Re: [PATCH 6.10 000/809] 6.10.3-rc3 review

[Guenter Roeck <linux@roeck-us.net>]

On 8/6/24 16:24, Thomas Gleixner wrote:
> 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:
> 

With the above added to my tree, I can also play around with the code.
Here is the next weird one:

diff --git a/mm/slub.c b/mm/slub.c
index 4927edec6a8c..b8a33966d858 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1385,6 +1385,9 @@ static int check_slab(struct kmem_cache *s, struct slab *slab)
         }

         maxobj = order_objects(slab_order(slab), s->size);
+
+       pr_info_once("##### slab->objects=%u maxobj=%u\n", slab->objects, maxobj);
+
         if (slab->objects > maxobj) {
                 slab_err(s, slab, "objects %u > max %u",
                         slab->objects, maxobj);

results in:

##### slab->objects=21 maxobj=21
=============================================================================
BUG kmem_cache_node (Not tainted): objects 21 > max 16

As Thomas noticed, this only happens if the divide assembler code is within a certain
address range.

Ok, now I am really lost.

Guenter