linux-ext4 - Re: [PATCH v2 01/10] string: introduce memweight

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Message-ID: <CAC5umyiLojOEuE6wyMk-Duf8C2skMhF2G4zw+3_qO_s2-F_nKg@mail.gmail.com>
Date:	Tue, 5 Jun 2012 18:03:14 +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>, Matthew Wilcox <matthew@....cx>
Subject: Re: [PATCH v2 01/10] string: introduce memweight

2012/6/4 Jan Kara <jack@...e.cz>:
> On Mon 04-06-12 20:46:14, Akinobu Mita wrote:
>> 2012/6/4 Jan Kara <jack@...e.cz>:
>> > 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. áUnlike 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@...il.com>
>> >> Cc: Anders Larsen <al@...rsen.net>
>> >> Cc: Alasdair Kergon <agk@...hat.com>
>> >> Cc: dm-devel@...hat.com
>> >> Cc: linux-fsdevel@...r.kernel.org
>> >> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
>> >> Cc: linux-media@...r.kernel.org
>> >> Cc: Mark Fasheh <mfasheh@...e.com>
>> >> Cc: Joel Becker <jlbec@...lplan.org>
>> >> Cc: ocfs2-devel@....oracle.com
>> >> Cc: Jan Kara <jack@...e.cz>
>> >> Cc: linux-ext4@...r.kernel.org
>> >> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> >> Cc: Andreas Dilger <adilger.kernel@...ger.ca>
>> >> Cc: "Theodore Ts'o" <tytso@....edu>
>> >> Cc: Matthew Wilcox <matthew@....cx>
>> >> ---
>> >>
>> >> v2: simplify memweight(), adviced by Jan Kara
>> >>
>> >> áinclude/linux/string.h | á á3 +++
>> >> álib/string.c á á á á á | á 32 ++++++++++++++++++++++++++++++++
>> >> á2 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)
>> >> á á á return strncmp(str, prefix, strlen(prefix)) == 0;
>> >> á}
>> >> á#endif
>> >> +
>> >> +extern size_t memweight(const void *ptr, size_t bytes);
>> >> +
>> >> á#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 @@
>> >> á#include <linux/export.h>
>> >> á#include <linux/bug.h>
>> >> á#include <linux/errno.h>
>> >> +#include <linux/bitmap.h>
>> >>
>> >> á#ifndef __HAVE_ARCH_STRNICMP
>> >> á/**
>> >> @@ -824,3 +825,34 @@ void *memchr_inv(const void *start, int c, size_t bytes)
>> >> á á á return check_bytes8(start, value, bytes % 8);
>> >> á}
>> >> áEXPORT_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)
>> >> +{
>> >> + á á size_t w = 0;
>> >> + á á size_t longs;
>> >> + á á const unsigned char *bitmap = ptr;
>> >> +
>> >> + á á for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
>> >> + á á á á á á á á á á bytes--, bitmap++)
>> >> + á á á á á á w += hweight8(*bitmap);
>> >> +
>> >> + á á longs = bytes / sizeof(long);
>> >> + á á if (longs) {
>> >> + á á á á á á BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
>> >> + á á á á á á w += bitmap_weight((unsigned long *)bitmap,
>> >> + á á á á á á á á á á á á á á longs * BITS_PER_LONG);
>> >> + á á á á á á bytes -= longs * sizeof(long);
>> >> + á á á á á á bitmap += longs * sizeof(long);
>> >> + á á }
>> >> +
>> >> + á á for (; bytes > 0; bytes--, bitmap++)
>> >> + á á á á á á w += hweight8(*bitmap);
>> > áLooking 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.
>  Ah, right. OK. Add this to the comment before the loop please. You save
> yourself some explanations :)

OK.  Thanks for your advice again.
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