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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , Uladzislau Rezki , Dmitry Vyukov , Andrey Konovalov , Jann Horn , Pedro Falcato , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-csky@vger.kernel.org, linux-mips@vger.kernel.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-mm@kvack.org, ntfs3@lists.linux.dev, kexec@lists.infradead.org, kasan-dev@googlegroups.com, iommu@lists.linux.dev, Kevin Tian , Will Deacon , Robin Murphy Subject: Re: [PATCH v3 11/13] mm: update mem char driver to use mmap_prepare Message-ID: <20250916174006.GS1086830@nvidia.com> References: <03d4fce5fd356022b5af453fc5ab702c2c3a36d6.1758031792.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <03d4fce5fd356022b5af453fc5ab702c2c3a36d6.1758031792.git.lorenzo.stoakes@oracle.com> 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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 Tue, Sep 16, 2025 at 03:11:57PM +0100, Lorenzo Stoakes wrote: > Update the mem char driver (backing /dev/mem and /dev/zero) to use > f_op->mmap_prepare hook rather than the deprecated f_op->mmap. > > The /dev/zero implementation has a very unique and rather concerning > characteristic in that it converts MAP_PRIVATE mmap() mappings anonymous > when they are, in fact, not. > > The new f_op->mmap_prepare() can support this, but rather than introducing > a helper function to perform this hack (and risk introducing other users), > simply set desc->vm_op to NULL here and add a comment describing what's > going on. > > We also introduce shmem_zero_setup_desc() to allow for the shared mapping > case via an f_op->mmap_prepare() hook, and generalise the code between > this and shmem_zero_setup(). > > We also use the desc->action_error_hook to filter the remap error to > -EAGAIN to keep behaviour consistent. Hurm, in practice this converts reserve_pfn_range()/etc conflicts into from EINVAL into EAGAIN and converts all the unlikely OOM ENOMEM failures to EAGAIN. Seems wrong/unnecessary to me, I wouldn't have preserved it. But oh well Reviewed-by: Jason Gunthorpe > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h > index 0e47465ef0fd..5b368f9549d6 100644 > --- a/include/linux/shmem_fs.h > +++ b/include/linux/shmem_fs.h This little bit should probably be its own patch "Add shmem_zero_setup_desc()", and I wonder if the caller from vma.c can call the desc version now? Too bad the usage in ppc is such an adventure through sysfs :\ But the code looks fine Jason