linux-kernel - Re: [PATCH] bitmap: Fix optimization of bitmap

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Message-ID: <20171025084602.GC25140@fergus.ozlabs.ibm.com>
Date:   Wed, 25 Oct 2017 19:46:02 +1100
From:   Paul Mackerras <paulus@...abs.org>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>
Subject: Re: [PATCH] bitmap: Fix optimization of bitmap_set/clear for
 big-endian machines

On Wed, Oct 25, 2017 at 07:39:48AM +0000, Matthew Wilcox wrote:
> Hang on, don't tell me you found this by inspection.  Are you not running the bitmap testcase, enabled by CONFIG_TEST_BITMAP?  Either that should be producing an error, or there's a missing test case, or your inspection is wrong ...

I did find it by inspection.  I was looking for a version of the
bitmap_* API that does little-endian style bitmaps on all systems, and
the inline bitmap_set() does that in the case where it calls memset,
but not in the case where it calls __bitmap_set.

I'll fire up a big-endian system tomorrow when I get to work to run
the test case.  (PPC64 is almost entirely little-endian these days as
far as the IBM POWER systems are concerned.)

In any case, it's pretty clearly wrong as it is.  On a big-endian
64-bit system, bitmap_set(p, 56, 16) should set bytes 0 and 15 to
0xff, and there's no way a single memset can do that.

Paul.

(and yes, I stuffed up the address for lkml)