Re: [PATCH] export the function kmap_flush_unused.

From: Zhang, Yanmin
Date: Sun Aug 10 2014 - 23:32:57 EST

On 2014/8/11 9:26, 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.

Thanks again!

Best Regards
Sha, Rui bin ( Robin )
+86 13817890945
Android System Integration Shanghai

-----Original Message-----
From: Chintan Pandya [mailto:cpandya@xxxxxxxxxxxxxx]
Sent: Friday, August 8, 2014 9:40 PM
To: Sha, Ruibin
Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; mel@xxxxxxxxx; a.p.zijlstra@xxxxxxxxx; mgorman@xxxxxxx; mingo@xxxxxxxxxx; Zhang, Yanmin; He, Bo
Subject: Re: [PATCH] export the function kmap_flush_unused.

On 08/08/2014 02:46 PM, Sha, Ruibin wrote:
export the function kmap_flush_unused.

Scenario: When graphic driver need high memory spece, we use
alloc_pages() to allocate. But if the allocated page has just been
mapped in the KMAP space(like first kmap then kunmap) and no flush
page happened on PKMAP, the page virtual address is not NULL.Then when
we get that page and set page attribute like set_memory_uc and
set_memory_wc, we hit error.
Could you explain your scenario with more details ? set_memory_* should be applied on mapped address. And in attempt to map your page (which was just kmap and kunmap'ed), it will overwrite the previous mappings.

Moreover, in my view, kmap_flush_unused is just helping us in keeping the cache clean for kmap virtual addresses if they are unmapped. Is it serving any more purpose here ?

It depends on how to define 'clean' here. It resets pkmap_count[i] to 0,
and cleans up page table entries used by PKMAP. Here, our scenario is
caused by the late page table entry cleanup as driver need avoid page
cache alias.

fix: For that scenario,when we get the allocated page and its virtual
address is not NULL, we would like first flush that page.
So need export that function kmap_flush_unused.

Signed-off-by: sha, ruibin <ruibin.sha@xxxxxxxxx>

mm/highmem.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/mm/highmem.c b/mm/highmem.c
index b32b70c..511299b 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -156,6 +156,7 @@ void kmap_flush_unused(void)
This symbol is already extern'ed. Is it not sufficient for your case ?

We want to call it in driver module. extern is not enough.


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