Re: [RFC PATCH 1/4] mm: pagewalk: add the ability to install PTEs

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Oct 14, 2024 at 11:47:58PM -0700, Christoph Hellwig wrote:
> Hi Lorenzo,
>
> sorry for only replying to this so late.

No worries, and thanks for taking a look! :)

>
> On Fri, Sep 27, 2024 at 01:51:11PM +0100, Lorenzo Stoakes wrote:
> > The existing generic pagewalk logic permits the walking of page tables,
> > invoking callbacks at individual page table levels via user-provided
> > mm_walk_ops callbacks.
> >
> > This is useful for traversing existing page table entries, but precludes
> > the ability to establish new ones.
> >
> > Existing mechanism for performing a walk which also installs page table
> > entries if necessary are heavily duplicated throughout the kernel, each
> > with semantic differences from one another and largely unavailable for use
> > elsewhere.
>
> I do like the idea of having common code for installing page tables!
>

Awesome.

> Minor nits below:
>
> > +int walk_page_range_mm(struct mm_struct *mm, unsigned long start,
> >  		unsigned long end, const struct mm_walk_ops *ops,
> >  		void *private)
>
> It would be good to have a minimum level of documentation for this
> function, including how it differs from walk_page_range and why
> it should remain internal.

Will add on respin!

>
> > +	/* For internal use only. */
> > +	if (ops->install_pte)
> > +		return -EINVAL;
>
> And this should probably be expanded a bit, including that no exported
> symbol should allow inserting arbitrary PTEs.  Maybe best done with
> a helper to share that comment with the other places that have this
> check.
>

Yeah a helper makes sense actually, a more general 'are these ops valid?'
thing. Will update on next respin with some explanation.

The next iteration I plan to un-RFC as seems generally the concept is
unopposed for this series, will make these changes then.

Thanks!