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os: don't corrupt pre-existing memory-backend data with prealloc 

When using a memory-backend object with prealloc turned on, QEMU
will memset() the first byte in every memory page to zero. While
this might have been acceptable for memory backends associated
with RAM, this corrupts application data for NVDIMMs.

Instead of setting every page to zero, read the current byte
value and then just write that same value back, so we are not
corrupting the original data. Directly write the value instead
of memset()ing it, since there's no benefit to memset for a
single byte write.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Message-id: 20170303113255.28262-1-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Daniel P. Berrange 2017-03-03 11:32:55 +00:00 committed by Stefan Hajnoczi
parent d84f714eaf
commit 9dc44aa582
1 changed files with 13 additions and 1 deletions
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14
util/oslib-posix.c
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@ -361,7 +361,19 @@ static void *do_touch_pages(void *arg)
memset_thread_failed = true;
} else {
for (i = 0; i < numpages; i++) {
memset(addr, 0, 1);
/*
* Read & write back the same value, so we don't
* corrupt existing user/app data that might be
* stored.
*
* 'volatile' to stop compiler optimizing this away
* to a no-op
*
* TODO: get a better solution from kernel so we
* don't need to write at all so we don't cause
* wear on the storage backing the region...
*/
*(volatile char *)addr = *addr;
addr += hpagesize;
}
}