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

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Message-ID: <c8f4e753-836d-4ca4-8a94-c54738b7db45@redhat.com>
Date: Wed, 5 Nov 2025 20:56:34 +0100
From: David Hildenbrand <dhildenb@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Gregory Price <gourry@...rry.net>
Cc: Matthew Wilcox <willy@...radead.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
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Subject: Re: [PATCH 02/16] mm: introduce leaf entry type and use to simplify
 leaf entry logic

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