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[2003:d8:2f05:e100:526f:9b8:bd2a:2997]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e2ab31f1dsm212404725e9.13.2025.09.29.00.25.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Sep 2025 00:25:35 -0700 (PDT) Message-ID: Date: Mon, 29 Sep 2025 09:25:33 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/rmap: fix soft-dirty bit loss when remapping zero-filled mTHP subpage to shared zeropage To: Lance Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com 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 References: <20250928044855.76359-1-lance.yang@linux.dev> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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. 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. 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. Acked-by: David Hildenbrand @Lance, can you double-check that the uffd-wp bit is handled correctly? I strongly assume we lose that as well here. -- Cheers David / dhildenb