[PATCH v3] mm/ksm: keep soft-dirty when merging a page

[PATCH v3] mm/ksm: keep soft-dirty when merging a page

[Samarth Narang]

replace_page() maps the KSM page with a PTE built from scratch, so the
soft-dirty bit of the PTE it replaces is lost. A page written since
"clear_refs 4" then no longer shows as soft-dirty once it is merged, and
a soft-dirty based dumper treats it as unchanged.

Copy the soft-dirty bit from the original PTE to the new PTE.

Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>
---

Notes:
    v2: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/
    
    Changes since v2:
    - Keep only the soft-dirty fix, as David suggested; Lance will be handling
      the uffd-wp/RWP part.
    - Fixes tag reduced to the soft-dirty commit.
    - Dropped the Tested-by tag, since the patch changed.

 mm/ksm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index dbfb8f98b189..910a1307ac9f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1463,6 +1463,10 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
 		dec_mm_counter(mm, MM_ANONPAGES);
 	}
 
+	/* Preserve soft-dirty when replacing the page. */
+	if (pte_soft_dirty(orig_pte))
+		newpte = pte_mksoft_dirty(newpte);
+
 	flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep)));
 	/*
 	 * No need to notify as we are replacing a read only page with another

base-commit: fcec4f4bcc1101dbd7e3aff267856b3bbfe128c6
-- 
2.53.0

Re: [PATCH v3] mm/ksm: keep soft-dirty when merging a page

[Lance Yang]

On Wed, Oct 21, 2026 at 11:21:57AM -0700, Samarth Narang wrote:
>replace_page() maps the KSM page with a PTE built from scratch, so the
>soft-dirty bit of the PTE it replaces is lost. A page written since
>"clear_refs 4" then no longer shows as soft-dirty once it is merged, and
>a soft-dirty based dumper treats it as unchanged.
>
>Copy the soft-dirty bit from the original PTE to the new PTE.
>
>Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")
>Cc: stable@vger.kernel.org
>Assisted-by: LLM
>Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>
>---

LGTM!

Reviewed-by: Lance Yang <lance.yang@linux.dev>

Re: [PATCH v3] mm/ksm: keep soft-dirty when merging a page

[Andrew Morton]

On Wed, 21 Oct 2026 11:21:57 -0700 Samarth Narang <samarth.colleges@gmail.com> wrote:

> replace_page() maps the KSM page with a PTE built from scratch, so the
> soft-dirty bit of the PTE it replaces is lost. A page written since
> "clear_refs 4" then no longer shows as soft-dirty once it is merged, and
> a soft-dirty based dumper treats it as unchanged.
> 
> Copy the soft-dirty bit from the original PTE to the new PTE.

Thanks.  My LLM review thing says:

: There's a separate, pre-existing problem with the KSM zero-page
: path which is worth pointing out.
: 
: When use_zero_pages is enabled, replace_page() constructs the
: replacement PTE using:
: 
:     newpte = pte_mkdirty(pte_mkspecial(pfn_pte(...)));
: 
: The dirty bit is deliberately set so that KSM can distinguish its
: own zero-page mappings from ordinary zero-page mappings. However,
: on x86, pte_mkdirty() sets both _PAGE_DIRTY and _PAGE_SOFT_DIRTY.
: 
: Consequently, a process can clear its soft-dirty bits through
: /proc/PID/clear_refs, perform no further writes, and subsequently
: have a page marked soft-dirty simply because KSM merged an all-zero
: page. Userspace then sees bit 55 set in /proc/PID/pagemap even though
: the page hasn't been modified since tracking was reset.
: 
: Your patch doesn't introduce this problem, but it doesn't fix it
: either. The new pte_soft_dirty(orig_pte) check cannot help in the
: zero-page case because pte_mkdirty() has already set the bit in
: newpte.
: 
: I don't think this should hold up v3. Losing soft-dirty when merging
: ordinary pages is a more serious problem, since it can cause
: incremental checkpointing to miss modified pages. The zero-page
: case instead produces false positives, causing unnecessary copying.
: 
: It would be worth addressing separately, preserving KSM's internal
: zero-page marker without manufacturing a soft-dirty indication.