Subject: Re: [PATCH v3 RESEND 4/6] bitmap: Introduce bitmap_off()

From: Yury Norov <[email-masked]>
Date: Mon, 12 Feb 2024 10:37:15 -0800

On Mon, Feb 12, 2024 at 08:56:32AM +0100, Herve Codina wrote:
> The bitmap_onto() function translates one bitmap relative to another but
> no function are present to perform the reverse translation.
> 
> Introduce bitmap_off() to fill this hole.
> 
> Signed-off-by: Herve Codina <[email-masked]>
> ---
>  include/linux/bitmap.h |  3 +++
>  lib/bitmap.c           | 42 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 99451431e4d6..5ecfcbbc91f4 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -65,6 +65,7 @@ struct device;
>   *  bitmap_remap(dst, src, old, new, nbits)     *dst = map(old, new)(src)
>   *  bitmap_bitremap(oldbit, old, new, nbits)    newbit = map(old, new)(oldbit)
>   *  bitmap_onto(dst, orig, relmap, nbits)       *dst = orig relative to relmap
> + *  bitmap_off(dst, orig, relmap, nbits)        *dst = bitmap_onto() reverse operation
>   *  bitmap_fold(dst, orig, sz, nbits)           dst bits = orig bits mod sz
>   *  bitmap_parse(buf, buflen, dst, nbits)       Parse bitmap dst from kernel buf
>   *  bitmap_parse_user(ubuf, ulen, dst, nbits)   Parse bitmap dst from user buf
> @@ -208,6 +209,8 @@ int bitmap_bitremap(int oldbit,
>  		const unsigned long *old, const unsigned long *new, int bits);
>  void bitmap_onto(unsigned long *dst, const unsigned long *orig,
>  		const unsigned long *relmap, unsigned int bits);
> +void bitmap_off(unsigned long *dst, const unsigned long *orig,
> +		const unsigned long *relmap, unsigned int bits);
>  void bitmap_fold(unsigned long *dst, const unsigned long *orig,
>  		unsigned int sz, unsigned int nbits);
>  
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 2feccb5047dc..71343967335e 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -682,6 +682,48 @@ void bitmap_onto(unsigned long *dst, const unsigned long *orig,
>  }
>  EXPORT_SYMBOL(bitmap_onto);
>  
> +/**
> + * bitmap_off - revert operation done by bitmap_onto()

This is definitely a bad name. I've no a better idea, but even
bitmap_onto_revert() would be better.

> + *     @dst: resulting translated bitmap
> + *     @orig: original untranslated bitmap
> + *     @relmap: bitmap relative to which translated
> + *     @bits: number of bits in each of these bitmaps
> + *
> + * Suppose onto computed using bitmap_onto(onto, src, relmap, n)
> + * The operation bitmap_off(result, onto, relmap, n) leads to a
> + * result equal or equivalent to src.

Agree with Rasmus. This should be well tested.

> + * The result can be 'equivalent' because bitmap_onto() and
> + * bitmap_off() are not bijective.
> + * The result and src values are equivalent in that sense that a
> + * call to bitmap_onto(onto, src, relmap, n) and a call to
> + * bitmap_onto(onto, result, relmap, n) will lead to the same onto
> + * value.

Did you mean "a call to bitmap_onto(onto, src, relmap, n) and a
call to bitmap_off(onto, result, relmap, n)"? 

I think the whole paragraph adds more confusion than explanations.
If a new function is supposed to revert the result of some other
function, I'd better focus on testing that it actually reverts as
advertised, and keep description as brief as possible.

> + * If either of @orig or @relmap is empty (no set bits), then @dst
> + * will be returned empty.

Is this an exception from the 'revert' policy? Doesn't look like that.
So, what for mentioning this specific case?

> + * All bits in @dst not set by the above rule are cleared.

The above rule is about empty @orig and @relmap, not about setting
bits. What did you mean here?

> + */
> +void bitmap_off(unsigned long *dst, const unsigned long *orig,
> +		const unsigned long *relmap, unsigned int bits)
> +{
> +	unsigned int n, m;      /* same meaning as in above comment */

In the above comment, n means the size of bitmaps, and m is not
mentioned at all.

> +	if (dst == orig)        /* following doesn't handle inplace mappings */
> +		return;
> +	bitmap_zero(dst, bits);

Can you add an empty line after 'return'.

> +	m = 0;
> +	for_each_set_bit(n, relmap, bits) {
> +		/* m == bitmap_pos_to_ord(relmap, n, bits) */

Don't think we need this comment here. If you want to underline that
m tracks bit order, can you just give it a more explanatory name. For
example, 'bit_order'.

> +		if (test_bit(n, orig))
> +			set_bit(m, dst);
> +		m++;
> +	}
> +}
> +EXPORT_SYMBOL(bitmap_off);
> +
>  #ifdef CONFIG_NUMA
>  /**
>   * bitmap_fold - fold larger bitmap into smaller, modulo specified size
> -- 
> 2.43.0