Re: Fw: [PATCH] ia64: race flushing icache in do_no_page path

Nick Piggin
2007-05-02T01:57:40Z

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.