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b=B7IXhROP5eZaXsujy91kFeFuniY2+1W+nwoEZozilAP2brfi+8rz1JbIlV6ol7P+p V6t5pR+3rlrrkGCvtbFw9bEEbbZQBxsm7lmTJr7vcpBareDfr6A4WlSwQNc/UwIEiO qnBl+RjBDaOPCF0hnKp0+P9NWjWc9zS9IvOE4KSGmx71BRJpo+kBm/PvY2sOalQtya 6XVS2O9/7+xXHk8awhoAnFPDfM5SRQUBzZ8F+p/WSAC8QFFZ477HgjcCjxQ/Vu6+sE M5np1cHKF+kjVfZf97LZP0j4P84yuGzmpX+fX4034G+TyFdcVAR7OmnUD1OroDGCbF XCZleASryZAJg== Message-ID: <57cf14b7-e5cc-4298-9d56-978b855e3276@kernel.org> Date: Mon, 9 Mar 2026 21:22:47 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate_device: document folio_get requirement before frozen PMD split To: Usama Arif , Andrew Morton , npache@redhat.com, ziy@nvidia.com, linux-mm@kvack.org, mpenttil@redhat.com, balbirs@nvidia.com Cc: matthew.brost@intel.com, joshua.hahnjy@gmail.com, hannes@cmpxchg.org, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260306104409.3915811-1-usama.arif@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 15F781C0002 X-Stat-Signature: kn9xke7uabtay59xdzb9k3o5cdeaj1qy X-Rspam-User: X-HE-Tag: 1773087777-302977 X-HE-Meta: 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 BvrFS38e /4Ih6t9Ps+mMmpYbrQmuZNsCV6CLrD9SokR+qk8EHNNKHGZHvtTudGjaH7BbS4L6Bjq50tfe4YqUaF/fZPYKpF5kPuy/+XBDlaykZNhUY3RzujOXTUGfn/cgeLH0GOJjSpzT5ikIRvUAVHlvtkezQJsY/20VTyc80O89xS/1YVai00UJJuyvY8f398cG/WIKHCLQfb92jYK5mj1pLZJK4x7RVNANL4Z6OP9166SJnzIct3r/iZj9PZbJnJ/I1fh2XXDvCuOcNFTHCh/Eu1WCorm0higeuaQYW1d2rzBQZt17u7mPKnymWsDtoueNNxRm301z2SEEAgys1my9Z1lYGGTCSMfb+kX+oO9goTvl1aU/Pb4PD4xjnO4LQU39ez5oe/aSOjHB9yD0ZT+bPGQ/XLFosOA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/9/26 20:11, Usama Arif wrote: > > > On 09/03/2026 18:18, David Hildenbrand (Arm) wrote: >> On 3/6/26 11:44, Usama Arif wrote: >>> split_huge_pmd_address() with freeze=true splits a PMD migration entry >>> into PTE migration entries, consuming one folio reference in the >>> process. The folio_get() before it provides this reference. >>> >>> Add a comment explaining this relationship and a VM_WARN_ON_ONCE to >>> catch an unexpected refcount != 1 entry state. >>> >>> Suggested-by: Zi Yan >>> Signed-off-by: Usama Arif >>> --- >>> mm/migrate_device.c | 5 +++++ >>> 1 file changed, 5 insertions(+) >>> >>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>> index 78c7acf024615..6fa2878848a7e 100644 >>> --- a/mm/migrate_device.c >>> +++ b/mm/migrate_device.c >>> @@ -908,6 +908,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, >>> unsigned long flags; >>> int ret = 0; >>> >>> + VM_WARN_ON_ONCE(folio_ref_count(folio) != 1); >> >> Can't we have speculative references here? In general, asserting that >> the refcount has an exact value (besides 0) is often shaky. > > > I hope not at this point in code. > > At this point, the folio is locked and unmapped (both done in migrate_vma_collect_huge_pmd()), > and the present PMD was set to migration entry. It is isolated from LRU in > migrate_device_unmap(). So the folio should not be visible to GUP or reclaim/compaction. > Only anon, non-swapcache folios should reach here. So it won't run into any folio_try_get > in page cache or swap cache. We have other pfn walkers that can just temporary grab a reference, to immediately back off. So it's not very reliable to depend on that. > > The folio_get() done in migrate_vma_split_unmapped_folio() is consumed by > split_huge_pmd_address(), and folio_split_unmapped() expects a folio_reference > of 1 after this [1]. > > If its not considered good to assert a non zero refcount value, I can change the > warning to a comment, but I think refcount should be 1 at this point, otherwise > folio_split_unmapped will fail. Well, yes. folio_split_unmapped() will handle this gracefully (as documented), without triggering a warning. So best to remove the VM_WARN_ON_ONCE(). -- Cheers, David