLOW_RETRY here, so sync RWP
gives us VM_FAULT_SIGBUS. Then break_ksm() does this:

static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
		unsigned long end, bool lock_vma)
{
	vm_fault_t ret = 0;
...
	do {
...
		ret = handle_mm_fault(vma, addr,
				      FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
				      NULL);
	} while (!(ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
...
	return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
}

So we report success with the page still merged. Async RWP does unshare
it, but only after clearing the uffd bit.

I'd let FAULT_FLAG_UNSHARE skip do_uffd_rwp(). We'd still need to carry
PAGE_NONE over in wp_page_copy(), though. That PTE is rebuilt from
vma->vm_page_prot, so the uffd bit alone won't stop reads.

>How to deal RWP properly is a good question (see Sashiko review). I feel like
>some more stuff might be missing there to handle this correctly.

Tested, this is what I had in mind:

---8<---
diff --git a/mm/memory.c b/mm/memory.c
index 976cd03954e3..d99f5ef1dd8c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4119,8 +4119,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
 		if (unlikely(unshare)) {
 			if (pte_soft_dirty(vmf->orig_pte))
 				entry = pte_mksoft_dirty(entry);
-			if (pte_uffd(vmf->orig_pte))
+			if (pte_uffd(vmf->orig_pte)) {
 				entry = pte_mkuffd(entry);
+				if (userfaultfd_rwp(vma))
+					entry = pte_modify(entry, PAGE_NONE);
+			}
 		} else {
 			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 		}
@@ -6656,9 +6659,12 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 * VM_UFFD_RWP VMA, a protnone PTE without the uffd bit is
 		 * NUMA hinting and must still fall through to do_numa_page().
 		 */
-		if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte))
-			return do_uffd_rwp(vmf);
-		return do_numa_page(vmf);
+		if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte)) {
+			if (!(vmf->flags & FAULT_FLAG_UNSHARE))
+				return do_uffd_rwp(vmf);
+		} else {
+			return do_numa_page(vmf);
+		}
 	}

 	spin_lock(vmf->ptl);
--

Cheers, Lance