Re: [PATCH v4 18/18] mm: factor out can_spin_trylock()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The refactoring to deduplicate spin_trylock safety checks into can_spin_trylock() missed __free_frozen_pages(), leaving a pre-existing UP NMI corruption bug unpatched.
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Note: The format of this report has been altered to provide a free-form
summary of the findings due to recitation restrictions.

commit a99a7a468132ab177e1233c7e2f5644523ace0a7
Author: Brendan Jackman <jackmanb@google.com>
Subject: mm: factor out can_spin_trylock()

This commit introduces can_spin_trylock() to deduplicate checks for whether 
the current context is safe for spin_trylock(). It updates 
alloc_nolock_allowed() in mm/page_alloc.c and __kmalloc_nolock_noprof() in 
mm/slub.c to use this newly factored helper.

Link: https://lore.kernel.org/all/397859cb-b127-4cc6-9c71-044afc99bf0c@kernel.org/

[Severity: High]

This isn't a bug introduced by this patch, but the deduplication refactoring 
appears to have missed an open-coded check in __free_frozen_pages() in 
mm/page_alloc.c. 

Could this open-coded check also be updated to use the new 
can_spin_trylock() helper to resolve a pre-existing UP NMI safety issue?

When __free_frozen_pages() handles FPI_TRYLOCK, it explicitly checks for
CONFIG_PREEMPT_RT and NMI/hardirq context. On UP systems where CONFIG_SMP 
is disabled, spin_trylock() and its variants are defined to unconditionally 
succeed because there is no actual lock state tracked.

If a caller invokes free_pages_nolock() from an NMI context on a UP system, 
the current RT-specific check fails. The execution then falls back to 
free_one_page(), where spin_trylock_irqsave() falsely succeeds even if the 
NMI interrupted another task that was actively modifying the free list under 
the zone lock. 

This allows concurrent modification of the free list, which can lead to 
corruption and kernel panics.

Updating the check in __free_frozen_pages() to use !can_spin_trylock() 
would catch the UP NMI case and safely route the allocation to the lockless 
add_page_to_zone_llist() path instead.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260702-alloc-trylock-v4-0-0af8ff387e80@google.com?part=18