On 7.10.19 г. 22:37 ч., David Sterba wrote:
> To remove use of extent_map::bdev we need to find a replacement, and the
> latest_bdev is the only one we can use here, because inode::i_bdev and
> superblock::s_bdev are NULL.
>
> The only thing that DIO code uses from the bdev is the blocksize to
> perform alignment checks in do_blockdev_direct_IO, but we do them in
> btrfs code before any call to DIO. We can't pass NULL because there are
nit: This is not entirely correct. In fact map_bh in
do_blockdev_direct_IO gets filled in :
do_direct_IO
get_more_blocks
sdio->get_block() <-- this is btrfs_get_blocks_direct
Subsequently the map_bh->b_dev member is used in
clean_bdev_aliases and dio_new_bio to set the bio's bdev to that of the
buffer_head. However, because we have provided a submit function
dio_bio_submit calls our submission function and ignores the bdev.
Furthermore __blockdev_direct_IO already uses latest_bdev as per
btrfs_direct_IO.
> dereferences of bdev in __blockdev_direct_IO, even though it's not going
> to be used later.
>
> So it's safe to pass any valid bdev that's used within the filesystem.
>
> Signed-off-by: David Sterba <dste...@suse.com>
The code is ok so :
Reviewed-by: Nikolay Borisov <nbori...@suse.com>
> ---
> fs/btrfs/inode.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 067cbd6e3923..8e085d21c3c5 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -7603,6 +7603,8 @@ static int btrfs_get_blocks_direct_read(struct
> extent_map *em,
> struct inode *inode,
> u64 start, u64 len)
> {
> + struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> +
> if (em->block_start == EXTENT_MAP_HOLE ||
> test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
> return -ENOENT;
> @@ -7612,7 +7614,7 @@ static int btrfs_get_blocks_direct_read(struct
> extent_map *em,
> bh_result->b_blocknr = (em->block_start + (start - em->start)) >>
> inode->i_blkbits;
> bh_result->b_size = len;
> - bh_result->b_bdev = em->bdev;
> + bh_result->b_bdev = fs_info->fs_devices->latest_bdev;
> set_buffer_mapped(bh_result);
>
> return 0;
> @@ -7695,7 +7697,7 @@ static int btrfs_get_blocks_direct_write(struct
> extent_map **map,
> bh_result->b_blocknr = (em->block_start + (start - em->start)) >>
> inode->i_blkbits;
> bh_result->b_size = len;
> - bh_result->b_bdev = em->bdev;
> + bh_result->b_bdev = fs_info->fs_devices->latest_bdev;
> set_buffer_mapped(bh_result);
>
> if (!test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
>
