linux-kernel - Re: [PATCH] bitmap: fix memset optimization on big-endian systems

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Message-ID: <20180402230006.GB18049@vader>
Date:   Mon, 2 Apr 2018 16:00:06 -0700
From:   Omar Sandoval <osandov@...ndov.com>
To:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     kernel-team@...com, Matthew Wilcox <mawilcox@...rosoft.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] bitmap: fix memset optimization on big-endian systems

On Mon, Apr 02, 2018 at 03:58:31PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@...com>
> 
> Commit 2a98dc028f91 introduced an optimization to bitmap_{set,clear}()
> which uses memset() when the start and length are constants aligned to a
> byte. This is wrong on big-endian systems; our bitmaps are arrays of
> unsigned long, so bit n is not at byte n / 8 in memory. This was caught
> by the Btrfs selftests, but the bitmap selftests also fail when run on a
> big-endian machine.
> 
> We can still use memset if the start and length are aligned to an
> unsigned long, so do that on big-endian. The same problem applies to the
> memcmp in bitmap_equal(), so fix it there, too.
> 
> Fixes: 2a98dc028f91 ("include/linux/bitmap.h: turn bitmap_set and bitmap_clear into memset when possible")
> Fixes: 2c6deb01525a ("bitmap: use memcmp optimisation in more situations")
> Cc: stable@...nel.org

This should be stable@...r.kernel.org, of course