Re: [PATCH] x86/mm: Do not split_large_page() for set_kernel_text_rw()

[Song Liu <songliubraving@fb.com>]

> On Aug 26, 2019, at 8:08 AM, Song Liu <songliubraving@fb.com> wrote:
> 
> 
> 
>> On Aug 26, 2019, at 2:23 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>> 
>> So only the high mapping is ever executable; the identity map should not
>> be. Both should be RO.
>> 
>>> kprobe (with CONFIG_KPROBES_ON_FTRACE) should work on kernel identity
>>> mapping. 
>> 
>> Please provide more information; kprobes shouldn't be touching either
>> mapping. That is, afaict kprobes uses text_poke() which uses a temporary
>> mapping (in 'userspace' even) to alias the high text mapping.
> 
> kprobe without CONFIG_KPROBES_ON_FTRACE uses text_poke(). But kprobe with
> CONFIG_KPROBES_ON_FTRACE uses another path. The split happens with
> set_kernel_text_rw() -> ... -> __change_page_attr() -> split_large_page().
> The split is introduced by commit 585948f4f695. do_split in 
> __change_page_attr() becomes true after commit 585948f4f695. This patch 
> tries to fix/workaround this part. 
> 
>> 
>> I'm also not sure how it would then result in any 4k text maps. Yes the
>> alias is 4k, but it should not affect the actual high text map in any
>> way.
> 
> I am confused by the alias logic. set_kernel_text_rw() makes the high map
> rw, and split the PMD in the high map. 
> 
>> 
>> kprobes also allocates executable slots, but it does that in the module
>> range (afaict), so that, again, should not affect the high text mapping.
>> 
>>> We found with 5.2 kernel (no CONFIG_PAGE_TABLE_ISOLATION, w/ 
>>> CONFIG_KPROBES_ON_FTRACE), a single kprobe will split _all_ PMDs in 
>>> kernel text mapping into pte-mapped pages. This increases iTLB 
>>> miss rate from about 300 per million instructions to about 700 per
>>> million instructions (for the application I test with). 
>>> 
>>> Per bisect, we found this behavior happens after commit 585948f4f695 
>>> ("x86/mm/cpa: Avoid the 4k pages check completely"). That's why I 
>>> proposed this PATCH to fix/workaround this issue. However, per
>>> Peter's comment and my study of the code, this doesn't seem the 
>>> real problem or the only here. 
>>> 
>>> I also tested that the PMD split issue doesn't happen w/o 
>>> CONFIG_KPROBES_ON_FTRACE. 
>> 
>> Right, because then ftrace doesn't flip the whole kernel map writable;
>> which it _really_ should stop doing anyway.
>> 
>> But I'm still wondering what causes that first 4k split...
> 
> Please see above. 

Another data point: we can repro the issue on Linus's master with just
ftrace:

# start with PMD mapped
root@virt-test:~# grep ffff81000000- /sys/kernel/debug/page_tables/kernel
0xffffffff81000000-0xffffffff81c00000          12M     ro         PSE         x  pmd

# enable single ftrace
root@virt-test:~# echo consume_skb > /sys/kernel/debug/tracing/set_ftrace_filter
root@virt-test:~# echo function > /sys/kernel/debug/tracing/current_tracer

# now the text is PTE mapped
root@virt-test:~# grep ffff81000000- /sys/kernel/debug/page_tables/kernel
0xffffffff81000000-0xffffffff81c00000          12M     ro                     x  pte

Song