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Re: [PATCH v2 01/10] string: introduce memweight



2012/6/4 Jan Kara <jack@xxxxxxx>:
> On Sat 02-06-12 22:40:07, Akinobu Mita wrote:
>> memweight() is the function that counts the total number of bits set
>> in memory area.  Unlike bitmap_weight(), memweight() takes pointer
>> and size in bytes to specify a memory area which does not need to be
>> aligned to long-word boundary.
>>
>> Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
>> Cc: Anders Larsen <al@xxxxxxxxxxx>
>> Cc: Alasdair Kergon <agk@xxxxxxxxxx>
>> Cc: dm-devel@xxxxxxxxxx
>> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
>> Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
>> Cc: linux-media@xxxxxxxxxxxxxxx
>> Cc: Mark Fasheh <mfasheh@xxxxxxxx>
>> Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
>> Cc: ocfs2-devel@xxxxxxxxxxxxxx
>> Cc: Jan Kara <jack@xxxxxxx>
>> Cc: linux-ext4@xxxxxxxxxxxxxxx
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
>> Cc: "Theodore Ts'o" <tytso@xxxxxxx>
>> Cc: Matthew Wilcox <matthew@xxxxxx>
>> ---
>>
>> v2: simplify memweight(), adviced by Jan Kara
>>
>>  include/linux/string.h |    3 +++
>>  lib/string.c           |   32 ++++++++++++++++++++++++++++++++
>>  2 files changed, 35 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/string.h b/include/linux/string.h
>> index e033564..ffe0442 100644
>> --- a/include/linux/string.h
>> +++ b/include/linux/string.h
>> @@ -145,4 +145,7 @@ static inline bool strstarts(const char *str, const char *prefix)
>>       return strncmp(str, prefix, strlen(prefix)) == 0;
>>  }
>>  #endif
>> +
>> +extern size_t memweight(const void *ptr, size_t bytes);
>> +
>>  #endif /* _LINUX_STRING_H_ */
>> diff --git a/lib/string.c b/lib/string.c
>> index e5878de..bf4d5a8 100644
>> --- a/lib/string.c
>> +++ b/lib/string.c
>> @@ -26,6 +26,7 @@
>>  #include <linux/export.h>
>>  #include <linux/bug.h>
>>  #include <linux/errno.h>
>> +#include <linux/bitmap.h>
>>
>>  #ifndef __HAVE_ARCH_STRNICMP
>>  /**
>> @@ -824,3 +825,34 @@ 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;
>> +     const unsigned char *bitmap = ptr;
>> +
>> +     for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
>> +                     bytes--, bitmap++)
>> +             w += hweight8(*bitmap);
>> +
>> +     longs = bytes / sizeof(long);
>> +     if (longs) {
>> +             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);
>  Looking at bitmap_weight() it seems this last loop is not needed. Just
> pass to bitmap_weight() bytes*BITS_PER_BYTE. Also generally this function
> doesn't seem necessary at all at least for ext2 & ext3 (sorry for not
> noticing this earlier...).

This last loop is necessary for big-endian architecture.
if bytes % sizeof(long) != 0, bitmap_weight() counts one-bits in wrong
byte-field
of the last long word.
--
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