On Thu, 19 Dec 2019 14:06:12 +0100 Vlastimil Babka vbabka@suse.cz 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?