Re: [PATCH] export the function kmap_flush_unused.

Zhang, Yanmin
2014-08-12T07:24:30Z

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