how to understand this bitmap_zero

how to understand this bitmap_zero

[Ming Zhang]

could anybody help me on this? thanks!

see if we call bitmap_zero(dst, 2), it will do *dst = 0UL and thus clear
whole *dst, but what if we intent to clear 2 bits? not 32bits?

static inline void bitmap_zero(unsigned long *dst, int nbits)
{
        if (nbits <= BITS_PER_LONG)
                *dst = 0UL;
        else {
                int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
                memset(dst, 0, len);
        }
}


Ming

Re: how to understand this bitmap_zero

[Neil Brown]

On Wednesday November 9, mingz@ele.uri.edu wrote:
> could anybody help me on this? thanks!
> 
> see if we call bitmap_zero(dst, 2), it will do *dst = 0UL and thus clear
> whole *dst, but what if we intent to clear 2 bits? not 32bits?

bitmap_zero isn't intended for zeroing just a few bits in a bitmap.
It is for zeroing an entire bitmap, and you tell it how big the bitmap
is.  So this function will always zero at least the whole bitmap, and
maybe a bit more.  as bitmaps are always allocated as an array of
'unsigned long', there it no risk it over-running the space allocated.

Hope that helps.
NeilBrown

> 
> static inline void bitmap_zero(unsigned long *dst, int nbits)
> {
>         if (nbits <= BITS_PER_LONG)
>                 *dst = 0UL;
>         else {
>                 int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
>                 memset(dst, 0, len);
>         }
> }
> 
> 
> Ming
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
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Re: how to understand this bitmap_zero

[Ming Zhang]

ic. thx!

so it seems that we still have to use set_bit in a for loop to set
certain particular area, rite?

Ming

On Thu, 2005-11-10 at 09:29 +1100, Neil Brown wrote:
> On Wednesday November 9, mingz@ele.uri.edu wrote:
> > could anybody help me on this? thanks!
> > 
> > see if we call bitmap_zero(dst, 2), it will do *dst = 0UL and thus clear
> > whole *dst, but what if we intent to clear 2 bits? not 32bits?
> 
> bitmap_zero isn't intended for zeroing just a few bits in a bitmap.
> It is for zeroing an entire bitmap, and you tell it how big the bitmap
> is.  So this function will always zero at least the whole bitmap, and
> maybe a bit more.  as bitmaps are always allocated as an array of
> 'unsigned long', there it no risk it over-running the space allocated.
> 
> Hope that helps.
> NeilBrown
> 
> > 
> > static inline void bitmap_zero(unsigned long *dst, int nbits)
> > {
> >         if (nbits <= BITS_PER_LONG)
> >                 *dst = 0UL;
> >         else {
> >                 int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> >                 memset(dst, 0, len);
> >         }
> > }
> > 
> > 
> > Ming
> > 
> > 
> > -
> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: how to understand this bitmap_zero

[Neil Brown]

On Wednesday November 9, mingz@ele.uri.edu wrote:
> ic. thx!
> 
> so it seems that we still have to use set_bit in a for loop to set
> certain particular area, rite?

Correct.

NeilBrown