Message-ID: <bc94d739-d66f-4cd6-a3f2-68938cfd08c1@gmail.com>
Date: Wed, 5 Nov 2025 22:29:14 +0100
From: "David Hildenbrand (Red Hat)" <davidhildenbrandkernel@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Gregory Price <gourry@...rry.net>, Matthew Wilcox <willy@...radead.org>,
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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 22:24, Lorenzo Stoakes wrote:
> On Wed, Nov 05, 2025 at 10:15:51PM +0100, David Hildenbrand (Red Hat) wrote:
>> On 05.11.25 22:08, Lorenzo Stoakes wrote:
>>> On Wed, Nov 05, 2025 at 09:11:45PM +0100, David Hildenbrand (Red Hat) wrote:
>>>> On 05.11.25 21:05, Lorenzo Stoakes wrote:
>>>>> On Wed, Nov 05, 2025 at 03:01:00PM -0500, Gregory Price wrote:
>>>>>> On Wed, Nov 05, 2025 at 07:52:36PM +0000, 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:
>>>>>>> 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.
>>>>>>>
>>>>>>
>>>>>> Reasonable - names are hard, but just about anything will be better than swp_entry.
>>>>>>
>>>>>> SWE / sw_entry seems perfectly reasonable.
>>>>>
>>>>> I'm not a lover of 'sw' in there it's just... eye-stabby. Is that a word?
>>>>>
>>>>> I am quite fond of my suggested soft_leaf_t, softleaf_xxx()
>>>>
>>>> We do have soft_dirty.
>>>>
>>>> It will get interesting with pte_swp_soft_dirty() :)
>>>
>>> Hmm but that's literally a swap entry, and is used against an actual PTE entry
>>> not an abstracted s/w leaf entry so I doubt that'd require renaming on any
>>> level.
>>
>> It's used on migration entries as well. Just like pte_swp_uffd_wp().
>>
>> So, it's ... tricky :)
>>
>> But maybe I am missing your point (my brain is exhausted from uffd code)
> 
> We'd either not rename it or rename it to something like pte_is_uffd_wp(). So
> it's not even so relevant.

We do have pte_uffd_wp() for present ptes.

> 
> We'd probably call that something like pte_is_soft_dirty() in the soft dirty
> case. The 'swp' part of that is pretty redundant.

We do have pte_soft_dirty() for present ptes.

So we'd need some indication that these are for softleaf entries (where 
the bit positions will differ).

> 
> If people were insistent on having softleaf in there we could call it
> pte_softleaf_is_soft_dirty() which isn't qutie so bad. But I'd not want to put
> softleaf in there anyway.
> 
> sw_entry or sw_leaf or sw_leaf_entry would all have the same weirdness.
> 
> I want it to be something that is readable + not hideous to look at but still
> clear as to what it's referring too. Softleaf covers all of that off... :)

I think you misunderstood me: I have nothing against softleaf, I was 
rather saying that we already use the "soft" terminology elsewhere 
(soft_dirt), so it's not too crazy.

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