Hmm, What if you do want to evict it from the cache for testing purposes?
Perhaps this functionality should be associated with POSIX_FADV_NOREUSE?
dd has been recently modified to support invalidating the cache for a file,
and it uses POSIX_FADV_DONTNEED for that.
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=5f311553

This change don't break dd. dd don't have a special privilege of file cache
dropping if it's also used by other processes.

if you want to drop a cache forcely (maybe for testing), you need to use
/proc/sys/vm/drop_caches. It's ok to ignore other processes activity because
it's privilege operation.

Thanks.

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