linux-kernel - Re: [PATCH] export the function kmap_flush

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Date:	Tue, 12 Aug 2014 15:24:25 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Sha, Ruibin" <ruibin.sha@...el.com>
CC:	Chintan Pandya <cpandya@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"mel@....ul.ie" <mel@....ul.ie>,
	"mgorman@...e.de" <mgorman@...e.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	"He, Bo" <bo.he@...el.com>
Subject: Re: [PATCH]  export the function kmap_flush_unused.

On 2014/8/11 19:54, Peter Zijlstra wrote:
> On Mon, Aug 11, 2014 at 01:26:45AM +0000, Sha, Ruibin wrote:
>> Hi Chintan,
>> Thank you very much for your timely and kindly response and comments.
>>
>> Here is more detail about our Scenario:
>>
>>      We have a big driver on Android product. The driver allocates lots of
>>      DDR pages. When applications mmap a file exported from the driver,
>>      driver would mmap the pages to the application space, usually with
>>      uncachable prot.
>>      On ia32/x86_64 arch, we have to avoid page cache alias issue. When
>>      driver allocates the pages, it would change page original mapping in
>>      page table with uncachable prot. Sometimes, the allocated page was
>>      used by kmap/kunmap. After kunmap, the page is still mapped in KMAP
>>      space. The entries in KMAP page table are not cleaned up until a
>>      kernel thread flushes the freed KMAP pages(usually it is woken up by kunmap).
>>      It means the driver need  force to flush the KMAP page table entries before mapping pages to
>>      application space to be used. Otherwise, there is a race to create
>>      cache alias.
>>
>>      To resolve this issue, we need export function kmap_flush_unused as
>>      the driver is compiled as module. Then, the driver calls
>>      kmap_flush_unused if the allocated pages are in HIGHMEM and being
>>      used by kmap.
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?

Sorry, Peter. Ruibin is a new guy in LKML community. He uses outlook
to send emails. He would improve that.

>
> That said, it sounds like you want set_memory_() to call
> kmap_flush_unused(). Because this race it not at all specific to your
> usage, it could happen to any set_memory_() site, right?
No. set_memory_() assumes the memory is not in HIGHMEM.
This scenario is driver allocates HIGHMEM pages, which are kmapped before.
Kernel uses a lazy method when kunmap a HIGHMEM page.
The pages are not unmapped from KMAP page table entries immediately.
When next kmap calling uses the same entry, kernel would change pte.
Or when change_page_attr_set_clr is called.

Our big driver doesn't call change_page_attr_set_clr when mmap the
pages with UNCACHABLE prot. It need call kmap_flush_unused directly after
allocating HIGHMEM pages.

Thanks for the kind comments.

Yanmin

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