linux-kernel - Re: [PATCH v3 03/16] mm: avoid unnecessary uses of is_swap

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Message-ID: <dfcf8c59-e836-4e35-8585-656b64149ad7@suse.cz>
Date: Wed, 12 Nov 2025 19:48:58 +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 03/16] mm: avoid unnecessary uses of is_swap_pte()

On 11/10/25 23:21, Lorenzo Stoakes wrote:
> There's an established convention in the kernel that we treat PTEs as
> containing swap entries (and the unfortunately named non-swap swap entries)
> should they be neither empty (i.e. pte_none() evaluating true) nor present
> (i.e. pte_present() evaluating true).
> 
> However, there is some inconsistency in how this is applied, as we also
> have the is_swap_pte() helper which explicitly performs this check:
> 
> 	/* check whether a pte points to a swap entry */
> 	static inline int is_swap_pte(pte_t pte)
> 	{
> 		return !pte_none(pte) && !pte_present(pte);
> 	}
> 
> As this represents a predicate, and it's logical to assume that in order to
> establish that a PTE entry can correctly be manipulated as a swap/non-swap
> entry, this predicate seems as if it must first be checked.
> 
> But we instead, we far more often utilise the established convention of
> checking pte_none() / pte_present() before operating on entries as if they
> were swap/non-swap.
> 
> This patch works towards correcting this inconsistency by removing all uses
> of is_swap_pte() where we are already in a position where we perform
> pte_none()/pte_present() checks anyway or otherwise it is clearly logical
> to do so.
> 
> We also take advantage of the fact that pte_swp_uffd_wp() is only set on
> swap entries.
> 
> Additionally, update comments referencing to is_swap_pte() and
> non_swap_entry().
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

LGTM (famous last words)

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