Re: [PATCH v3 1/2] erofs: update on-disk format for xattr name filter

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2023/7/12 19:51, Jingbo Xu wrote:
> The xattr name bloom filter feature is going to be introduced to speed
> up the negative xattr lookup, e.g. system.posix_acl_[access|default]
> lookup when running "ls -lR" workload.
> 
> There are some commonly used extended attributes (n) and the total
> number of these is approximately 30.
> 
> 	trusted.overlay.opaque
> 	trusted.overlay.redirect
> 	trusted.overlay.origin
> 	trusted.overlay.impure
> 	trusted.overlay.nlink
> 	trusted.overlay.upper
> 	trusted.overlay.metacopy
> 	trusted.overlay.protattr
> 	user.overlay.opaque
> 	user.overlay.redirect
> 	user.overlay.origin
> 	user.overlay.impure
> 	user.overlay.nlink
> 	user.overlay.upper
> 	user.overlay.metacopy
> 	user.overlay.protattr
> 	security.evm
> 	security.ima
> 	security.selinux
> 	security.SMACK64
> 	security.SMACK64IPIN
> 	security.SMACK64IPOUT
> 	security.SMACK64EXEC
> 	security.SMACK64TRANSMUTE
> 	security.SMACK64MMAP
> 	security.apparmor
> 	security.capability
> 	system.posix_acl_access
> 	system.posix_acl_default
> 	user.mime_type
> 
> Given the number of bits of the bloom filter (m) is 32, the optimal
> value for the number of the hash functions (k) is 1 (ln2 * m/n = 0.74).
> 
> The single hash function is implemented as:
> 
> 	xxh32(name, strlen(name), EROFS_XATTR_FILTER_SEED + index)
> 
> where `index` represents the index of corresponding predefined short name
> prefix, while `name` represents the name string after stripping the above
> predefined name prefix.
> 
> The constant magic number EROFS_XATTR_FILTER_SEED, i.e. 0x25BBE08F, is
> used to give a better spread when mapping these 30 extended attributes
> into 32-bit bloom filter as:
> 
> 	bit  0: security.ima
> 	bit  1:
> 	bit  2: trusted.overlay.nlink
> 	bit  3:
> 	bit  4: user.overlay.nlink
> 	bit  5: trusted.overlay.upper
> 	bit  6: user.overlay.origin
> 	bit  7: trusted.overlay.protattr
> 	bit  8: security.apparmor
> 	bit  9: user.overlay.protattr
> 	bit 10: user.overlay.opaque
> 	bit 11: security.selinux
> 	bit 12: security.SMACK64TRANSMUTE
> 	bit 13: security.SMACK64
> 	bit 14: security.SMACK64MMAP
> 	bit 15: user.overlay.impure
> 	bit 16: security.SMACK64IPIN
> 	bit 17: trusted.overlay.redirect
> 	bit 18: trusted.overlay.origin
> 	bit 19: security.SMACK64IPOUT
> 	bit 20: trusted.overlay.opaque
> 	bit 21: system.posix_acl_default
> 	bit 22:
> 	bit 23: user.mime_type
> 	bit 24: trusted.overlay.impure
> 	bit 25: security.SMACK64EXEC
> 	bit 26: user.overlay.redirect
> 	bit 27: user.overlay.upper
> 	bit 28: security.evm
> 	bit 29: security.capability
> 	bit 30: system.posix_acl_access
> 	bit 31: trusted.overlay.metacopy, user.overlay.metacopy
> 
> h_name_filter is introduced to the on-disk per-inode xattr header to
> place the corresponding xattr name filter, where bit value 1 indicates
> non-existence for compatibility.
> 
> This feature is indicated by EROFS_FEATURE_COMPAT_XATTR_FILTER
> compatible feature bit.
> 
> Reserve one byte in on-disk superblock as the on-disk format for xattr
> name filter may change in the future.  With this flag we don't need
> bothering these compatible bits again at that time.
> 
> Suggested-by: Alexander Larsson <alexl@redhat.com>
> Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>

Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Thanks,
Gao Xiang