Re: [PATCH v2 4/4] btrfs: subpage: pack all subpage bitmaps into a larger bitmap

[David Sterba <dsterba@suse.cz>]

On Tue, Aug 17, 2021 at 04:43:56PM +0300, Nikolay Borisov wrote:
> 
> 
> On 17.08.21 г. 12:38, Qu Wenruo wrote:
> > Currently we use u16 bitmap to make 4k sectorsize work for 64K page
> > size.
> > 
> > But this u16 bitmap is not large enough to contain larger page size like
> > 128K, nor is space efficient for 16K page size.
> > 
> > To handle both cases, here we pack all subpage bitmaps into a larger
> > bitmap, now btrfs_subpage::bitmaps[] will be the ultimate bitmap for
> > subpage usage.
> > 
> > Each sub-bitmap will has its start bit number recorded in
> > btrfs_subpage_info::*_start, and its bitmap length will be recorded in
> > btrfs_subpage_info::bitmap_nr_bits.
> > 
> > All subpage bitmap operations will be converted from using direct u16
> > operations to bitmap operations, with above *_start calculated.
> > 
> > For 64K page size with 4K sectorsize, this should not cause much
> > difference.
> > 
> > While for 16K page size, we will only need 1 unsigned long (u32) to
> > store all the bitmaps, which saves quite some space.
> > 
> > Furthermore, this allows us to support larger page size like 128K and
> > 258K.
> > 
> > Signed-off-by: Qu Wenruo <wqu@suse.com>
> 
> <snip>
> 
> > +#define GANG_LOOKUP_SIZE	16
> >  static struct extent_buffer *get_next_extent_buffer(
> >  		struct btrfs_fs_info *fs_info, struct page *page, u64 bytenr)
> >  {
> > -	struct extent_buffer *gang[BTRFS_SUBPAGE_BITMAP_SIZE];
> > +	struct extent_buffer *gang[GANG_LOOKUP_SIZE];
> >  	struct extent_buffer *found = NULL;
> >  	u64 page_start = page_offset(page);
> > -	int ret;
> > -	int i;
> > +	u64 cur = page_start;
> >  
> >  	ASSERT(in_range(bytenr, page_start, PAGE_SIZE));
> > -	ASSERT(PAGE_SIZE / fs_info->nodesize <= BTRFS_SUBPAGE_BITMAP_SIZE);
> >  	lockdep_assert_held(&fs_info->buffer_lock);
> >  
> > -	ret = radix_tree_gang_lookup(&fs_info->buffer_radix, (void **)gang,
> > -			bytenr >> fs_info->sectorsize_bits,
> > -			PAGE_SIZE / fs_info->nodesize);
> > -	for (i = 0; i < ret; i++) {
> > -		/* Already beyond page end */
> > -		if (gang[i]->start >= page_start + PAGE_SIZE)
> > -			break;
> > -		/* Found one */
> > -		if (gang[i]->start >= bytenr) {
> > -			found = gang[i];
> > -			break;
> > +	while (cur < page_start + PAGE_SIZE) {
> > +		int ret;
> > +		int i;
> > +
> > +		ret = radix_tree_gang_lookup(&fs_info->buffer_radix,
> > +				(void **)gang, cur >> fs_info->sectorsize_bits,
> > +				min_t(unsigned int, GANG_LOOKUP_SIZE,
> > +				      PAGE_SIZE / fs_info->nodesize));
> > +		if (ret == 0)
> > +			goto out;
> > +		for (i = 0; i < ret; i++) {
> > +			/* Already beyond page end */
> > +			if (gang[i]->start >= page_start + PAGE_SIZE)
> 
> nit: this could be rewritten as (in_range(gang[i]->start, page_start,
> PAGE_SIZE)

Yes it could though I won't do that myself as it touches the logic and
code flow so I don't count it as trivial fixups I usually do. I'll fold
a fixup if you or Qu send it.