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([2a01:599:901:4a65:f2e2:845:f3d2:404d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e2ab6a514sm231311635e9.22.2025.09.29.09.11.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Sep 2025 09:12:00 -0700 (PDT) Message-ID: <1718aee4-1201-4362-885b-e707f536a065@redhat.com> Date: Mon, 29 Sep 2025 18:11:58 +0200 Precedence: bulk X-Mailing-List: stable@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> <69b463e5-9854-496d-b461-4bf65e82bc0a@redhat.com> <0701c9d9-b9b3-4313-8783-8e6d1dbec94d@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 15:22, Lance Yang wrote: > > > On 2025/9/29 20:08, David Hildenbrand wrote: >> 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 :) > > My bad! Please accept my apologies for the earlier confusion :( > > I messed up my test environment (forgot to enable mTHP), which > led me to a completely wrong conclusion... > > You're spot on. With mTHP enabled, the WP fault was not caught > on the shared zeropage after it replaced a zero-filled subpage > during an mTHP split. > > This is because do_wp_page() requires userfaultfd_pte_wp() to > be true, which in turn needs both userfaultfd_wp(vma) and > pte_uffd_wp(pte). > > static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma, > pte_t pte) > { > return userfaultfd_wp(vma) && pte_uffd_wp(pte); > } > > userfaultfd_pte_wp() fails as we lose the uffd-wp bit on the PTE ... That's my understanding. And FWIW, that's a much more important fix. (in contrast to soft-dirty, uffd-wp actually is precise) Can you test+send a fix ... please? :) -- Cheers David / dhildenb