linux-ext4 - Re: [PATCH 01/10] string: introduce memweight

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Message-ID: <CAC5umyi=ridqRZGGh0+_xw0-GCN+69B33Qz82-9x4dVODGGx6w@mail.gmail.com>
Date:	Wed, 23 May 2012 21:12:18 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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

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));

>        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);

> 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;
}
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