linux-kernel - Re: [RFC/PATCH] Use mmu_gather for fork() instead of flush_tlb

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Message-ID: <46920B7D.5090100@yahoo.com.au>
Date:	Mon, 09 Jul 2007 20:18:37 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	unlisted-recipients:; (no To-header on input)
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-mm@...ck.org,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/PATCH] Use mmu_gather for fork() instead of flush_tlb_mm()

Nick Piggin wrote:
> Benjamin Herrenschmidt wrote:
> 
>> On Mon, 2007-07-09 at 19:29 +1000, Nick Piggin wrote:
>>
>>> They could just #define one to the other though, there are only a
>>> small
>>> number of them. Is there a downside to not making them distinct? i386
>>> for example probably would just keep doing a tlb flush for fork and
>>> not
>>> want to worry about touching the tlb gather stuff.
>>
>>
>>
>> But the tlb gather stuff just does ... a flush_tlb_mm() on x86 :-)
> 
> 
> But it still does the get_cpu of the mmu gather data structure and

To elaborate on this one... I realise for this one that in the kernel
where this is currently used everything is non-preemptible anyway
because of the ptl. And I also realise that -rt kernel issues don't
really have a bearing on mainline kernel.. but the generic
implementation of this API is fundamentally used to operate on a
per-cpu data structure that is only required when tearing down page
tables. That makes this necessarily non-preemptible.

Which shows that it adds more restrictions that may not otherwise be
required.

> has to look in there and touch the cacheline. You're also having to
> do more work when unlocking/relocking the ptl etc.
> 
> 
>> I really think it's the right API

OK, the *form* of the API is fine, I have no arguments. I just don't
know why you have to reuse the same thing. If you provided a new set of
names then you can trivially do a generic implementation which compiles
to exactly the same code for all architectures right now. That seems to
me like the right way to go...

-- 
SUSE Labs, Novell Inc.

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