linux-kernel - Re: [PATCH v3 3/4] lib/find_bit: optimize find_next

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Message-ID: <xhsmhfsh3b15v.mognet@vschneid.remote.csb>
Date:   Wed, 07 Sep 2022 17:27:08 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Alexey Klimov <klimov.linux@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andy Whitcroft <apw@...onical.com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Laight <David.Laight@...LAB.COM>,
        Dennis Zhou <dennis@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Kees Cook <keescook@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH v3 3/4] lib/find_bit: optimize find_next_bit() functions

On 27/08/22 10:58, Yury Norov wrote:
> +#define FIND_NEXT_BIT(FETCH, MUNGE, size, start)				\
> +({										\
> +	unsigned long mask, idx, tmp, sz = (size), __start = (start);		\
> +										\
> +	if (unlikely(__start >= sz))						\
> +		goto out;							\
> +										\
> +	mask = MUNGE(BITMAP_FIRST_WORD_MASK(__start));				\
> +	idx = __start / BITS_PER_LONG;						\
> +										\
> +	for (tmp = (FETCH) & mask; !tmp; tmp = (FETCH)) {			\
> +		if (idx > sz / BITS_PER_LONG)					\

Does that want to be

                if (idx + 1 >= sz / BITS_PER_LONG)

?

Consider this as used in _find_next_bit() for an all-zero 128-bit wide
bitmap (two ULL's), providing the memory contiguous to the bitmap is also
zero then this will only stop at idx=3, so that's fetching two ULLs too
far.

> +			goto out;						\
> +		idx++;								\
> +	}									\
> +										\
> +	sz = min(idx * BITS_PER_LONG + __ffs(MUNGE(tmp)), sz);			\
> +out:										\
> +	sz;									\
> +})