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    SubjectRe: [PATCH] Flush dcache before writing into page to avoid alias
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    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.
    >
    >
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