Re: [linux-next:master 5002/7443] include/linux/compiler_types.h:357:45: error: call to '__compiletime_assert_474' declared with attribute error: BUILD_BUG_ON failed: PERCPU_DYNAMIC_EARLY_SIZE < NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH * sizeof(struct kmem_cache_cpu)

[Dennis Zhou <dennis@kernel.org>]

Hi Vlastimil & Baoquan,

On Mon, Nov 14, 2022 at 06:58:13PM +0100, Vlastimil Babka wrote:
> On 11/14/22 08:44, Baoquan He wrote:
> > Hi,
> > 
> > I reproduced the build failure according to lkp report and made a patch
> > as below to fix it.
> > 
> > From dae7dd9705015ce36db757e88c78802584f949b1 Mon Sep 17 00:00:00 2001
> > From: Baoquan He <bhe@redhat.com>
> > Date: Sun, 13 Nov 2022 18:08:27 +0800
> > Subject: [PATCH] percpu: adjust the value of PERCPU_DYNAMIC_EARLY_SIZE
> > Content-type: text/plain
> > 
> > LKP reported a build failure as below on the patch "mm/slub, percpu:
> > correct the calculation of early percpu allocation size"
> 
> Since I have that patch in slab.git exposed to -next, should I take this fix
> too, to make things simpler? Dennis?
> 

I don't have any problems with you running a fix, but I'm not quite sure
this is the right fix. Though this might cause a trivial merge conflict
with: d667c94962c1 ("mm/percpu: remove unused PERCPU_DYNAMIC_EARLY_SLOTS")
in my percpu#for-6.2 branch.

If I'm understanding this correctly, slub requires additional percpu
memory due to the use of 64k pages. By increasing
PERCPU_DYNAMIC_EARLY_SIZE, we solve the problem for 64k page users, but
require a few unnecessary pages that can bloat the size of subsequent
percpu chunks. Though, I'm not sure if that's an issue today for
embedded devices.

I think adding parity to PERCPU_DYNAMIC_EARLY_SIZE with
PERCPU_DYNAMIC_RESERVE is defined by BITS_PER_LONG is a safer option
here. A small TODO item would be to make PERCPU_DYNAMIC_RESERVE be a +
value instead of a max() with PERCPU_DYNAMIC_EARLY_SIZE.

Thanks,
Dennis

---
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index f1ec5ad1351c..22ce3271eed2 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -42,7 +42,11 @@
  * larger than PERCPU_DYNAMIC_EARLY_SIZE.
  */
 #define PERCPU_DYNAMIC_EARLY_SLOTS	128
+#if BITS_PER_LONG > 32
+#define PERCPU_DYNAMIC_EARLY_SIZE	(20 << 10)
+#else
 #define PERCPU_DYNAMIC_EARLY_SIZE	(12 << 10)
+#endif
 
 /*
  * PERCPU_DYNAMIC_RESERVE indicates the amount of free area to piggy