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charset="UTF-8" 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: On Thu, Oct 8, 2020 at 1:13 AM Daniel Vetter wrote: > > On Thu, Oct 8, 2020 at 9:50 AM Dan Williams wrote: > > > > On Wed, Oct 7, 2020 at 4:25 PM Jason Gunthorpe wrote: > > > > > > On Wed, Oct 07, 2020 at 12:33:06PM -0700, Dan Williams wrote: > > > > On Wed, Oct 7, 2020 at 11:11 AM Daniel Vetter wrote: > > > > > > > > > > Since 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims > > > > > the region") /dev/kmem zaps ptes when the kernel requests exclusive > > > > > acccess to an iomem region. And with CONFIG_IO_STRICT_DEVMEM, this is > > > > > the default for all driver uses. > > > > > > > > > > Except there's two more ways to access pci bars: sysfs and proc mmap > > > > > support. Let's plug that hole. > > > > > > > > Ooh, yes, lets. > > > > > > > > > > > > > > For revoke_devmem() to work we need to link our vma into the same > > > > > address_space, with consistent vma->vm_pgoff. ->pgoff is already > > > > > adjusted, because that's how (io_)remap_pfn_range works, but for the > > > > > mapping we need to adjust vma->vm_file->f_mapping. Usually that's done > > > > > at ->open time, but that's a bit tricky here with all the entry points > > > > > and arch code. So instead create a fake file and adjust vma->vm_file. > > > > > > > > I don't think you want to share the devmem inode for this, this should > > > > be based off the sysfs inode which I believe there is already only one > > > > instance per resource. In contrast /dev/mem can have multiple inodes > > > > because anyone can just mknod a new character device file, the same > > > > problem does not exist for sysfs. > > > > > > The inode does not come from the filesystem char/mem.c creates a > > > singular anon inode in devmem_init_inode() > > > > That's not quite right, An inode does come from the filesystem I just > > arranged for that inode's i_mapping to be set to a common instance. > > > > > Seems OK to use this more widely, but it feels a bit weird to live in > > > char/memory.c. > > > > Sure, now that more users have arrived it should move somewhere common. > > > > > This is what got me thinking maybe this needs to be a bit bigger > > > generic infrastructure - eg enter this scheme from fops mmap and > > > everything else is in mm/user_iomem.c > > > > It still requires every file that can map physical memory to have its > > ->open fop do > > > > inode->i_mapping = devmem_inode->i_mapping; > > filp->f_mapping = inode->i_mapping; > > > > I don't see how you can centralize that part. > > btw, why are you setting inode->i_mapping? The inode is already > published, changing that looks risky. And I don't think it's needed, > vma_link() only looks at filp->f_mapping, and in our drm_open() we > only set that one. I think you're right it is unnecessary for devmem, but I don't think it's dangerous to do it from the very first open before anything is using the address space. It's copy-paste from what all the other "shared address space" implementers do. For example, block-devices in bd_acquire(). However, the rationale for block_devices to do it is so that page cache pages can be associated with the address space in the absence of an f_mapping. Without filesystem page writeback to coordinate I don't see any devmem code paths that would operate on the inode->i_mapping.