From mboxrd@z Thu Jan  1 00:00:00 1970
From: anfei.zhou@gmail.com (anfei zhou)
Date: Tue, 26 Jan 2010 09:01:10 +0800
Subject: [PATCH] Flush dcache before writing into page to avoid alias
In-Reply-To: <20100125115814.156d401d.akpm@linux-foundation.org>
References: <979dd0561001202107v4ddc1eb7xa59a7c16c452f7a2@mail.gmail.com>
	<20100125133308.GA26799@desktop>
	<20100125115814.156d401d.akpm@linux-foundation.org>
Message-ID: <979dd0561001251701y76f35b8as545c390135b34da2@mail.gmail.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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.
>
>