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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: DABD080007 X-Stat-Signature: 5hcpesbymzptaxsjk5gzid5398ojrc9q X-HE-Tag: 1759207826-975666 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. > 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));