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

From: Yury Norov <[email-masked]>
Date: Mon, 12 Feb 2024 10:41:42 -0800

On Mon, Feb 12, 2024 at 10:37:18AM -0800, Yury Norov wrote:
> 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++;

Forgot to mention - we need a __set_bit() and __test_bit(), because the
whole function is not atomic. This applies to the bitmap_onto() as
well. Can you please send a patch fixing it for bitmap_onto() in the
next iteration?

> > +	}
> > +}
> > +EXPORT_SYMBOL(bitmap_off);
> > +
> >  #ifdef CONFIG_NUMA
> >  /**
> >   * bitmap_fold - fold larger bitmap into smaller, modulo specified size
> > -- 
> > 2.43.0