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

Lance Yang
2025-09-30T01:53:35Z

On 2025/9/30 00:11, David Hildenbrand wrote:
> 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)

Got it, and thanks for setting me straight on that!

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

Certainly, I'm on it ;)