Re: [PATCH v41 10/24] mm: Add 'mprotect' hook to struct vm_operations_struct

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Nov 16, 2020 at 10:09:57AM +0000, Mel Gorman wrote:
> On Sun, Nov 15, 2020 at 10:36:51AM -0800, Dave Hansen wrote:
> > On 11/15/20 9:32 AM, Matthew Wilcox wrote:
> > > On Fri, Nov 13, 2020 at 12:01:21AM +0200, Jarkko Sakkinen wrote:
> > >> +++ b/include/linux/mm.h
> > >> @@ -559,6 +559,13 @@ struct vm_operations_struct {
> > >>  	void (*close)(struct vm_area_struct * area);
> > >>  	int (*split)(struct vm_area_struct * area, unsigned long addr);
> > >>  	int (*mremap)(struct vm_area_struct * area);
> > >> +	/*
> > >> +	 * Called by mprotect() to make driver-specific permission
> > >> +	 * checks before mprotect() is finalised.   The VMA must not
> > >> +	 * be modified.  Returns 0 if eprotect() can proceed.
> > >> +	 */

Wonder if this should also document the negative case for the return
value, i.e. -EACCES is returned otherwise.

> > > 
> > > This is the wrong place for this documentation, and it's absurdly
> > > specific to your implementation.  It should be in
> > > Documentation/filesystems/locking.rst.
> > 
> > I'll let you and Mel duke that one out:
> > 
> 
> I suggested placing the comment there to make it clear what the expected
> semantics of the hook was to reduce the chances of abuse or surprises. The
> hook does not affect locking so Documentation/filesystems/locking.rst
> didn't appear appropriate other than maybe adding a note there
> that it doesn't affect locks. The hook also is not expecting any
> filesystems-specific action that I aware of but a note could be added to
> the effect that filesystems should not need to take special action for it.
> Protections on the filesystem level are for the inode, I can't imagine what
> a filesystem would do with a protection change on the page table level
> but maybe I'm not particularly imaginative today.

I try to decipher this in generic context.

In a permission check of a filesystem, truncated pages should be
encapsulated in to the permission decision. It's a just a query.

So maybe I'll add something like:

"This callback does only a permission query, and thus does never
return locked pages."

> -- 
> Mel Gorman
> SUSE Labs

/Jarkko