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3ceaccdf92
The MPX code needs to clear out bounds tables for memory which
is no longer in use. We do this when a userspace mapping is
torn down (unmapped).
There are two modes:
1. An entire bounds table becomes unused, and can be freed
and its pointer removed from the bounds directory. This
happens either when a large mapping is torn down, or when
a small mapping is torn down and it is the last mapping
"covered" by a bounds table.
2. Only part of a bounds table becomes unused, in which case
we free the backing memory as if MADV_DONTNEED was called.
The old code was a spaghetti mess of "edge" bounds tables
where the edges were handled specially, even if we were
unmapping an entire one. Non-edge bounds tables are always
fully unmapped, but share a different code path from the edge
ones. The old code had a bug where it was unmapping too much
memory. I worked on fixing it for two days and gave up.
I didn't write the original code. I didn't particularly like
it, but it worked, so I left it. After my debug session, I
realized it was undebuggagle *and* buggy, so out it went.
I also wrote a new unmapping test program which uncovers bugs
pretty nicely.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave@sr71.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150607183706.DCAEC67D@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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