Re: [PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc

["John Thomson" <lists@johnthomson.fastmail.com.au>]

On Tue, 1 Nov 2022, at 07:57, Feng Tang wrote:
> Hi Thomson,
>
> Thanks for testing!
>
> + mips maintainer and mail list. The original report is here
>
> https://lore.kernel.org/lkml/becf2ac3-2a90-4f3a-96d9-a70f67c66e4a@app.fastmail.com/



I am guessing my issue comes from __kmem_cache_alloc_lru accessing s->object_size when (kmem_cache) s is NULL?
If that is the case, this change is not to blame, it only exposes the issue?

I get the following dmesg (note very early NULL kmem_cache) with the below change atop v6.1-rc3:

transfer started ......................................... transfer ok, time=2.02s
setting up elf image... OK
jumping to kernel code
zimage at:     80B842A0 810B4EFC

Uncompressing Linux at load address 80001000

Copy device tree to address  80B80EE0

Now, booting the kernel...

[    0.000000] Linux version 6.1.0-rc3+ (john@john) (mipsel-buildroot-linux-gnu-gcc.br_real (Buildroot 2021.11-4428-g6b6741b) 12.2.0, GNU ld (GNU Binutils) 2.39) #61 SMP Tue Nov  1 18:04:13 AEST 2022
[    0.000000] slub: kmem_cache_alloc called with kmem_cache: 0x0
[    0.000000] slub: __kmem_cache_alloc_lru called with kmem_cache: 0x0
[    0.000000] SoC Type: MediaTek MT7621 ver:1 eco:3
[    0.000000] printk: bootconsole [early0] enabled
[    0.000000] CPU0 revision is: 0001992f (MIPS 1004Kc)
[    0.000000] MIPS: machine is MikroTik RouterBOARD 760iGS

normal boot


diff --git a/mm/slub.c b/mm/slub.c
index 157527d7101b..10fcdf2520d2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3410,7 +3410,13 @@ static __always_inline
 void *__kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
 			     gfp_t gfpflags)
 {
-	void *ret = slab_alloc(s, lru, gfpflags, _RET_IP_, s->object_size);
+	void *ret;
+	if (IS_ERR_OR_NULL(s)) {
+		pr_warn("slub: __kmem_cache_alloc_lru called with kmem_cache: %pSR\n", s);
+		ret = slab_alloc(s, lru, gfpflags, _RET_IP_, 0);
+	} else {
+		ret = slab_alloc(s, lru, gfpflags, _RET_IP_, s->object_size);
+	}
 
 	trace_kmem_cache_alloc(_RET_IP_, ret, s, gfpflags, NUMA_NO_NODE);
 
@@ -3419,6 +3425,8 @@ void *__kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
 
 void *kmem_cache_alloc(struct kmem_cache *s, gfp_t gfpflags)
 {
+	if (IS_ERR_OR_NULL(s))
+		pr_warn("slub: kmem_cache_alloc called with kmem_cache: %pSR\n", s);
 	return __kmem_cache_alloc_lru(s, NULL, gfpflags);
 }
 EXPORT_SYMBOL(kmem_cache_alloc);
@@ -3426,6 +3434,8 @@ EXPORT_SYMBOL(kmem_cache_alloc);
 void *kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
 			   gfp_t gfpflags)
 {
+	if (IS_ERR_OR_NULL(s))
+		pr_warn("slub: __kmem_cache_alloc_lru called with kmem_cache: %pSR\n", s);
 	return __kmem_cache_alloc_lru(s, lru, gfpflags);
 }
 EXPORT_SYMBOL(kmem_cache_alloc_lru);


Any hints on where kmem_cache_alloc would be being called from this early?
I will start looking from /init/main.c around pr_notice("%s", linux_banner);

Thank you for your help.

Let me know if you want me to stop replying to this mm/slub debug memory wasting email,
and take this to a new email?

Cheers,
-- 
  John Thomson