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

[Nikolay Borisov <nborisov@suse.com>]

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)

> +				goto out;
> +			/* Found one */
> +			if (gang[i]->start >= bytenr) {
> +				found = gang[i];
> +				goto out;
> +			}
>  		}
> +		cur = gang[ret - 1]->start + gang[ret - 1]->len;
>  	}
> +out:
>  	return found;
>  }
>  

<snip>