linux-kernel - Re: [BUGFIX][PATCH] DO flush icache before set

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Date:	Fri, 6 Jul 2007 07:18:53 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mike Stroyan <mike@...oyan.net>
Cc:	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	tony.luck@...el.com, linux-mm@...ck.org, clameter@....com,
	y-goto@...fujitsu.com, dmosberger@...il.com, hugh@...itas.com,
	nickpiggin@...oo.com.au
Subject: Re: [BUGFIX][PATCH] DO flush icache before set_pte() on ia64.

On Thu, 5 Jul 2007 12:13:09 -0600
Mike Stroyan <mike@...oyan.net> wrote:
>   The L3 cache is involved in the HP-UX defect description because the
> earlier HP-UX patch PHKL_33781 added flushing of the instruction cache
> when an executable mapping was removed.  Linux never added that
> unsuccessfull attempt at montecito cache coherency.  In the current
> linux situation it can execute old cache lines straight from L2 icache.
> 
Hmm... I couldn't understand "why icache includes old lines in a new page."
This happens at
 - a file is newly loaded into page-cache.
 - only on NFS.
 - happens very *often* if the program is unlucky.

So I wrote my understainding as I think.

> > Now, I think icache should be flushed before set_pte().
> > This is a patch to try that.
> > 
> > 1. remove all lazy_mmu_prot_update()...which is used by only ia64.
> > 2. implements flush_cache_page()/flush_icache_page() for ia64.
> > 
> > Something unsure....
> > 3. mprotect() flushes cache before removing pte. Is this sane ?
> >    I added flush_icache_range() before set_pte() here.
> > 
> > Any comments and advices ?
> 
>   I am concerned about performance consequences.  With the change
> from lazy_mmu_prot_update to __flush_icache_page_ia64 you dropped
> the code that avoids icache flushes for non-executable pages.

Hmm? I added VM_EXEC check in flush_(d|i)cache_page(). Isn't it enough ?

> Section 4.6.2 of David Mosberger and Stephane Eranian's
> "ia-64 linux kernel design and implementation" goes into some
> detail about the performance penalties avoided by limiting icache
> flushes to executable pages and defering flushes until the first
> fault for execution.
> 
>   Have you done any benchmarking to measure the performance
> effect of these additional cache flushes?  It would be particularly
> interesting to measure on large systems with many CPUs.  The fc.i
> instruction needs to be broadcast to all CPUs in the system.

no benchmarks yet.

> 
>   The only defect that I see in the current implementation of
> lazy_mmu_prot_update() is that it is called too late in some
> functions that are already calling it.  Are your large changes
> attempting to correct other defects?  Or are you simplifying
> away potentially valuable code because you don't understand it?
> 
I know your *simple* patch in April wasn't included. So I wrote this.
In April thread, commenter's advices was "implement flush_icache_page()" I think.  
If you have a better patch, please post.

Thanks,
-Kame




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