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


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