Re: [PATCH v19 38/40] mm: percpu: increase PERCPU_DYNAMIC_SIZE_SHIFT on DEPT and large PAGE_SIZE

On Tue, Jul 07, 2026 at 07:33:56AM +0000, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] The patch addresses a compile-time BUILD_BUG_ON failure that no longer exists, referencing a deleted structure (`struct kmem_cache_cpu`) and function (`alloc_kmem_cache_cpus()`). Both have been removed from the upstream kernel. The only remaining BUILD_BUG_ON checking `PERCPU_DYNAMIC_EARLY_SIZE` in SLUB is in `alloc_kmem_cache_stats()`, which evaluates `sizeof(struct kmem_cache_stats)`. Because `struct kmem_cache_stats` consists solely of an integer array, its size is entirely unaffected by `CONFIG_DEPT` or `local_lock_t`. Thus, this patch needlessly increases the dynamically reserved per-CPU memory based on an obsolete and misleading justification.
> --
> 
> commit 279fb8fb2919a407c18ab625ec67d2a3430b7b4d
> Author: Byungchul Park <[email protected]>
> 
> mm: percpu: increase PERCPU_DYNAMIC_SIZE_SHIFT on DEPT and large PAGE_SIZE
> 
> This commit increases PERCPU_DYNAMIC_SIZE_SHIFT to 11 when both CONFIG_DEPT
> and a PAGE_SIZE larger than 4KiB are enabled. It aims to prevent a build
> failure related to the size of struct kmem_cache_cpu.
> 
> >     Yunseong reported a build failure due to the BUILD_BUG_ON() statement in
> >     alloc_kmem_cache_cpus().  In the following test:
> >
> >       PERCPU_DYNAMIC_EARLY_SIZE < NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH * sizeof(struct kmem_cache_cpu)
> 
> [Severity: Medium]
> Are we referencing structures and functions that have been removed from the
> upstream kernel?
> 
> Both alloc_kmem_cache_cpus() and struct kmem_cache_cpu no longer exist
> upstream. The only remaining BUILD_BUG_ON() checking PERCPU_DYNAMIC_EARLY_SIZE
> in SLUB is in alloc_kmem_cache_stats():
> 
> mm/slub.c:alloc_kmem_cache_stats() {
>         BUILD_BUG_ON(PERCPU_DYNAMIC_EARLY_SIZE <
>                         NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH *
>                         sizeof(struct kmem_cache_stats));
> }
> 
> Since struct kmem_cache_stats consists solely of an integer array, its size
> is entirely unaffected by CONFIG_DEPT or local_lock_t. Does this mean the
> increase in dynamically reserved per-CPU memory is no longer needed?

Agree.  It seems no longer needed.  I will fix it.

	Byungchul

> > diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> > index 85bf8dd9f0874..dd74321d4bbd0 100644
> > --- a/include/linux/percpu.h
> > +++ b/include/linux/percpu.h
> > @@ -43,7 +43,11 @@
> >  # define PERCPU_DYNAMIC_SIZE_SHIFT      12
> >  #endif /* LOCKDEP and PAGE_SIZE > 4KiB */
> >  #else
> > +#if defined(CONFIG_DEPT) && !defined(CONFIG_PAGE_SIZE_4KB)
> > +#define PERCPU_DYNAMIC_SIZE_SHIFT      11
> > +#else
> >  #define PERCPU_DYNAMIC_SIZE_SHIFT      10
> > +#endif /* DEPT and PAGE_SIZE > 4KiB */
> >  #endif
> 
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=38