On 2024-06-06 23:03, Christoph Hellwig wrote: > On Thu, Jun 06, 2024 at 10:54:06PM +0200, Greg Kroah-Hartman wrote: >> On Wed, Jun 05, 2024 at 01:29:29PM -0600, Martin Oliveira wrote: >>> The standard kernfs vm_ops installs a page_mkwrite() operator which >>> modifies the file update time on write. >>> >>> This not always required (or makes sense), such as in the P2PDMA, which >>> uses the sysfs file as an allocator from userspace. >> >> That's not a good idea, please don't do that. sysfs binary files are >> "pass through", why would you want to use this as an allocator? > > I think the real question is why sysfs binary files implement > page_mkwrite by default. page_mkwrite is needed for file systems that > need to allocate space from a free space pool, which seems odd for > sysfs. The default page_mkwrite in kernfs just calls file_update_time() but, as I understand it, the fault code should call file_update_time() if page_mkwrite isn't set. So perhaps the easiest thing is to simply not add a page_mkwrite unless the vm_ops adds one. It's not the easiest thing to trace, but as best as I can tell there are no kernfs binary attributes that use page_mkwrite. So alternatively, perhaps we could just disallow page_mkwrite in kernfs entirely? Logan
