mm: slab: Do not create kmalloc caches smaller than arch_slab_minalign()

Commit b035f5a ("mm: slab: reduce the kmalloc() minimum alignment
if DMA bouncing possible") allows architectures with non-coherent DMA to
define a small ARCH_KMALLOC_MINALIGN (e.g. sizeof(unsigned long long))
and this has been enabled on arm64. With KASAN_HW_TAGS enabled, however,
ARCH_SLAB_MINALIGN becomes 16 on arm64 (arch_slab_minalign() dynamically
selects it since commit d949a81 ("mm: make minimum slab alignment a
runtime property")). This can lead to a situation where kmalloc-8 caches
are attempted to be created with a kmem_caches.size aligned to 16. When
the cache is mergeable, it can lead to kernel warnings like:

sysfs: cannot create duplicate filename '/kernel/slab/:d-0000016'
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.6.0-rc1-00001-gda98843cd306-dirty #5
Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
Call trace:
 dump_backtrace+0x90/0xe8
 show_stack+0x18/0x24
 dump_stack_lvl+0x48/0x60
 dump_stack+0x18/0x24
 sysfs_warn_dup+0x64/0x80
 sysfs_create_dir_ns+0xe8/0x108
 kobject_add_internal+0x98/0x264
 kobject_init_and_add+0x8c/0xd8
 sysfs_slab_add+0x12c/0x248
 slab_sysfs_init+0x98/0x14c
 do_one_initcall+0x6c/0x1b0
 kernel_init_freeable+0x1c0/0x288
 kernel_init+0x24/0x1e0
 ret_from_fork+0x10/0x20
kobject: kobject_add_internal failed for :d-0000016 with -EEXIST, don't try to register things with the same name in the same directory.
SLUB: Unable to add boot slab dma-kmalloc-8 to sysfs

Limit the __kmalloc_minalign() return value (used to create the
kmalloc-* caches) to arch_slab_minalign() so that kmalloc-8 caches are
skipped when KASAN_HW_TAGS is enabled (both config and runtime).

Reported-by: Mark Rutland <[email protected]>
Fixes: b035f5a ("mm: slab: reduce the kmalloc() minimum alignment if DMA bouncing possible")
Signed-off-by: Catalin Marinas <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: [email protected] # 6.5.x
Signed-off-by: Vlastimil Babka <[email protected]>

mm/slab_common.c

@@ -895,10 +895,13 @@ void __init setup_kmalloc_cache_index_table(void)
 
 static unsigned int __kmalloc_minalign(void)
 {
+	unsigned int minalign = dma_get_cache_alignment();
+
 	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
 	    is_swiotlb_allocated())
-		return ARCH_KMALLOC_MINALIGN;
-	return dma_get_cache_alignment();
+		minalign = ARCH_KMALLOC_MINALIGN;
+
+	return max(minalign, arch_slab_minalign());
 }
 
 void __init