linux-kernel - Re: bitmap_find_free_region and bitmap

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Message-ID: <3a5b1be00612010040r5ee24696h71119adb0a9bbffe@mail.gmail.com>
Date:	Fri, 1 Dec 2006 10:40:27 +0200
From:	"Komal Shah" <komal.shah802003@...il.com>
To:	"Paul Jackson" <pj@....com>
Cc:	"Paul Mundt" <lethal@...ux-sh.org>,
	"M. R. Brown" <mrbrown@...6.org>, linux-kernel@...r.kernel.org,
	akpm@...l.org
Subject: Re: bitmap_find_free_region and bitmap_full arg doubts

On 12/1/06, Paul Jackson <pj@....com> wrote:
>
>   M. R. Brown and Paul Mundt -- take a look at the question at end of this post.
>
> Komal wrote:
> >
> > I have attached the test module code. I have confusion about the what
> > to pass as second argument of bitmap_find_free_region for 2nd layer
> > bitmap.
> >
> > Do we have to pass "total size of the block...here 128MB" as the
> > second argument or no. of bits in that block, which 32768 (128 >>
> > PAGE_SHIFT) ?
> >
> > This confusion arised as store queue implementation (sq.c) of sh arch,
> > passes total size of the bitmap (64M) as the second argument instead
> > of bits.
> >
> > Same is the case with bitmap_full, where I have to pass total size of
> > the block (here 128MB) instead of no. of bits, in-order it to make my
> > test module work correctly.
>
> A bitmap is essentially an array of bits.  The bitmap_* routines should
> take a count of how many bits are in the bitmap array (or in your case,
> how many bits are in the segment that you expect to be used in that call.)
>
> So I'm guessing you will be passing L2_BM_BITS for that second argument.
>
> The call to bitmap_find_free_region(), in arch/sh/kernel/cpu/sh4/sq.c
> looks bogus:
>
>         page = bitmap_find_free_region(sq_bitmap, 0x04000000,
>                                        get_order(map->size));
>
> It says the bitmap has 0x04000000 bits.  This would take 0x04000000 / 8
> which is 8388608 (decimal) bytes to hold.  That's an insanely
> huge bitmap - 8 million bytes worth of bits.
>
> I can't make entire sense of the the code you attached.  I get confused
> over the various sizes of maps and what the maps represent, and of bits
> and bytes, and of the two levels.
>

Thanx for clarifying. I am now able to understand it properly, and I
will modify my code, so that it will be easy to understand/read :)

-- 
---Komal Shah
http://komalshah.blogspot.com
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