Re: [PATCH 01/10] string: introduce memweight
From: Jan Kara <jack@suse.cz>
Date: 2012-05-23 09:21:13
Also in:
dm-devel, linux-fsdevel, linux-media, lkml
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.
quoted hunk ↗ jump to hunk
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com> Cc: Anders Larsen <al@alarsen.net> Cc: Alasdair Kergon <agk@redhat.com> Cc: dm-devel@redhat.com Cc: linux-fsdevel@vger.kernel.org Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Cc: linux-media@vger.kernel.org Cc: Mark Fasheh <redacted> Cc: Joel Becker <jlbec@evilplan.org> Cc: ocfs2-devel@oss.oracle.com Cc: Jan Kara <jack@suse.cz> Cc: linux-ext4@vger.kernel.org Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andreas Dilger <adilger.kernel@dilger.ca> Cc: "Theodore Ts'o" <tytso@mit.edu> --- include/linux/string.h | 3 +++ lib/string.c | 37 +++++++++++++++++++++++++++++++++++++ 2 files changed, 40 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..c8b92a0 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 +825,39 @@ 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);
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); 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).
+ + w += bitmap_weight(bitmap.ptr, bits); + bytes -= bits / BITS_PER_BYTE; + bitmap.address += bits / BITS_PER_BYTE; + longs -= bits / BITS_PER_LONG; + } + + for (; bytes > 0; bytes--, bitmap.address++) + w += hweight8(*bitmap.b); + + return w; +} +EXPORT_SYMBOL(memweight);
Honza -- Jan Kara [off-list ref] SUSE Labs, CR