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From: Lance Yang 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 To: Dev Jain Cc: peterx@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, ryan.roberts@arm.com, npache@redhat.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, jannh@google.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, usamaarif642@gmail.com, yuzhao@google.com, lorenzo.stoakes@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org, akpm@linux-foundation.org, david@redhat.com References: <20250930043351.34927-1-lance.yang@linux.dev> <0c498301-7434-4b2b-b7bc-73abe2057b67@arm.com> Content-Language: en-US In-Reply-To: <0c498301-7434-4b2b-b7bc-73abe2057b67@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/9/30 12:50, Dev Jain wrote: > > On 30/09/25 10:03 am, Lance Yang wrote: >> From: Lance Yang >> >> 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: >> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage >> when splitting isolated thp") >> Suggested-by: David Hildenbrand >> Suggested-by: Dev Jain >> Acked-by: David Hildenbrand >> Signed-off-by: Lance Yang >> --- >> 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@linux.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? > >>       pte_t newpte; >>       if (PageCompound(page)) >>           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(oldpte), 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(oldpte)) >> +        newpte = pte_mksoft_dirty(newpte); >> +    if (pte_swp_uffd_wp(oldpte)) >> +        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));