linux-kernel - Re: [PATCH] mm: fix up a spurious page fault whenever it happens

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Date:	Thu, 23 May 2013 08:19:38 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Stanislav Meduna <stano@...una.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Hai Huang <hhuang@...hat.com>
Subject: Re: [PATCH] mm: fix up a spurious page fault whenever it happens

On 05/23/2013 04:07 AM, Stanislav Meduna wrote:
> On 22.05.2013 20:43, Rik van Riel wrote:
>
>>> Some CPUs have had errata when it comes to flushing large pages that
>>> have been split into small pages by hardware, e.g. due to MTRR
>>> conflicts.  In that case, fragments of the large page may have been left
>>> in the TLB.
>
> Can I somehow find if this is the case? The memory mapping
> for the failing process has two regions slightly larger than
> 4 MB - code and heap.
>
> The process also does not access any funny memory regions
> from userspace - it is basically networking (both TCP/IP
> and raw sockets) and crunching of the data received.
> No mmapped devices or something like that.
>
>> static inline void __native_flush_tlb_single(unsigned long addr)
>> {
>>          __flush_tlb();
>> }
>>
>> This on top of the other two patches.
>
> It did not crash overnight, but it also does not show any
> minor fault counted for the threads, so I'm afraid the situation
> just did not happen - there should be at least one visible in
> the ps -o min_flt output, right?

If all the page faults are done by he main thread,
and the TLB gets properly flushed now, the other
threads might not see minor faults.

> I will give it some more testing time.

That is a good idea.

Now to figure out how we properly fix this
issue in the kernel...

We can add a bit in the architecture bits that
we use to check against other CPU and system
errata, and conditionally flush the whole TLB
from __native_flush_tlb_single().

The question is, how do we identify what CPUs
need the extra flushing?

And in what circumstances do they require it?
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