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[2003:d8:2f27:ec00:4f4d:d38:ba97:9aa2]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a526057278sm2657445f8f.63.2025.06.05.05.21.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Jun 2025 05:21:52 -0700 (PDT) Message-ID: <897590f7-99a5-4053-8566-76623b929c7c@redhat.com> Date: Thu, 5 Jun 2025 14:21:50 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls To: Jason Gunthorpe , Dan Williams Cc: Alistair Popple , linux-mm@kvack.org, gerald.schaefer@linux.ibm.com, willy@infradead.org, linux-kernel@vger.kernel.org, nvdimm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, zhang.lyra@gmail.com, debug@rivosinc.com, bjorn@kernel.org, balbirs@nvidia.com, lorenzo.stoakes@oracle.com, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-cxl@vger.kernel.org, dri-devel@lists.freedesktop.org, John@groves.net References: <2ee5a64581d2c78445e5c4180d7eceed085825ca.1748500293.git-series.apopple@nvidia.com> <6841026c50e57_249110022@dwillia2-xfh.jf.intel.com.notmuch> <20250605120909.GA44681@ziepe.ca> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 05.06.25 14:09, Jason Gunthorpe wrote: > On Wed, Jun 04, 2025 at 07:35:24PM -0700, Dan Williams wrote: > >> If all dax pages are special, then vm_normal_page() should never find >> them and gup should fail. >> >> ...oh, but vm_normal_page_p[mu]d() is not used in the gup path, and >> 'special' is not set in the pte path. > > That seems really suboptimal?? Why would pmd and pte be different? > >> I think for any p[mu]d where p[mu]d_page() is ok to use should never set >> 'special', right? > > There should be dedicated functions for installing pages and PFNs, > only the PFN one would set the special bit. > > And certainly your tests *should* be failing as special entries should > never ever be converted to struct page. Worth reviewing [1] where I clean that up and describe the current impact. ;) What's even worse about this pte_devmap()/pmd_devmap()/... shit (sorry! but it's absolute shit) is that some pte_mkdev() set the pte special, while others ... don't. E.g., loongarch static inline pte_t pte_mkdevmap(pte_t pte) { pte_val(pte) |= _PAGE_DEVMAP; return pte; } I don't even know how it can (could) survive vm_normal_page(). Of course, a wild (and different) mixture on pmd_mkdevmap() as well. So happy to see that go away. [1] https://lkml.kernel.org/r/20250603211634.2925015-1-david@redhat.com -- Cheers, David / dhildenb