Subject: Re: [PATCH 2/8] bitmap: Introduce bitmap_set, bitmap_clear, bitmap_find_next_zero_area

From: Andrew Morton <[email-masked]>
Date: Fri, 9 Oct 2009 16:41:00 -0700

On Fri,  9 Oct 2009 17:29:15 +0900
Akinobu Mita <[email-masked]> wrote:

> This introduces new bitmap functions:
> 
> bitmap_set: Set specified bit area
> bitmap_clear: Clear specified bit area
> bitmap_find_next_zero_area: Find free bit area
> 
> These are stolen from iommu helper.
> 
> I changed the return value of bitmap_find_next_zero_area if there is
> no zero area.
> 
> find_next_zero_area in iommu helper: returns -1
> bitmap_find_next_zero_area: return >= bitmap size

I'll plan to merge this patch into 2.6.32 so we can trickle all the
other patches into subsystems in an orderly fashion.

> +void bitmap_set(unsigned long *map, int i, int len)
> +{
> +	int end = i + len;
> +
> +	while (i < end) {
> +		__set_bit(i, map);
> +		i++;
> +	}
> +}

This is really inefficient, isn't it?  It's a pretty trivial matter to
romp through memory 32 or 64 bits at a time.

> +EXPORT_SYMBOL(bitmap_set);
> +
> +void bitmap_clear(unsigned long *map, int start, int nr)
> +{
> +	int end = start + nr;
> +
> +	while (start < end) {
> +		__clear_bit(start, map);
> +		start++;
> +	}
> +}
> +EXPORT_SYMBOL(bitmap_clear);

Ditto.

> +unsigned long bitmap_find_next_zero_area(unsigned long *map,
> +					 unsigned long size,
> +					 unsigned long start,
> +					 unsigned int nr,
> +					 unsigned long align_mask)
> +{
> +	unsigned long index, end, i;
> +again:
> +	index = find_next_zero_bit(map, size, start);
> +
> +	/* Align allocation */
> +	index = (index + align_mask) & ~align_mask;
> +
> +	end = index + nr;
> +	if (end >= size)
> +		return end;
> +	i = find_next_bit(map, end, index);
> +	if (i < end) {
> +		start = i + 1;
> +		goto again;
> +	}
> +	return index;
> +}
> +EXPORT_SYMBOL(bitmap_find_next_zero_area);

This needs documentation, please.  It appears that `size' is the size
of the bitmap and `nr' is the number of zeroed bits we're looking for,
but an inattentive programmer could get those reversed.

Also the semantics of `align_mask' could benefit from spelling out.  Is
the alignment with respect to memory boundaries or with respect to
`map' or with respect to map+start or what?

And why does align_mask exist at all?  I was a bit surprised to see it
there.  In which scenarios will it be non-zero?