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, sashiko-bot@kernel.org 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 [off-list ref]

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.

quoted

    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

quoted

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/20260706061928.66713-1-byungchul@sk.com?part=38