Re: [PATCH 1/6] kernfs: create vm_operations_struct without page_mkwrite()

[Christoph Hellwig]

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.