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[149.14.88.26]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4b2b8c693a3sm94776351cf.1.2025.08.27.07.34.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Aug 2025 07:34:39 -0700 (PDT) Message-ID: <2aa3f478-9c87-4102-b83e-bf235372d834@redhat.com> Date: Wed, 27 Aug 2025 16:34:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] huge_mm.h: is_huge_zero_folio(NULL) should return false To: Max Kellermann Cc: Andrew Morton , lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shikemeng@huaweicloud.com, kasong@tencent.com, nphamcs@gmail.com, bhe@redhat.com, chrisl@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250826231626.218675-1-max.kellermann@ionos.com> <20250826185515.7fbe1821713195b170ac1b31@linux-foundation.org> <11a5d391-3a13-4376-98f1-34b529d3c583@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 9AC0A12000A X-Stat-Signature: ssop7135i636mr5cdhp7hhwprp1fofdp X-Rspam-User: X-HE-Tag: 1756305283-842005 X-HE-Meta: U2FsdGVkX189YuJZXdvXZvadsC7gk4xNdkCT+MiIrN8YsfaLT6LZVvJupnne4yJoOYfQjy7H3tGvLtStshLlbDOZ8Ll1bNfs5y9SmaRtmT8pNPnNjG0/YUD4iC5k0qZ0nCBglV12QuAZNQBwotbBK78oT1iRJhV+JfRJk32v2/EgzxnYweboqOUiJWM5j9vmdhEhyo+44JbL10DxBAkUCd/9akVZVips9Vv4ohnaGHTW43LeqiuslAjjNd1w3QCdTZ0gB7DcGfZsEi1xIqdJX1BUEO7y3ibd4iL0h0JvoyYZvf+u7SQfJjuSxATSl09zX+rEIgiYnZiwpOVxiFtjbIaOj6Sxtcl5vmEMweHxAzkFja19JWeKCarr9Q/XNZvAnpCb3QlKrw5ejoaLOxqiU03VyzJrp4diIkSqC2k0Aat4tHCEENK8rzk5JuFioQVr31Q4/uwvpE4iGVOWpXW4IOenQ9EdWK08R05e/EjENRSssW2aRCoOabYfJFYWm8YTxFfFEKm7OBcr8SX4CrLN2dbeS5EBtkgqwy0WL45cfp1OAfaSeXUbGcT2AgrooyOOCOEAQLNP9cQ0t59l5/AXtWaLGVa0aMPVuywoXb77e9iXgQM507KgRba+EahZTncByQHucFnqdxv3sFGCJX6JEC2d/j9Qi3MwKuMxy21GrD6V76w3HGRW2ky0wfBB6q0wdRWalrnoIyAjEh9I7bLJEFeQJ/twnF4921bMpaCKSf9JBsLmb9nO2IHIcJXRL2tVGu9IBWYo6aoObG3hSyOT8XXD7dllTAEg11oLKr44HrXxBB3w2zF/tUyPnAC7gaGTnnunrX/xh+TvLjzSJn+a2t0Y7ZC6jTPzJRV0nj1CGOcXOU67mi5NUf5kGC+1oYSbGkfz3nfleho9/NweRrc4fB7gg7pTlB0pvg+HCxEhllZwYhA2Y7zBiegNVGpxNUydk9g4T8T6lzUe6x4ALTQ EGbHEpOL 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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 27.08.25 15:06, Max Kellermann wrote: > On Wed, Aug 27, 2025 at 1:56 PM David Hildenbrand wrote: >>> This patch isn't about the function parameter but about the global >>> variable being NULL. >>> (Don't mix up with my other patch.) >> >> Huh? > > Oh, I see the misunderstanding. You are right here, yes my patch is > indeed about passing NULL to is_huge_zero_folio(). My previous reply > was confusing. > > But you were replying to a sub-thread by Andrew who questioned whether > a call is_huge_zero_folio() is legal at all before the huge zero folio > has been created. Sure it is, I mean on some systems it is never initialized. > The question I asked Andrew, which you replied to, was not about > passing NULL to is_huge_zero_folio(), but about whether the call to > is_huge_zero_folio() is legal at all, no matter which parameter value. Yes, see how get_huge_zero_folio() dynamically allocates it on demand to then update huge_zero_folio. > > I agree with you that is_huge_zero_folio(NULL) should not be legal and > makes no sense. It's a bug somewhere in the caller. > But I saw that the current implementation effectively (randomly) hides > a bug in another part of the kernel (= Ceph), one that I'd rather like > to be visible (and get fixed). And that is the point of my patch, and > is the reason why I chose to have the additional debug-only code > inside an #ifdef - I don't want normal users to pay the price for a > debugging feature, no matter how small the price. > > David, what is your opinion here? > > - leave it as it is; is_huge_zero_folio(NULL) may randomly return true > or false (= reject this patch) > - add the huge_zero_folio==NULL check, but guarded with #ifdef DEBUG > (= this patch) > - add the huge_zero_folio==NULL check unconditionally (suggested by Zi Yan) VM_WARN_ON_ONCE(!folio); Should allow for catching such bugs without causing any runtime overhead on non-debug kernels. -- Cheers David / dhildenb