Re: [PATCH v8 8/8] btrfs: remove buffer_heads form superblock mirror integrity checking

[David Sterba <dsterba@suse.cz>]

On Fri, Feb 14, 2020 at 12:24:36AM +0900, Johannes Thumshirn wrote:
> The integrity checking code for the superblock mirrors is the last remaining
> user of buffer_heads in BTRFS, change it to using plain BIOs as well.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
> 
> ---
> Changes to v7:
> - Use read_Cache_page_gfp()
> - Don't kmap() block device mappings (David)
> 
> Changes to v4:
> - Remove mapping_gfp_constraint()
> 
> Changes to v2:
> - Open-code kunmap() + put_page() (David)
> - Remove __GFP_NOFAIL from allocation (Josef)
> - Merge error paths (David)
> 
> Changes to v1:
> - Convert from alloc_page() to find_or_create_page()
> ---
>  fs/btrfs/check-integrity.c | 42 +++++++++++++++++++++-----------------
>  1 file changed, 23 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/btrfs/check-integrity.c b/fs/btrfs/check-integrity.c
> index 4f6db2fe482a..d8d915d7beda 100644
> --- a/fs/btrfs/check-integrity.c
> +++ b/fs/btrfs/check-integrity.c
> @@ -77,7 +77,6 @@
>  
>  #include <linux/sched.h>
>  #include <linux/slab.h>
> -#include <linux/buffer_head.h>
>  #include <linux/mutex.h>
>  #include <linux/genhd.h>
>  #include <linux/blkdev.h>
> @@ -762,29 +761,33 @@ static int btrfsic_process_superblock_dev_mirror(
>  	struct btrfs_fs_info *fs_info = state->fs_info;
>  	struct btrfs_super_block *super_tmp;
>  	u64 dev_bytenr;
> -	struct buffer_head *bh;
>  	struct btrfsic_block *superblock_tmp;
>  	int pass;
>  	struct block_device *const superblock_bdev = device->bdev;
> +	struct page *page;
> +	struct bio bio;
> +	struct bio_vec bio_vec;
> +	struct address_space *mapping = superblock_bdev->bd_inode->i_mapping;
> +	int ret;
>  
>  	/* super block bytenr is always the unmapped device bytenr */
>  	dev_bytenr = btrfs_sb_offset(superblock_mirror_num);
>  	if (dev_bytenr + BTRFS_SUPER_INFO_SIZE > device->commit_total_bytes)
>  		return -1;
> -	bh = __bread(superblock_bdev, dev_bytenr / BTRFS_BDEV_BLOCKSIZE,
> -		     BTRFS_SUPER_INFO_SIZE);
> -	if (NULL == bh)
> +
> +	page = reed_cache_page_gfp(mapping, dev_bytenr >> PAGE_SHIFT, GFP_NOFS);

Reed-Solomon error correction in page cache? Have I missed the news? :)

  CC [M]  fs/btrfs/check-integrity.o
fs/btrfs/check-integrity.c: In function ‘btrfsic_process_superblock_dev_mirror’:
fs/btrfs/check-integrity.c:778:9: error: implicit declaration of function ‘reed_cache_page_gfp’; did you mean ‘read_cache_page_gfp’? [-Werror=implicit-function-declaration]
  778 |  page = reed_cache_page_gfp(mapping, dev_bytenr >> PAGE_SHIFT, GFP_NOFS);
      |         ^~~~~~~~~~~~~~~~~~~
      |         read_cache_page_gfp

And with that fixed there are still the following warnings.

fs/btrfs/check-integrity.c:778:7: warning: assignment to ‘struct page *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
  778 |  page = reed_cache_page_gfp(mapping, dev_bytenr >> PAGE_SHIFT, GFP_NOFS);
      |       ^
fs/btrfs/check-integrity.c:769:17: warning: unused variable ‘bio_vec’ [-Wunused-variable]
  769 |  struct bio_vec bio_vec;
      |                 ^~~~~~~
fs/btrfs/check-integrity.c:768:13: warning: unused variable ‘bio’ [-Wunused-variable]
  768 |  struct bio bio;
      |             ^~~