Messages in this thread | | | Date | Mon, 25 Jan 2010 11:58:14 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] Flush dcache before writing into page to avoid alias |
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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.
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