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[2003:d8:2f39:1500:c90f:7794:3c33:4164]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e56f53596sm10484755e9.7.2025.09.29.05.08.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Sep 2025 05:08:19 -0700 (PDT) Message-ID: <69b463e5-9854-496d-b461-4bf65e82bc0a@redhat.com> Date: Mon, 29 Sep 2025 14:08:17 +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 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> <1f66374a-a901-49e7-95c8-96b1e5a5f22d@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: 8bit On 29.09.25 13:29, Lance Yang wrote: > > > 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. That's not how VM_UFFD_WP works :) -- Cheers David / dhildenb