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b=oJXKl2svOLUDhtBKsHZtOlCviRAoJzYkTYfxOHmirlaQu47y3rzTVhCFXBs/Q4yJH UgYlVKaz7zL0FyHV31QoO2GceLTO2fO/N3dpxb6QG0qX8n+xeUZFfekhhKNC31wmyP RfWl9xi/kdbnetynthznxRTw6VqmNeYRQ2jR8gtur64EEoHULSVf9k1f4p6/zXWroR l9pu9odzmuvYbJcpY6IVw73ORSgeJEybArCZBO9woXFzQ+nWeLdtrgnl3hDiFuUNCy MzC+xg/QN3TjhDLmH6UaIBYAMT5iE3pnrZi7KF69GLIqi4tCBvTId+bfYxP/wdBy3O TM7eo9CW5PoIA== Message-ID: <3b62ce89-2b58-48d3-9c19-c742f211c88f@kernel.org> Date: Mon, 9 Mar 2026 23:00:33 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] 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 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, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260309212502.3922825-1-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: rspam01 X-Rspamd-Queue-Id: BC8BD20019 X-Stat-Signature: be6kdwuzofifm87qabbtfyp576t91bri X-Rspam-User: X-HE-Tag: 1773093642-838185 X-HE-Meta: U2FsdGVkX18wJcNyJv7ty5T43Uw2bxI2RyHupce6aV9GanQRhPU7chakxAah/R9WjXt3/YCwEOAOJcux3KjVd6HfYYiHNAopBvzai0ntUBZg5oecHoIGXXOBvtA17MVjHNjvXFeoBie6uVH1KONE4m23ZKo4kCpS37UFuFmF3h8HkMl7kYgHwsTc7NROg2/t2nPrTuxbdAZh93m0wzQicsdEX2YaftGmFSsgo26700VKQtOTFJG5sQRYgUMZF1R9mchnmlc/AYxUzbwkL2h+vPF6v3SI3WKRMGcsv7t/SQ2TXQWAVI9AYGGdfEWj2b7dgsMsx3IwYTwzay0lhfnzr77GZFvif/WQmzxG7VBi9FerJPIe1cMjAuESpLrtgJMYFincM7KK/Nvsw6W6rwbg4Xeh3wKCfPSDqBreTNgDwjgrn+59ZekY4vo8sCBI9q0Uff4tGlZ2Uo8OOvfi4wI6MbtsEPr6O5xQwmdWFDVaFx0yd6YfO8IIlv9UBoqC2FUnQrSw5wsEZvXH99L/ThSqQPgj41+lPHTSq73rtlDqe2km9Mgz+Vj4X0Ta93LX/LI12FQ6SRdSx15rQZOWCOzDOoryCz0HkEyBnKVQfVTP886kLY43B5hBR5ro3uNCYiCaOpiotWI6ibTZ2bZvG8lxON6vLVrmUnmEaf2pKuxYC/pPOM4hw5OON4jV2AKE9axqUtCSOQkbaTeUB9LVMbtcFiecmxO5X5d/PSPee4Ys5UMV2UB0rsG5TX070AsAETZOoy1lon+edhJoU4HK39QkaxsCgenUiolZTbbHEMxoPhEqS/Cn90RfEutKD+JJ1a+bi0tQKGJqtOL04boGIqORchw2NtStg5y5h1eA7CQxxuWgZc/2agk+nsBZaTb0yZpGddit01VxE0qYwpJFsNQiBi1uPm9aamhbd+OKfDS7WxnO8Kj4I5v2yh8/AMj4I2bSs0TP+7dwFyvLH8Jv4sm gJvjYBHs uWVsDDeiO/qY5mkSZwlVdfgNf2n57Ie87KgKp9CGMKUIGyUAYQuEAXlHJKyqXLoEl5yZ7teffvdCYVXw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/9/26 22:25, 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. The expected folio refcount > at the start of migrate_vma_split_unmapped_folio() is 1. > > Suggested-by: Zi Yan > Reviewed-by: Zi Yan > Reviewed-by: Nico Pache > Signed-off-by: Usama Arif > --- > v1 -> v2: > - Remove warning if folio refcount !=1 at the start of > migrate_vma_split_unmapped_folio() (David) > --- > mm/migrate_device.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 78c7acf02461..fbfe5715f635 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -908,6 +908,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, > unsigned long flags; > int ret = 0; > > + /* > + * 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); Acked-by: David Hildenbrand (Arm) -- Cheers, David