linux-kernel - Re: [PATCH 01/10] string: introduce memweight

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Date:	Wed, 23 May 2012 14:29:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Anders Larsen <al@...rsen.net>,
	Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
	linux-fsdevel@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	linux-media@...r.kernel.org, Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>, ocfs2-devel@....oracle.com,
	linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 01/10] string: introduce memweight

On Wed 23-05-12 21:12:18, Akinobu Mita wrote:
> 2012/5/23 Jan Kara <jack@...e.cz>:
> > On Sun 20-05-12 22:23:14, Akinobu Mita wrote:
> >> memweight() is the function that counts the total number of bits set
> >> in memory area.  The memory area doesn't need to be aligned to
> >> long-word boundary unlike bitmap_weight().
> >  Thanks for the patch. I have some comments below.
> 
> Thanks for the review.
> 
> >> @@ -824,3 +825,39 @@ void *memchr_inv(const void *start, int c, size_t bytes)
> >>       return check_bytes8(start, value, bytes % 8);
> >>  }
> >>  EXPORT_SYMBOL(memchr_inv);
> >> +
> >> +/**
> >> + * memweight - count the total number of bits set in memory area
> >> + * @ptr: pointer to the start of the area
> >> + * @bytes: the size of the area
> >> + */
> >> +size_t memweight(const void *ptr, size_t bytes)
> >> +{
> >> +     size_t w = 0;
> >> +     size_t longs;
> >> +     union {
> >> +             const void *ptr;
> >> +             const unsigned char *b;
> >> +             unsigned long address;
> >> +     } bitmap;
> >  Ugh, this is ugly and mostly unnecessary. Just use "const unsigned char
> > *bitmap".
> >
> >> +
> >> +     for (bitmap.ptr = ptr; bytes > 0 && bitmap.address % sizeof(long);
> >> +                     bytes--, bitmap.address++)
> >> +             w += hweight8(*bitmap.b);
> >  This can be:
> >        count = ((unsigned long)bitmap) % sizeof(long);
> 
> The count should be the size of unaligned area and it can be greater than
> bytes. So
> 
>         count = min(bytes,
>                     sizeof(long) - ((unsigned long)bitmap) % sizeof(long));
  You are right, I didn't quite think this through.
 
> >        while (count--) {
> >                w += hweight(*bitmap);
> >                bitmap++;
> >                bytes--;
> >        }
> >> +
> >> +     for (longs = bytes / sizeof(long); longs > 0; ) {
> >> +             size_t bits = min_t(size_t, INT_MAX & ~(BITS_PER_LONG - 1),
> >> +                                     longs * BITS_PER_LONG);
> >  I find it highly unlikely that someone would have such a large bitmap
> > (256 MB or more on 32-bit). Also the condition as you wrote it can just
> > overflow so it won't have the desired effect. Just do
> >        BUG_ON(longs >= ULONG_MAX / BITS_PER_LONG);
> 
> The bits argument of bitmap_weight() is int type. So this should be
> 
>         BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
  OK, I didn't check and thought it's size_t.

> > and remove the loop completely. If someone comes with such a huge bitmap,
> > the code can be modified easily (after really closely inspecting whether
> > such a huge bitmap is really well justified).
> 
> size_t memweight(const void *ptr, size_t bytes)
> {
> 	size_t w = 0;
> 	size_t longs;
> 	const unsigned char *bitmap = ptr;
> 
> 	for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
> 			bytes--, bitmap++)
> 		w += hweight8(*bitmap);
> 
> 	longs = bytes / sizeof(long);
> 	BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
> 	w += bitmap_weight((unsigned long *)bitmap, longs * BITS_PER_LONG);
> 	bytes -= longs * sizeof(long);
> 	bitmap += longs * sizeof(long);
> 
> 	for (; bytes > 0; bytes--, bitmap++)
> 		w += hweight8(*bitmap);
> 
> 	return w;
> }
  Yup, this looks much more readable. Thanks!

								Honza
  
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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