[PATCH] Flush dcache before writing into page to avoid alias

[PATCH] Flush dcache before writing into page to avoid alias

[Andrew Morton]

On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou@gmail.com> wrote:

> Hi Andrew,
> 
> On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote:
> > The cache alias problem will happen if the changes of user shared mapping
> > is not flushed before copying, then user and kernel mapping may be mapped
> > into two different cache line, it is impossible to guarantee the coherence
> > after iov_iter_copy_from_user_atomic.  So the right steps should be:
> > 	flush_dcache_page(page);
> > 	kmap_atomic(page);
> > 	write to page;
> > 	kunmap_atomic(page);
> > 	flush_dcache_page(page);
> > More precisely, we might create two new APIs flush_dcache_user_page and
> > flush_dcache_kern_page to replace the two flush_dcache_page accordingly.
> > 
> > Here is a snippet tested on omap2430 with VIPT cache, and I think it is
> > not ARM-specific:
> > 	int val = 0x11111111;
> > 	fd = open("abc", O_RDWR);
> > 	addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> > 	*(addr+0) = 0x44444444;
> > 	tmp = *(addr+0);
> > 	*(addr+1) = 0x77777777;
> > 	write(fd, &val, sizeof(int));
> > 	close(fd);
> > The results are not always 0x11111111 0x77777777 at the beginning as expected.
> > 
> Is this a real bug or not necessary to support?

Bug.  If variable `addr' has type int* then the contents of that file
should be 0x11111111 0x77777777.  You didn't tell us what the contents
were in the incorrect case, but I guess it doesn't matter.

[PATCH] Flush dcache before writing into page to avoid alias

[Russell King - ARM Linux]

On Mon, Jan 25, 2010 at 11:58:14AM -0800, Andrew Morton wrote:
> On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou@gmail.com> wrote:
> 
> > Hi Andrew,
> > 
> > On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote:
> > > The cache alias problem will happen if the changes of user shared mapping
> > > is not flushed before copying, then user and kernel mapping may be mapped
> > > into two different cache line, it is impossible to guarantee the coherence
> > > after iov_iter_copy_from_user_atomic.  So the right steps should be:
> > > 	flush_dcache_page(page);
> > > 	kmap_atomic(page);
> > > 	write to page;
> > > 	kunmap_atomic(page);
> > > 	flush_dcache_page(page);
> > > More precisely, we might create two new APIs flush_dcache_user_page and
> > > flush_dcache_kern_page to replace the two flush_dcache_page accordingly.
> > > 
> > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is
> > > not ARM-specific:
> > > 	int val = 0x11111111;
> > > 	fd = open("abc", O_RDWR);
> > > 	addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> > > 	*(addr+0) = 0x44444444;
> > > 	tmp = *(addr+0);
> > > 	*(addr+1) = 0x77777777;
> > > 	write(fd, &val, sizeof(int));
> > > 	close(fd);
> > > The results are not always 0x11111111 0x77777777 at the beginning as expected.
> > > 
> > Is this a real bug or not necessary to support?
> 
> Bug.  If variable `addr' has type int* then the contents of that file
> should be 0x11111111 0x77777777.  You didn't tell us what the contents
> were in the incorrect case, but I guess it doesn't matter.

FYI, from a previous email from anfei:

0x44444444 0x77777777

[PATCH] Flush dcache before writing into page to avoid alias

[Sebastian Andrzej Siewior]

* Russell King - ARM Linux | 2010-01-25 20:00:04 [+0000]:

>On Mon, Jan 25, 2010 at 11:58:14AM -0800, Andrew Morton wrote:
>> On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou@gmail.com> wrote:
>> 
>> > Hi Andrew,
>> > 
>> > On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote:
>> > > The cache alias problem will happen if the changes of user shared mapping
>> > > is not flushed before copying, then user and kernel mapping may be mapped
>> > > into two different cache line, it is impossible to guarantee the coherence
>> > > after iov_iter_copy_from_user_atomic.  So the right steps should be:
>> > > 	flush_dcache_page(page);
>> > > 	kmap_atomic(page);
>> > > 	write to page;
>> > > 	kunmap_atomic(page);
>> > > 	flush_dcache_page(page);
>> > > More precisely, we might create two new APIs flush_dcache_user_page and
>> > > flush_dcache_kern_page to replace the two flush_dcache_page accordingly.
>> > > 
>> > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is
>> > > not ARM-specific:
>> > > 	int val = 0x11111111;
>> > > 	fd = open("abc", O_RDWR);
>> > > 	addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>> > > 	*(addr+0) = 0x44444444;
>> > > 	tmp = *(addr+0);
>> > > 	*(addr+1) = 0x77777777;
>> > > 	write(fd, &val, sizeof(int));
>> > > 	close(fd);
>> > > The results are not always 0x11111111 0x77777777 at the beginning as expected.
>> > > 
>> > Is this a real bug or not necessary to support?
>> 
>> Bug.  If variable `addr' has type int* then the contents of that file
>> should be 0x11111111 0x77777777.  You didn't tell us what the contents
>> were in the incorrect case, but I guess it doesn't matter.
>
>FYI, from a previous email from anfei:
>
>0x44444444 0x77777777

I just wanted to query what the status of this patch is. This patch
seems to fix a real bug which causes a test suite to fail on ARM [0].
The test suite passes on my VIVT ARM with this patch.

[0] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=524003

Sebastian

[PATCH] Flush dcache before writing into page to avoid alias

[Andrew Morton]

On Wed, 27 Jan 2010 22:59:59 +0100
Sebastian Andrzej Siewior <sebastian@breakpoint.cc> wrote:

> I just wanted to query what the status of this patch is. This patch
> seems to fix a real bug which causes a test suite to fail on ARM [0].
> The test suite passes on my VIVT ARM with this patch.
> 
> [0] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=524003

I have it queued for 2.6.33, backportable to 2.6.32.x, assuming that
nobody sees any issues with it.

[PATCH] Flush dcache before writing into page to avoid alias

[anfei zhou]

On Tue, Jan 26, 2010 at 3:58 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou@gmail.com> wrote:
>
>> Hi Andrew,
>>
>> On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote:
>> > The cache alias problem will happen if the changes of user shared mapping
>> > is not flushed before copying, then user and kernel mapping may be mapped
>> > into two different cache line, it is impossible to guarantee the coherence
>> > after iov_iter_copy_from_user_atomic. ?So the right steps should be:
>> > ? ? flush_dcache_page(page);
>> > ? ? kmap_atomic(page);
>> > ? ? write to page;
>> > ? ? kunmap_atomic(page);
>> > ? ? flush_dcache_page(page);
>> > More precisely, we might create two new APIs flush_dcache_user_page and
>> > flush_dcache_kern_page to replace the two flush_dcache_page accordingly.
>> >
>> > Here is a snippet tested on omap2430 with VIPT cache, and I think it is
>> > not ARM-specific:
>> > ? ? int val = 0x11111111;
>> > ? ? fd = open("abc", O_RDWR);
>> > ? ? addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>> > ? ? *(addr+0) = 0x44444444;
>> > ? ? tmp = *(addr+0);
>> > ? ? *(addr+1) = 0x77777777;
>> > ? ? write(fd, &val, sizeof(int));
>> > ? ? close(fd);
>> > The results are not always 0x11111111 0x77777777 at the beginning as expected.
>> >
>> Is this a real bug or not necessary to support?
>
> Bug. ?If variable `addr' has type int* then the contents of that file
> should be 0x11111111 0x77777777. ?You didn't tell us what the contents
> were in the incorrect case, but I guess it doesn't matter.
>
Sorry, I didn't give the details, here is the old thread with more details:
  http://linux.derkeiler.com/Mailing-Lists/Kernel/2010-01/msg07124.html

Regards,
Anfei.
>
>