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Miller" , Andreas Larsson , Arnd Bergmann , Greg Kroah-Hartman , "Dan Williams" , Vishal Verma , Dave Jiang , Nicolas Pitre , "Muchun Song" , Oscar Salvador , "David Hildenbrand" , Konstantin Komarov , Baoquan He , Vivek Goyal , Dave Young , Tony Luck , Reinette Chatre , "Dave Martin" , James Morse , "Alexander Viro" , Christian Brauner , "Jan Kara" , "Liam R . Howlett" , "Vlastimil Babka" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , "Uladzislau Rezki" , Dmitry Vyukov , "Andrey Konovalov" , Jann Horn , Pedro Falcato , , , , , , , , , , , , , , Jason Gunthorpe Subject: Re: [PATCH v2 11/16] mm: update mem char driver to use mmap_prepare Date: Thu, 18 Sep 2025 12:11:05 -0700 Message-ID: <20250918191119.3622358-1-clm@meta.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain X-Originating-IP: [2620:10d:c085:108::150d] X-Authority-Analysis: v=2.4 cv=Wr8rMcfv c=1 sm=1 tr=0 ts=68cc596e cx=c_pps a=CB4LiSf2rd0gKozIdrpkBw==:117 a=CB4LiSf2rd0gKozIdrpkBw==:17 a=yJojWOMRYYMA:10 a=yPCof4ZbAAAA:8 a=QLIaiyCFZkuJlwHvl48A:9 X-Proofpoint-ORIG-GUID: yzU4eNHLhAUfSh4h33V6r2BXOldJckJZ X-Proofpoint-GUID: yzU4eNHLhAUfSh4h33V6r2BXOldJckJZ X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTE4MDE3MCBTYWx0ZWRfX+OH6/CZXfYF1 ZQZSd3MY5d1+huXM92tmUuaixGbvXh9eslNEM1bRcbWXduWBJkY9hM7pAwYK1UmVGB6+KtMLK3+ 5xu0OvXnTYTE5X3/j7r0YUT9jTEQIyg7vs2wzGv7et2oYHzPIQqFSONTUbrUfKZtQGE+5Ql2G/m FB+fFTcxSnJqueTYbtn/kFdO7ZXWcwZvUvPEHQAajQYNZXBbU5RIDDKbQMkKIDQGqkR8eZiO5xs q0uwGSCKiMwePOEm2hD3yphFf7LSfGFQnTUqyn0uufzwKaznKzrNA1c2BW91Ht07S88ASQbLxJd y3Z61QWwzOnbaToH1kdrhAuNayIEpInurTQIzwra/DJzb/lInqX9Bhz50sofEw= X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1117,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-09-18_02,2025-09-18_02,2025-03-28_01 X-Rspamd-Queue-Id: E2331180010 X-Stat-Signature: 8wnu1f4qgpswfs5fept9zt5rr8j11dpp X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1758222731-4001 X-HE-Meta: U2FsdGVkX18SAZ90JZzBrdJMYBoMpO5Oz8oV5rBoVCTPhhL2NJ/rrVgrm5VlsohCmn3q4PrcrbH9TL4/8Zp3XwpA7NPM6Z/B346KkijCE9k9YsrrlJi4wJGATGaZU/S+uS3GkVfHB2y3l1kA5xDtz1hEVY9YQ81SXl8asdPmyZqKxxXzNZ+VFl0QFZz5VapkKIyix3vSDu9K+DSI7X4ka0JOICCHntYwmHcKH4LbmG9JZI2OibJpA788fK1hEOar8LqZIgKCWdv3F9A15FY0tUW9cJlhJb/WAO4QYlsN1rdF7+CyP/QpGV+8T39i8z/www/w4y/IA6i1t4yYyLSEwCSpt4ZhPy0to1B1KmSpuUKAHKKR4qbGu6D4Vy7noCduvegeORhkY0vC76I0WugRjPGtwHPtezDsJApd0sq3R6AbgK5kaET+MtXk461c9/g8IUKN5OOpSD1OV3OMUhG2cZ0hsPyQUd4UVi+olXKuGYNRSI6xV+ZzXGInhwGbzA8D6rytixnQRlflV4wR/1NWy78OMWk4eEA0vpOxP6WBpLjLxzsctb3XD+oulPJcfizT2N+2r0BEbrm3f3bsPCuriXGs2ZlWnsq3oCFgOog1NJ8gIf1g+uR4gr7Yej+Il9j1OC0N6W9mCZ1XUp03Yfk7Fbem89agWGOMcomS1iVl5rk16Hl7jLUF2IrsobpV0EOhCGg/fbw21qGtIKZQtAZiax2gECw3nENlrep+i4t/8boRcq30huAhjXX6HZdS6mw3fAGIJvVVnRjdCqNbBMQCYI2IbSfea++KVpPqhOQuEvf2qekmyDACJ4RAL6Hr9B8KO3N+AxoqDYroaKmRx82+G8t9BuM0MBCLlnBc/vbmT+HCpS9CI8+bGHsqK6miTNQRXJn1oIz/pAqdP2+epKVaFKmYnLyW4tHzV6mrmP6ajVqIGAycdLOGgQJfXojDLk4CwpT1B8Jg1t4nrT7DFBq XsFrpSSY 2a9bfC52z6CP2H0n2TkpZgDq0FZ6OqkM8kl6/fam1d0BdXAtIb4iox3OtdvICqxsuYaBi4wsJmhweRbpmdr7jJU1xtLCWc+6e+dSRbJUIAnQkgDmy248bCysysgq6OoxB/NmMLJcr49QTLhgM8XzytqB1Cb/gVdrvDn1F4NdG63yhLpzBI+ozCLnUk9QbmpdqoIUjdEMa5ynfSSALR7+Nvi0MVO570eiK4gkXCmdgODG/ibcee1Y95o8h5Rw/It0P+yuX3WzNws//L/ng2PeGO0gfwfVakVhSFOrgFNYAaO5TAVPf0XO8E2tFXvWxOut3lsDdOZOG7PYUzDs4j1iVg5qbtODRIxTBeGyQLBM+Ecn648Yd4yuHT79glsH7WbVqoHIM3aR6gVHz1rJyGUPrBbEEevWbT6MyopOXFR1rP6fUt++BecsJJhFiF3hXFrkZoZ8zUmw24veSm73Xn2jTG5yg62a+3Wk62LRwvKIhasfGZdo+eVTvTmT/Z6Fe4h4a1Si1eTgmE7W+oX3uZYnskWtW9LqQ261kYAEsGPH8N6sz/bYc7rEaCdibiHYvWgnpAeLag7pHC7tYD5z7Bo2mHtrOwUxJmg1L7JUS 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 Wed, 10 Sep 2025 21:22:06 +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. > > Signed-off-by: Lorenzo Stoakes > --- > drivers/char/mem.c | 75 ++++++++++++++++++++++------------------ > include/linux/shmem_fs.h | 3 +- > mm/shmem.c | 40 ++++++++++++++++----- > 3 files changed, 76 insertions(+), 42 deletions(-) > [ ... ] > diff --git a/mm/shmem.c b/mm/shmem.c > index 990e33c6a776..cb6ff00eb4cb 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c [ ... ] > @@ -5920,6 +5925,25 @@ int shmem_zero_setup(struct vm_area_struct *vma) > return 0; > } > > +/** > + * shmem_zero_setup_desc - same as shmem_zero_setup, but determined by VMA > + * descriptor for convenience. > + * @desc: Describes VMA > + * Returns: 0 on success, or error > + */ > +int shmem_zero_setup_desc(struct vm_area_desc *desc) > +{ > + struct file *file = __shmem_zero_setup(desc->start, desc->end, desc->vm_flags); > + > + if (IS_ERR(file)) > + return PTR_ERR(file); > + > + desc->vm_file = file; > + desc->vm_ops = &shmem_anon_vm_ops; > + > + return 0; > +} > + Hi Lorenzo, shmem_zero_setup() does a if (vma->vm_file) fput(vma->vm_file) dance. It looks like we need one here too? -chris