From: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>,
"Sha, Ruibin" <ruibin.sha@intel.com>
Cc: Chintan Pandya <cpandya@codeaurora.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"mel@csn.ul.ie" <mel@csn.ul.ie>,
"mgorman@suse.de" <mgorman@suse.de>,
"mingo@redhat.com" <mingo@redhat.com>,
"Zhang, Yanmin" <yanmin.zhang@intel.com>,
"He, Bo" <bo.he@intel.com>
Subject: Re: [PATCH] export the function kmap_flush_unused.
Date: Tue, 12 Aug 2014 15:24:25 +0800 [thread overview]
Message-ID: <53E9C129.2020902@linux.intel.com> (raw)
In-Reply-To: <20140811115431.GW9918@twins.programming.kicks-ass.net>
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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prev parent reply other threads:[~2014-08-12 7:24 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-08-08 9:16 [PATCH] export the function kmap_flush_unused Sha, Ruibin
2014-08-08 13:39 ` Chintan Pandya
2014-08-11 1:26 ` Sha, Ruibin
2014-08-11 3:32 ` Zhang, Yanmin
2014-08-11 11:54 ` Peter Zijlstra
2014-08-12 7:24 ` Zhang, Yanmin [this message]
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