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List-Unsubscribe: On Thu, Sep 18, 2025 at 12:11:05PM -0700, Chris Mason wrote: > 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? No we don't, it's intentionally designed to avoid this because mmap_prepare is done at a time prior to the file pointer having had been pinned like this. This is necessary in mmap() but not in mmap_prepare(), equally you can just assign VMA flags or any other field without any need for special helpers or lock/refcount dances etc. > > -chris Cheers, Lorenzo