linux-kernel - Re: [PATCH] udf: convert some macros to functions

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Message-ID: <20080108100941.GA17794@duck.suse.cz>
Date:	Tue, 8 Jan 2008 11:09:41 +0100
From:	Jan Kara <jack@...e.cz>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ben Fennema <bfennema@...con.csc.calpoly.edu>
Subject: Re: [PATCH] udf: convert some macros to functions

On Mon 07-01-08 21:44:26, Marcin Slusarz wrote:
> On Mon, Jan 07, 2008 at 12:26:18PM +0000, Christoph Hellwig wrote:
> > On Sun, Jan 06, 2008 at 01:44:34AM +0100, marcin.slusarz@...il.com wrote:
> > > +static struct udf_bitmap *udf_sb_alloc_bitmap(struct super_block *sb, __u32 index)
> > > +{
> > > +	struct udf_part_map *map = &UDF_SB(sb)->s_partmaps[index];
> > > +	int nr_groups = (map->s_partition_len + (sizeof(struct spaceBitmapDesc) << 3) +
> > > +		(sb->s_blocksize * 8) - 1) / (sb->s_blocksize * 8);
> > > +	int size = sizeof(struct udf_bitmap) + (sizeof(struct buffer_head *) * nr_groups);
> > > +	struct udf_bitmap *bitmap;
> > > +
> > > +	if (size <= PAGE_SIZE)
> > > +		bitmap = kmalloc(size, GFP_KERNEL);
> > > +	else
> > > +		bitmap = vmalloc(size);
> > > +	if (bitmap != NULL) {
> > > +		memset(bitmap, 0x00, size);
> > > +		bitmap->s_block_bitmap = (struct buffer_head **)(bitmap + 1);
> > > +		bitmap->s_nr_groups = nr_groups;
> > > +	} else
> > > +		udf_error(sb, __FUNCTION__, "Unable to allocate space for bitmap and %d buffer_head pointers", nr_groups);
> > > +	return bitmap;
> > > +}
> > 
> > There's some overly long lines here and some odd style, this should look
> > more like:
> These long lines were split later in "[PATCH 1/7] udf: fix coding style"
> (but I will fix it in next version of this patch).
> 
> > static struct udf_bitmap *udf_sb_alloc_bitmap(struct super_block *sb,
> > 		u32 index)
> > {
> > 	struct udf_part_map *map = &UDF_SB(sb)->s_partmaps[index];
> > 	struct udf_bitmap *bitmap;
> > 	int nr_groups;
> > 	int size;
> > 
> > 	nr_groups = (map->s_partition_len +
> > 		(sizeof(struct spaceBitmapDesc) << 3) +
> > 		 (sb->s_blocksize * 8) - 1) /
> > 		(sb->s_blocksize * 8);
> > 	size = sizeof(struct udf_bitmap) +
> > 		(sizeof(struct buffer_head *) * nr_groups);
> > 	if (size <= PAGE_SIZE)
> > 		bitmap = kmalloc(size, GFP_KERNEL);
> > 	else
> > 		bitmap = vmalloc(size);
> > 
> > 	if (!bitmap) {
> > 		udf_error(sb, __FUNCTION__,
> > 			  "Unable to allocate space for bitmap "
> > 			  "and %d buffer_head pointers", nr_groups);
> > 		return NULL;
> > 	}
> > 
> > 	memset(bitmap, 0, size);
> > 	bitmap->s_block_bitmap = (struct buffer_head **)(bitmap + 1);
> > 	bitmap->s_nr_groups = nr_groups;
> > 	return bitmap;
> > }
> Yep. This looks better.
> 
> > But even that is not quite optimal.  The nr_groups calculation should
> > probably move to a helper (I suspect it's used elsewhere too anyway),
> I will look for them. I've seen many weird calculations in udf code
> and I think it's a good idea to move some of them into helpers.
> 
> > and instead of using vmalloc for large allocations I'd rather split
> > the allocation of the bitmap from s->block_bitmap and use individual
> > smaller allocations.  But that latter part is probably better left
> > for a separate patch.
> So every struct buffer_head * in bitmap will be indexed by a pair of:
> idx >> ilog2(PAGE_SIZE / sizeof(struct buffer_head *))
> idx & ((PAGE_SIZE / sizeof(struct buffer_head *)) - 1)
> Am I right? Or did I misunderstand something?
> I don't know how big nr_groups could be, but it might still need vmalloc
> when array of pages won't fit on one page...
  Size of group is a number of bits in a block blocks. So in case of 1 KB
blocks a group has 8192 blocks => 8MB. Since you can fit 1024 pointers to a
page (or even only 512 in 64-bit arch), that gives you 8GB (or 4GB,
respectively). So it's likely that pointers won't fit one page.
Personally, I don't think removing the vmalloc is worth the trouble. At
least for now...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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