cache alias in mmap + write

From: anfei
Date: Wed Jan 20 2010 - 03:31:38 EST


The below test case is easy to reproduce the cache alias problem on
omap2430 with the VIPT cache. The steps as these:

$ dd if=/dev/zero of=abc bs=4k count=1
$ ./a.out # this program
$ xxd abc | head -1 # check the result

I expect it's always 0x11111111 0x77777777 at the beginning of file
"abc", but the result is not (run multiple times):

0x11111111 0x77777777
0x44444444 0x77777777
0x11111111 0x77777777
0x44444444 0x77777777
0x44444444 0x77777777

If I add munmap()/msync() before write(), I can see it's always as
expected (0x11111111 0x77777777).

Does Linux guarantee the coherence between write and the shared mappings
w/o the help of munmap/msync? If not, what kind of the coherence are
ensured? Can anyone give a clear definition?

And if I apply the below patch to write (only for the fs using this
generic function), the problem disappeared. That's another question,
why do we only do flush for read (see flush_dcache_page in
do_generic_file_read), but not write too?


#include <stdio.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(void)
int fd;
int *addr;
int tmp;
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));

return 0;

diff --git a/mm/filemap.c b/mm/filemap.c
index 96ac6b0..07056fb 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2196,6 +2196,9 @@ again:
if (unlikely(status))

+ if (mapping_writably_mapped(mapping))
+ flush_dcache_page(page);
copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
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