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

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Message-ID: <aQukruJP6CyG7UNx@gourry-fedora-PF4VCD3F>
Date: Wed, 5 Nov 2025 14:25:34 -0500
From: Gregory Price <gourry@...rry.net>
To: Matthew Wilcox <willy@...radead.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 02/16] mm: introduce leaf entry type and use to simplify
 leaf entry logic

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