Re: [PATCH] mm, debug_pagealloc: don't rely on static keys too early

[Andrew Morton]

On Thu, 19 Dec 2019 14:06:12 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:

> Commit 96a2b03f281d ("mm, debug_pagelloc: use static keys to enable debugging")
> has introduced a static key to reduce overhead when debug_pagealloc is compiled
> in but not enabled. It relied on the assumption that jump_label_init() is
> called before parse_early_param() as in start_kernel(), so when the
> "debug_pagealloc=on" option is parsed, it is safe to enable the static key.
> 
> However, it turns out multiple architectures call parse_early_param() earlier
> from their setup_arch(). x86 also calls jump_label_init() even earlier, so no
> issue was found while testing the commit, but same is not true for e.g. ppc64
> and s390 where the kernel would not boot with debug_pagealloc=on as found by
> our QA.
> 
> To fix this without tricky changes to init code of multiple architectures, this
> patch partially reverts the static key conversion from 96a2b03f281d. Init-time
> and non-fastpath calls (such as in arch code) of debug_pagealloc_enabled() will
> again test a simple bool variable. Fastpath mm code is converted to a new
> debug_pagealloc_enabled_static() variant that relies on the static key, which
> is enabled in a well-defined point in mm_init() where it's guaranteed that
> jump_label_init() has been called, regardless of architecture.

I'm seeing this with x86_64 allmodconfig:

ERROR: "_debug_pagealloc_enabled_early" [sound/drivers/pcsp/snd-pcsp.ko] undefined!

Not sure why.  It's there:

q:/usr/src/25> nm mm/page_alloc.o|grep _debug_pagealloc_enabled
...
00000000000028a0 B _debug_pagealloc_enabled
...

and exported:

0000000000000072 r __kstrtab__debug_pagealloc_enabled

Odd.  Does this happen to you as well?