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Message-ID: <68cff704-bbdb-41a7-81d4-7195f71ba73a@arm.com>
Date: Tue, 30 Sep 2025 10:59:15 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: peterx@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
baohua@...nel.org, ryan.roberts@....com, npache@...hat.com,
riel@...riel.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
harry.yoo@...cle.com, jannh@...gle.com, matthew.brost@...el.com,
joshua.hahnjy@...il.com, rakie.kim@...com, byungchul@...com,
gourry@...rry.net, ying.huang@...ux.alibaba.com, apopple@...dia.com,
usamaarif642@...il.com, yuzhao@...gle.com, lorenzo.stoakes@...cle.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org, ioworker0@...il.com,
stable@...r.kernel.org, akpm@...ux-foundation.org, david@...hat.com
Subject: Re: [PATCH v2 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when
remapping zero-filled mTHP subpage to shared zeropage
On 30/09/25 10:52 am, Lance Yang wrote:
>
>
> On 2025/9/30 12:50, Dev Jain wrote:
>>
>> On 30/09/25 10:03 am, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@...ux.dev>
>>>
>>> When splitting an mTHP and replacing a zero-filled subpage with the
>>> shared
>>> zeropage, try_to_map_unused_to_zeropage() currently drops several
>>> important
>>> PTE bits.
>>>
>>> For userspace tools like CRIU, which rely on the soft-dirty
>>> mechanism for
>>> incremental snapshots, losing the soft-dirty bit means modified
>>> pages are
>>> missed, leading to inconsistent memory state after restore.
>>>
>>> As pointed out by David, the more critical uffd-wp bit is also dropped.
>>> This breaks the userfaultfd write-protection mechanism, causing writes
>>> to be silently missed by monitoring applications, which can lead to
>>> data
>>> corruption.
>>>
>>> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
>>> creating the new zeropage mapping to ensure they are correctly tracked.
>>>
>>> Cc: <stable@...r.kernel.org>
>>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage
>>> when splitting isolated thp")
>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>> Suggested-by: Dev Jain <dev.jain@....com>
>>> Acked-by: David Hildenbrand <david@...hat.com>
>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>> ---
>>> v1 -> v2:
>>> - Avoid calling ptep_get() multiple times (per Dev)
>>> - Double-check the uffd-wp bit (per David)
>>> - Collect Acked-by from David - thanks!
>>> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-
>>> lance.yang@...ux.dev/
>>>
>>> mm/migrate.c | 9 ++++++++-
>>> 1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index ce83c2c3c287..50aa91d9ab4e 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -300,13 +300,14 @@ static bool
>>> try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>>> unsigned long idx)
>>> {
>>> struct page *page = folio_page(folio, idx);
>>> + pte_t oldpte = ptep_get(pvmw->pte);
>>
>> What I meant to say was, you can pass oldpte from
>> remove_migration_pte to this
>> function. Basically define old_pte = ptep_get(pvmw.pte) in the
>> declarations of
>> the start of the while block in remove_migration_pte and remove the
>> existing
>> one. That will ensure ptep_get() gets called only once per iteration.
>
> Ah, got it. Thanks for the clarification!
>
> IIUC, you mean something like this:
>
> ```
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..bafd8cb3bebe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio,
> struct list_head *list)
>
> static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk
> *pvmw,
> struct folio *folio,
> + pte_t old_pte,
> unsigned long idx)
> {
> struct page *page = folio_page(folio, idx);
> @@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct
> page_vma_mapped_walk *pvmw,
> return false;
> VM_BUG_ON_PAGE(!PageAnon(page), page);
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> - VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
> + VM_BUG_ON_PAGE(pte_present(old_pte), page);
>
> if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags &
> VM_LOCKED) ||
> mm_forbids_zeropage(pvmw->vma->vm_mm))
> @@ -322,6 +323,12 @@ 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(old_pte))
> + newpte = pte_mksoft_dirty(newpte);
> + if (pte_swp_uffd_wp(old_pte))
> + newpte = pte_mkuffd_wp(newpte);
> +
> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>
> dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
> @@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio,
>
> while (page_vma_mapped_walk(&pvmw)) {
> rmap_t rmap_flags = RMAP_NONE;
> - pte_t old_pte;
> + pte_t old_pte = ptep_get(pvmw.pte);
> pte_t pte;
> swp_entry_t entry;
> struct page *new;
> @@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio
> *folio,
> }
> #endif
> if (rmap_walk_arg->map_unused_to_zeropage &&
> - try_to_map_unused_to_zeropage(&pvmw, folio, idx))
> + try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
> continue;
>
> folio_get(folio);
> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
> - old_pte = ptep_get(pvmw.pte);
>
> entry = pte_to_swp_entry(old_pte);
> if (!is_migration_entry_young(entry))
> ```
>
> ptep_get() gets called only once per iteration, right?
Yup.
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