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b=UJf0p1+TzB+pXOrVGFSCej1LqFgXVQgcbvxFKJSlTrtAz29CkovetwiQerry37Vad oxlwScXzf4hdF0Mjg0qDZY/pWrvBBlQi7I6jpkzaY2P1tMNpk+WhhkK+e65sEafFB/ nv5aBYPF6G4CnY8jzaLL7CBqSr6/y5R1kwYsSCDi7wRBLYuqBlDJ5DEqxxB1IiCm+D ssaqVEz5/qPuZexvnxnHzXVOi+jHF1XtJ4Y4N1ZtcV7vCn8idJIXXFBOzdywuz9eqc 3KeMPQaO1CDjUp5Hq6FhesO9gJaddulH3QaRadR7/KbpAeun5YerHwe4XkPGoCEEQF I1Ksl33mCy1lw== Message-ID: Date: Mon, 9 Mar 2026 16:18:29 +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: rspam02 X-Rspamd-Queue-Id: 774E3180012 X-Stat-Signature: a7nxown9qkui885co43qd1ujw9jnodtu X-Rspam-User: X-HE-Tag: 1773069520-643277 X-HE-Meta: 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 zttWM7C4 Gjq5xIYpQGCySBN1WdLn9IPZQ9KaJd0wIUBQwrtfk0KGM/Qr/EnL8n5fz9U9cAs4tksdk4MwQCZ4RJPaAHgSb+xOCphKNZiZyARrAkYfjMaXfB1nUDGjn08MDzyemLPeurhwPIQCfsNs06lSyInzmjgI6LUtI6H7z2Pl68nWSiikmgJ1O7FfY4VqfZg7GYZTTO2IL3/F24QXVjKXKNaC0IHt8mViL609ndOuvskXN99LxZmqsAa1rYZiBjeR0Kc7NHOLsWNFqLDIpC4IR2WPMHi0Gg1/vQCszp57jBssoua/LSUNHXU5cFOf2SAzvlymoa/pYnC81aQrON+Zv8Y0++YclLjgPK/8WqvsQoOyZPTOi8dQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. > + /* > + * take a reference, since split_huge_pmd_address() with freeze = true > + * drops a reference at the end. > + */ > folio_get(folio); > split_huge_pmd_address(migrate->vma, addr, true); > ret = folio_split_unmapped(folio, 0); -- Cheers, David