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[2003:d8:2f25:bd00:949:b5a9:e02a:f265]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b28240sm11292833f8f.72.2025.06.16.07.26.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Jun 2025 07:26:38 -0700 (PDT) Message-ID: <51662e0b-4ca2-4841-b8ba-cbf32178eea2@redhat.com> Date: Mon, 16 Jun 2025 16:26:37 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/4] mm/mempolicy: Expose policy_nodemask() in include/linux/mempolicy.h To: Bijan Tabatabai Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, sj@kernel.org, akpm@linux-foundation.org, corbet@lwn.net, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, bijantabatab@micron.com, venkataravis@micron.com, emirakhur@micron.com, ajayjoshi@micron.com, vtavarespetr@micron.com, damon@lists.linux.dev References: <20250612181330.31236-1-bijan311@gmail.com> <20250612181330.31236-2-bijan311@gmail.com> <5a50eeba-b26d-4913-8016-45278608a1ee@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 16.06.25 16:16, Bijan Tabatabai wrote: > On Mon, Jun 16, 2025 at 4:46 AM David Hildenbrand wrote: >> >> On 13.06.25 18:33, Bijan Tabatabai wrote: >>> On Fri, Jun 13, 2025 at 8:45 AM David Hildenbrand wrote: >>>> >>>> On 12.06.25 20:13, Bijan Tabatabai wrote: > [...] >> Hi, >> >>> >>> I did not use get_vma_policy or mpol_misplaced, which I believe is the >>> closest function that exists for what I want in this patch, because >>> those functions >> >> I think what you mean is, that you are performing an rmap walk. But >> there, you do have a VMA + MM available (stable). >> >>> seem to assume they are called inside of the task that the folio/vma >>> is mapped to. >> >> But, we do have a VMA at hand, so why would we want to ignore any set >> policy? (I think VMA policies so far only apply to shmem, but still). >> >> I really think you want to use get_vma_policy() instead of the task policy. > > Sorry, I think I misunderstood you before. You are right, we should > consider the VMA policy before using the task policy. I will do this > in the next revision. > >> >>> More specifically, mpol_misplaced assumes it is being called within a >>> page fault. >>> This doesn't work for us, because we call it inside of a kdamond process. >> >> Right. >> >> But it uses the vmf only for ... >> >> 1) Obtaining the VMA >> 2) Sanity-checking that the ptlock is held. >> >> Which, you also have during the rmap walk. > > There is another subtle dependency in get_vma_policy. > It first checks if a VMA policy exists, and if it doesn't, it uses the > task policy of the current task, which doesn't make sense when called > by a kdamond thread. Indeed, anything that depends on current task / nid / cpu might require care. -- Cheers, David / dhildenb