Re: [PATCH 02/16] mm: introduce leaf entry type and use to simplify leaf entry logic

[David Hildenbrand]

On 05.11.25 20:52, Lorenzo Stoakes wrote:
> On Wed, Nov 05, 2025 at 02:25:34PM -0500, Gregory Price wrote:
> > On Wed, Nov 05, 2025 at 07:06:11PM +0000, Matthew Wilcox wrote:
> > > On Mon, Nov 03, 2025 at 12:31:43PM +0000, Lorenzo Stoakes wrote:
> > > > The kernel maintains leaf page table entries which contain either:
> > > >
> > > > - Nothing ('none' entries)
> > > > - Present entries (that is stuff the hardware can navigate without fault)
> > > > - Everything else that will cause a fault which the kernel handles
> > >
> > > The problem is that we're already using 'pmd leaf entries' to mean "this
> > > is a pointer to a PMD entry rather than a table of PTEs".
> >
> > Having not looked at the implications of this for leafent_t prototypes
> > ...
> > Can't this be solved by just adding a leafent type "Pointer" which
> > implies there's exactly one leaf-ent type which won't cause faults?
> >
> > is_present() => (table_ptr || leafent_ptr)
> > else():      => !leafent_ptr
> >
> > if is_none()
> > 	do the none-thing
> > if is_present()
> > 	if is_leafent(ent)  (== is_leafent_ptr)
> > 		do the pointer thing
> > 	else
> > 		do the table thing
> > else()
> > 	type = leafent_type(ent)
> > 	switch(type)
> > 		do the software things
> > 		can't be a present entry (see above)
> >
> >
> > A leaf is a leaf :shrug:
> >
> > ~Gregory
>
> I thought about doing this but it doesn't really work as the type is
> _abstracted_ from the architecture-specific value, _and_ we use what is
> currently the swp_type field to identify what this is.
>
> So we would lose the architecture-specific information that any 'hardware leaf'
> entry would require and not be able to reliably identify it without losing bits.
>
> Trying to preserve the value _and_ correctly identify it as a present entry
> would be difficult.
>
> And I _really_ didn't want to go on a deep dive through all the architectures to
> see if we could encode it differently to allow for this.
>
> Rather I think it's better to differentiate between s/w + h/w leaf entries.

(Being rather silent because I'm busy with all kinds of other stuff)

I agree :)

As Willy said, something that spells out "sw leaf" would be nice.

--
Cheers

David