Possible bug in BTRFS w/ Duplication

[Heinrich Schuchardt]

On 12/28/22 21:51, Sam Winchenbach wrote:
> Hello,
>
> Hello, I have hit the following situation when trying to load files from a BTRFS partition with duplication enabled.
>
> In the first example I read a 16KiB file - __btrfs_map_block() changes the length to something larger than the file being read. This works fine, as length is later clamped to the file size.
>
> In the second example, __btrfs_map_block() changes the length parameter to something smaller than the file (the size of a stripe).  This seems to break this check here:
>
>      read = len;
>      num_copies = btrfs_num_copies(fs_info, logical, len);
>      for (i = 1; i <= num_copies; i++) {
>          ret = read_extent_data(fs_info, dest, logical, &read, i);
>          if (ret < 0 || read != len) {
>              continue;
>          }
>          finished = true;
>          break;
>      }
>
> The problem being that read is always less than len.
>
> I am not sure if __btrfs_map_block is changing "len" to the incorrect value, or if there is some logic in "read_extent_data" that isn't correct. Any pointers on how this code is supposed to work would be greatly appreciated.
> Thanks.

Thanks for reporting the issue

$ scripts/get_maintainer.pl -f fs/btrfs/volumes.c

suggests to include

"Marek Behún" <kabel at kernel.org> (maintainer:BTRFS)
Qu Wenruo <wqu at suse.com> (reviewer:BTRFS)
linux-btrfs at vger.kernel.org

to the communication.

Best regards

Heinrich

>
> === EXAMPLE 2 ===
> Zynq> load mmc 1:0 0 16K
> [btrfs_file_read,fs/btrfs/inode.c:710] === read the aligned part ===
> [btrfs_read_extent_reg,fs/btrfs/inode.c:458] before read_extent_data (ret = 0, read = 16384, len = 16384)
> [read_extent_data,fs/btrfs/disk-io.c:547] before __btrfs_map_block (len = 16384)
> [read_extent_data,fs/btrfs/disk-io.c:550] after __btrfs_map_block (len = 28672)
> [read_extent_data,fs/btrfs/disk-io.c:565] before __btrfs_devread (len = 16384)
> [read_extent_data,fs/btrfs/disk-io.c:568] after __btrfs_devread (len = 16384)
> [btrfs_read_extent_reg,fs/btrfs/inode.c:460] after read_extent_data (ret = 0, read = 16384, len = 16384)
> cur: 0, extent_num_bytes: 16384, aligned_end: 16384
> 16384 bytes read in 100 ms (159.2 KiB/s)
>
> === EXAMPLE 2 ===
> Zynq> load mmc 1:0 0 32K
> [btrfs_file_read,fs/btrfs/inode.c:710] === read the aligned part ===
> [btrfs_read_extent_reg,fs/btrfs/inode.c:458] before read_extent_data (ret = 0, read = 32768, len = 32768)
> [read_extent_data,fs/btrfs/disk-io.c:547] before __btrfs_map_block (len = 32768)
> [read_extent_data,fs/btrfs/disk-io.c:550] after __btrfs_map_block (len = 12288)
> [read_extent_data,fs/btrfs/disk-io.c:565] before __btrfs_devread (len = 12288)
> [read_extent_data,fs/btrfs/disk-io.c:568] after __btrfs_devread (len = 12288)
> [btrfs_read_extent_reg,fs/btrfs/inode.c:460] after read_extent_data (ret = 0, read = 12288, len = 32768)
> [btrfs_read_extent_reg,fs/btrfs/inode.c:458] before read_extent_data (ret = 0, read = 12288, len = 32768)
> [read_extent_data,fs/btrfs/disk-io.c:547] before __btrfs_map_block (len = 12288)
> [read_extent_data,fs/btrfs/disk-io.c:550] after __btrfs_map_block (len = 12288)
> [read_extent_data,fs/btrfs/disk-io.c:565] before __btrfs_devread (len = 12288)
> [read_extent_data,fs/btrfs/disk-io.c:568] after __btrfs_devread (len = 12288)
> [btrfs_read_extent_reg,fs/btrfs/inode.c:460] after read_extent_data (ret = 0, read = 12288, len = 32768)
> file: fs/btrfs/inode.c, line: 468
> cur: 0, extent_num_bytes: 32768, aligned_end: 32768
> -----> btrfs_read_extent_reg: -5, line: 758
> BTRFS: An error occurred while reading file 32K
> Failed to load '32K'
>
>
>
>
>
> Sam Winchenbach
> Embedded Software Engineer III
> Tethers Unlimited, Inc. | Connect Your Universe | www.tethers.com
> swinchenbach at tethers.com | C: 207-974-6934
> 11711 North Creek Pkwy # D113, Bothell, WA 98011-8808, USA