linux-kernel - Re: [PATCH v3 02/16] mm: introduce leaf entry type and use to simplify leaf entry logic

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Message-ID: <c113069a-8581-4e44-b7a1-7bfe9de623a5@suse.cz>
Date: Wed, 12 Nov 2025 16:36:22 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>,
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Subject: Re: [PATCH v3 02/16] mm: introduce leaf entry type and use to
 simplify leaf entry logic

On 11/10/25 23:21, 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
> 
> In the 'everything else' group we include swap entries, but we also include
> a number of other things such as migration entries, device private entries
> and marker entries.
> 
> Unfortunately this 'everything else' group expresses everything through
> a swp_entry_t type, and these entries are referred to swap entries even
> though they may well not contain a... swap entry.
> 
> This is compounded by the rather mind-boggling concept of a non-swap swap
> entry (checked via non_swap_entry()) and the means by which we twist and
> turn to satisfy this.
> 
> This patch lays the foundation for reducing this confusion.
> 
> We refer to 'everything else' as a 'software-define leaf entry' or
> 'softleaf'. for short And in fact we scoop up the 'none' entries into this
> concept also so we are left with:
> 
> - Present entries.
> - Softleaf entries (which may be empty).
> 
> This allows for radical simplification across the board - one can simply
> convert any leaf page table entry to a leaf entry via softleaf_from_pte().
> 
> If the entry is present, we return an empty leaf entry, so it is assumed
> the caller is aware that they must differentiate between the two categories
> of page table entries, checking for the former via pte_present().
> 
> As a result, we can eliminate a number of places where we would otherwise
> need to use predicates to see if we can proceed with leaf page table entry
> conversion and instead just go ahead and do it unconditionally.
> 
> We do so where we can, adjusting surrounding logic as necessary to
> integrate the new softleaf_t logic as far as seems reasonable at this
> stage.
> 
> We typedef swp_entry_t to softleaf_t for the time being until the
> conversion can be complete, meaning everything remains compatible
> regardless of which type is used. We will eventually remove swp_entry_t
> when the conversion is complete.
> 
> We introduce a new header file to keep things clear - leafops.h - this
> imports swapops.h so can direct replace swapops imports without issue, and
> we do so in all the files that require it.
> 
> Additionally, add new leafops.h file to core mm maintainers entry.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Seems like a reasonable first step of the conversion.

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>