The EROFS on-disk format uses a tiny, plain metadata design that
prioritizes performance and minimizes complex inconsistencies against
common writable disk filesystems (almost all serious metadata
inconsistency cannot happen in well-designed immutable filesystems like
EROFS). EROFS deliberately avoids artificial design flaws to eliminate
serious security risks from untrusted remote sources by design,
although human-made implementation bugs can still happen sometimes.
Currently, there is no strict check to prevent compressed inodes,
especially LZ4-compressed inodes, from being read in plain filesystems.
Starting with erofs-utils 1.0 and Linux 5.3, LZ4_0PADDING sb feature
is automatically enabled for LZ4-compressed EROFS images to support
in-place decompression. Furthermore, since Linux 5.4 LTS is no longer
supported, we no longer need to handle ancient LZ4-compressed EROFS
images generated by erofs-utils prior to 1.0.
To formally distinguish different filesystem types for improved
security:
- Use the presence of LZ4_0PADDING or a non-zero
`dsb->u1.lz4_max_distance` as a marker for compressed filesystems
containing LZ4-compressed inodes only;
- For other algorithms, use `dsb->u1.available_compr_algs` bitmap.
Note: LZ4_0PADDING has been supported since Linux 5.4 (the first formal
kernel version), so exposing it via sysfs is no longer necessary and is
now deprecated (but remain it for five more years until 2031):
`dsb->u1` has been strictly non-zero for all EROFS images containing
compressed inodes starting with erofs-utils v1.3 and it is actually
a much better marker for compressed filesystems.
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
- Get rid of `sbi->opt.max_sync_decompress_pages` since it's fixed as
3 all the time;
- Add Z_EROFS_MAX_SYNC_DECOMPRESS_BYTES in bytes instead of in pages,
since for non-4K pages, 3-page limitation makes no sense;
- Move `sync_decompress` to sbi to avoid unexpected remount impact;
- Fold z_erofs_is_sync_decompress() into its caller;
- Better description of sysfs entry `sync_decompress`.
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
This patch supports to readahead more blocks in erofs_readdir(), it can
enhance readdir performance in large direcotry.
readdir test in a large directory which contains 12000 sub-files.
files_per_second
Before: 926385.54
After: 2380435.562
Meanwhile, let's introduces a new sysfs entry to control readahead
bytes to provide more flexible policy for readahead of readdir().
- location: /sys/fs/erofs/<disk>/dir_ra_bytes
- default value: 16384
- disable readahead: set the value to 0
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250721021352.2495371-1-chao@kernel.org
[ Gao Xiang: minor styling adjustment. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
