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

[Guenter Roeck]

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@xxxxxxxxxxxx
>
> 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