linux-kernel - Re: Fw: [PATCH] ia64: race flushing icache in do_no

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Date:	Tue, 01 May 2007 17:36:18 -0700
From:	Rohit Seth <rohitseth@...gle.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	"'Mike Stroyan'" <mike.stroyan@...com>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>,
	"'Hugh Dickins'" <hugh@...itas.com>,
	"'Luck, Tony'" <tony.luck@...el.com>, linux-ia64@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Fw: [PATCH] ia64: race flushing icache in do_no_page path

On Tue, 2007-05-01 at 21:39 +1000, Nick Piggin wrote:
> Rohit Seth wrote:

> > 
> > If a user is requesting kernel to do (for example) write on a page that is
> > already mapped with execute and write permissions then it should be treated
> > as if the user space is doing modifications to that page.  There is no
> > change in protections so lazy_prot_mmu_update shouldn't be called even
> > though PG_arch_1 is (I think) set.  Does it answer your concern?
> 
> I'm not sure that I would agree. For direct modifications of memory via
> a passed in user virtual address, perhaps. For operations on pagecache,
> we may not even have a handle to issue the flush cache instruction on (ie.
> a user virtual address), let alone know whether anyone else is mapping
> the page.
> 

Can you please describe the page cache scenario in more detail?  IMO, if
a page is user mapped with at least one execute and write permission
then the responsibility of update caches lies with user.

> >>What if you were to say remove all the PG_arch_1 code, and do 
> >>something really simple like flush icache in 
> >>flush_dcache_page? Would performance suffer horribly?
> > 
> > 
> > On Itanium, I think it will have some performance penalty (horrible or not I
> > don't know) as you will be invalidating the caches more often.  And they
> > alsways look for last 0.1% performance that they can get.
> 
> Sure, but if we _only_ flushed when page_mapcount was raised,

You will need this every time there is change in protection (e.g.
mprotect) not only when page_mapcount is raised.

-rohit


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