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From: Lance Yang Subject: Re: [PATCH 1/1] mm/rmap: fix soft-dirty bit loss when remapping zero-filled mTHP subpage to shared zeropage To: David Hildenbrand Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, ryan.roberts@arm.com, dev.jain@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, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.kernel.org, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com References: <20250928044855.76359-1-lance.yang@linux.dev> <900d0314-8e9a-4779-a058-9bb3cc8840b8@linux.dev> Content-Language: en-US In-Reply-To: <900d0314-8e9a-4779-a058-9bb3cc8840b8@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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 >>> >>> 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: >>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage >>> when splitting isolated thp") >>> Signed-off-by: Lance Yang >>> --- >>>   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 > > 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. IMHO, explicitly preserving the uffd-wp bit on the PTE is still necessary, since we're not sure if losing that bit is safe in all cases :) > > Certainly, I'll check the uffd-wp bit as well and get back to you soon. > > Cheers, > Lance