Re: [PATCH] export the function kmap_flush_unused.

Peter Zijlstra
2014-08-11T11:54:51Z

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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?

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?

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