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

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Date:	Wed, 02 May 2007 11:57:40 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	rohitseth@...gle.com
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

Rohit Seth wrote:
> 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 a different user write(2)s the underlying page?


>>>>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.

Yeah, you would retain the flush on fault, I meant you would
introduce a flush in flush_dcache_page for when mapcount is raised.

-- 
SUSE Labs, Novell Inc.
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