linux-kernel - Re: [PATCH 17/19] x86, mpx: rewrite unmap code

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Date:	Mon, 18 May 2015 23:55:24 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave@...1.net>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH 17/19] x86, mpx: rewrite unmap code

On Fri, 8 May 2015, Dave Hansen wrote:
> 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.

Very appreciated. This is really readable now.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
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