Re: [PATCH 1/1] mm/rmap: fix soft-dirty bit loss when remapping zero-filled mTHP subpage to shared zeropage

David Hildenbrand
2025-09-29T16:12:09Z

On 29.09.25 15:22, Lance Yang wrote:
>
>
> On 2025/9/29 20:08, David Hildenbrand wrote:
>> On 29.09.25 13:29, Lance Yang wrote:
>>>
>>>
>>> On 2025/9/29 18:29, Lance Yang wrote:
>>>>
>>>>
>>>> On 2025/9/29 15:25, David Hildenbrand wrote:
>>>>> On 28.09.25 06:48, Lance Yang wrote:
>>>>>> From: Lance Yang <lance.yang@linux.dev>
>>>>>>
>>>>>> When splitting an mTHP and replacing a zero-filled subpage with the
>>>>>> shared
>>>>>> zeropage, try_to_map_unused_to_zeropage() currently drops the soft-
>>>>>> dirty
>>>>>> bit.
>>>>>>
>>>>>> For userspace tools like CRIU, which rely on the soft-dirty mechanism
>>>>>> for
>>>>>> incremental snapshots, losing this bit means modified pages are
>>>>>> missed,
>>>>>> leading to inconsistent memory state after restore.
>>>>>>
>>>>>> Preserve the soft-dirty bit from the old PTE when creating the
>>>>>> zeropage
>>>>>> mapping to ensure modified pages are correctly tracked.
>>>>>>
>>>>>> Cc: <stable@vger.kernel.org>
>>>>>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage
>>>>>> when splitting isolated thp")
>>>>>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>>>>>> ---
>>>>>>    mm/migrate.c | 4 ++++
>>>>>>    1 file changed, 4 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>>>> index ce83c2c3c287..bf364ba07a3f 100644
>>>>>> --- a/mm/migrate.c
>>>>>> +++ b/mm/migrate.c
>>>>>> @@ -322,6 +322,10 @@ static bool try_to_map_unused_to_zeropage(struct
>>>>>> page_vma_mapped_walk *pvmw,
>>>>>>        newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
>>>>>>                        pvmw->vma->vm_page_prot));
>>>>>> +
>>>>>> +    if (pte_swp_soft_dirty(ptep_get(pvmw->pte)))
>>>>>> +        newpte = pte_mksoft_dirty(newpte);
>>>>>> +
>>>>>>        set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>>>>>>        dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
>>>>>
>>>>> It's interesting that there isn't a single occurrence of the stof-
>>>>> dirty flag in khugepaged code. I guess it all works because we do the
>>>>>
>>>>>       _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
>>>>>
>>>>> and the pmd_mkdirty() will imply marking it soft-dirty.
>>>>>
>>>>> Now to the problem at hand: I don't think this is particularly
>>>>> problematic in the common case: if the page is zero, it likely was
>>>>> never written to (that's what the unerused shrinker is targeted at),
>>>>> so the soft-dirty setting on the PMD is actually just an over-
>>>>> indication for this page.
>>>>
>>>> Cool. Thanks for the insight! Good to know that ;)
>>>>
>>>>>
>>>>> For example, when we just install the shared zeropage directly in
>>>>> do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
>>>>>
>>>>> Now, one could argue that if the content was changed from non-zero to
>>>>> zero, it ould actually be soft-dirty.
>>>>
>>>> Exactly. A false negative could be a problem for the userspace tools,
>>>> IMO.
>>>>
>>>>>
>>>>> Long-story short: I don't think this matters much in practice, but
>>>>> it's an easy fix.
>>>>>
>>>>> As said by dev, please avoid double ptep_get() if possible.
>>>>
>>>> Sure, will do. I'll refactor it in the next version.
>>>>
>>>>>
>>>>> Acked-by: David Hildenbrand <david@redhat.com>
>>>>
>>>> Thanks!
>>>>
>>>>>
>>>>>
>>>>> @Lance, can you double-check that the uffd-wp bit is handled
>>>>> correctly? I strongly assume we lose that as well here.
>>>
>>> Yes, the uffd-wp bit was indeed being dropped, but ...
>>>
>>> The shared zeropage is read-only, which triggers a fault. IIUC,
>>> The kernel then falls back to checking the VM_UFFD_WP flag on
>>> the VMA and correctly generates a uffd-wp event, masking the
>>> fact that the uffd-wp bit on the PTE was lost.
>>
>> That's not how VM_UFFD_WP works :)
>
> My bad! Please accept my apologies for the earlier confusion :(
>
> I messed up my test environment (forgot to enable mTHP), which
> led me to a completely wrong conclusion...
>
> You're spot on. With mTHP enabled, the WP fault was not caught
> on the shared zeropage after it replaced a zero-filled subpage
> during an mTHP split.
>
> This is because do_wp_page() requires userfaultfd_pte_wp() to
> be true, which in turn needs both userfaultfd_wp(vma) and
> pte_uffd_wp(pte).
>
> static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
> pte_t pte)
> {
> return userfaultfd_wp(vma) && pte_uffd_wp(pte);
> }
>
> userfaultfd_pte_wp() fails as we lose the uffd-wp bit on the PTE ...

That's my understanding. And FWIW, that's a much more important fix. (in
contrast to soft-dirty, uffd-wp actually is precise)

Can you test+send a fix ... please? :)

--
Cheers

David / dhildenb