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

[Vlastimil Babka]

On 12/19/19 2:16 PM, Qian Cai wrote:
> 
> 
>> On Dec 19, 2019, at 8:06 AM, 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.
> 
> This was daily tested on linux-next here for those arches and never saw an issue.

Well I assume nobody has booted the kernel specifically with
debug_pagealloc=on. In case a randconfig run enabled
CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT then the problem wouldn't manifest
as the static key would be initialized with DEFINE_STATIC_KEY_TRUE by
the compiler itself, and the enabling-by-param would be a no-op.

> Are you able to reproduce it on mainline or linux-next?

I didn't try, but our kernel is 5.3-based so that's quite recent. The
offending commit was also introduced in 5.3-rc1 so there's that.