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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Miaohe Lin , Naoya Horiguchi , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 2/3] mm/huge_memory: add kernel-doc for folio_split_supported() Date: Thu, 20 Nov 2025 09:48:31 -0500 X-Mailer: MailMate (2.0r6290) Message-ID: <9D88B1A6-1D00-4770-9B66-A9A0F1B6AC15@nvidia.com> In-Reply-To: References: <20251120035953.1115736-1-ziy@nvidia.com> <20251120035953.1115736-3-ziy@nvidia.com> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BL1PR13CA0326.namprd13.prod.outlook.com (2603:10b6:208:2c1::31) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|SA3PR12MB9131:EE_ X-MS-Office365-Filtering-Correlation-Id: 142f2584-9005-4a91-1627-08de2843e0f7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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 49LX12Sn 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 20 Nov 2025, at 4:27, David Hildenbrand (Red Hat) wrote: > On 11/20/25 04:59, Zi Yan wrote: >> It clarifies that folio_split_supported() does not check folio->mappin= g and >> can dereference it. >> >> Signed-off-by: Zi Yan >> --- >> mm/huge_memory.c | 17 +++++++++++++++++ >> 1 file changed, 17 insertions(+) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index efea42d68157..15e555f1b85d 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio = *folio, int new_order, >> return 0; >> } >> +/** >> + * folio_split_supported() - check if a folio can be split to a given= order >> + * @folio: folio to be split >> + * @new_order: the smallest order of the after split folios (since bu= ddy >> + * allocator like split generates folios with orders from= @folio's >> + * order - 1 to new_order). >> + * @split_type: uniform or non-uniform split >> + * @warns: whether gives warnings or not for the checks in the functi= on >> + * >> + * folio_split_supported() checks if @folio can be split to @new_orde= r using >> + * @split_type method. >> + * >> + * Context: Caller must make sure folio->mapping is not NULL, since t= he >> + * function does not check it and can dereference folio->map= ping > > Only for anon folios. Also, I would drop the detail about dereference. OK. > > I guess we really need the folio lock to prevent concurrent truncation.= > > Maybe something like: > > "The folio must be locked. For non-anon folios, the caller must make su= re that folio->mapping is not NULL (e.g., not truncated)." Sure. Do you think it is worth adding VM_WARN_ONCE_ON(!folio_test_locked)= ; and VM_WARN_ONCE_ON(!folio->mapping); ? Best Regards, Yan, Zi