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Message-ID: <87frrh44mf.ffs@tglx>
Date: Wed, 07 Aug 2024 01:24:08 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Vlastimil Babka <vbabka@...e.cz>, Linus Torvalds
<torvalds@...ux-foundation.org>, Guenter Roeck <linux@...ck-us.net>
Cc: linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>, Helge
Deller <deller@....de>, linux-parisc@...r.kernel.org
Subject: Re: [PATCH 6.10 000/809] 6.10.3-rc3 review
Cc+: Helge, parisc ML
We're chasing a weird failure which has been tracked down to the
placement of the division library functions (I assume they are imported
from libgcc).
See the thread starting at:
https://lore.kernel.org/all/718b8afe-222f-4b3a-96d3-93af0e4ceff1@roeck-us.net
On Tue, Aug 06 2024 at 21:25, Vlastimil Babka wrote:
> On 8/6/24 19:33, Thomas Gleixner wrote:
>>
>> So this change adds 16 bytes to __softirq() which moves the division
>> functions up by 16 bytes. That's all it takes to make the stupid go
>> away....
>
> Heh I was actually wondering if the division is somhow messed up because
> maxobj = order_objects() and order_objects() does a division. Now I suspect
> it even more.
check_slab() calls into that muck, but I checked the disassembly of a
working and a broken kernel and the only difference there is the
displacement offset when the code calculates the call address, but
that's as expected a difference of 16 bytes.
Now it becomes interesting.
I added a unused function after __do_softirq() into the softirq text
section and filled it with ASM nonsense so that it occupies exactly one
page. That moves $$divoI, which is what check_slab() calls, exactly one
page forward:
-0000000041218c70 T $$divoI
+0000000041219c70 T $$divoI
Guess what happens? If falls on it's nose again.
Now with that ASM gunk I can steer the size conveniently. It works up
to:
0000000041219c50 T $$divoI
and fails for
0000000041219c60 T $$divoI
0000000041219c70 T $$divoI
and works again at
0000000041219c80 T $$divoI
So I added the following:
+extern void testme(void);
+extern unsigned int testsize;
+
+unsigned int testsize = 192;
+
+void __init testme(void)
+{
+ pr_info("TESTME: %lu\n", PAGE_SIZE / testsize);
+}
called that _before_ mm_core_init() from init/main.c and adjusted my ASM
hack to make $$divoI be at:
0000000041219c70 T $$divoI
again and surprisingly the output is:
[ 0.000000] softirq: TESTME: 21
Now I went back to the hppa64 gcc version 12.2.0 again and did the same
ASM gunk adjustment so that $$divoI ends up at the offset 0xc70 in the
page and the same happens.
So it's not a compiler dependent problem.
But then I added a testme() call to the error path and get:
[ 0.000000] softirq: TESTME: 21
[ 0.000000] =============================================================================
[ 0.000000] BUG kmem_cache_node (Not tainted): objects 21 > max 16 size 192 sorder 0
Now what's wrong?
Adding more debug:
[ 0.000000] BUG kmem_cache_node (Not tainted): objects 21 > max 16 size 192 sorder 0 21
where the last '21' is the output of the same call which made maxobj go
south:
static int check_slab(struct kmem_cache *s, struct slab *slab)
{
int maxobj;
@@ -1386,8 +1388,10 @@ static int check_slab(struct kmem_cache
maxobj = order_objects(slab_order(slab), s->size);
if (slab->objects > maxobj) {
- slab_err(s, slab, "objects %u > max %u",
- slab->objects, maxobj);
+ testme();
+ slab_err(s, slab, "objects %u > max %u size %u sorder %u %u",
+ slab->objects, maxobj, s->size, slab_order(slab),
+ order_objects(slab_order(slab), s->size));
return 0;
}
if (slab->inuse > slab->objects) {
I don't know and I don't want to know TBH...
Thanks,
tglx
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