linux-kernel - Re: [RFC 1/2] erofs: update on-disk format for xattr bloom filter

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Date:   Tue, 27 Jun 2023 10:12:58 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Jingbo Xu <jefflexu@...ux.alibaba.com>, chao@...nel.org,
        huyue2@...lpad.com, linux-erofs@...ts.ozlabs.org
Cc:     linux-kernel@...r.kernel.org, alexl@...hat.com
Subject: Re: [RFC 1/2] erofs: update on-disk format for xattr bloom filter



On 2023/6/21 16:32, Jingbo Xu wrote:
> The xattr 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.
> 
> The number of common used xattr (n) is approximately 8, including
> system.[posix_acl_access|posix_acl_default], security.[capability|selinux]
> and security.[SMACK64|SMACK64TRANSMUTE|SMACK64EXEC|SMACK64MMAP].  Given the
> number of bits of the bloom filter (m) is 32, the optimal value for the
> number of the hash functions (k) is 2 (ln2 * m/n = 2.7).
> 
> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>
> ---
>   fs/erofs/erofs_fs.h | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
> index 2c7b16e340fe..9daea86cdb52 100644
> --- a/fs/erofs/erofs_fs.h
> +++ b/fs/erofs/erofs_fs.h
> @@ -13,6 +13,7 @@
>   
>   #define EROFS_FEATURE_COMPAT_SB_CHKSUM          0x00000001
>   #define EROFS_FEATURE_COMPAT_MTIME              0x00000002
> +#define EROFS_FEATURE_COMPAT_XATTR_BLOOM	0x00000003

#define EROFS_FEATURE_COMPAT_XATTR_BLOOM 0x00000004

>   
>   /*
>    * Any bits that aren't in EROFS_ALL_FEATURE_INCOMPAT should
> @@ -200,7 +201,7 @@ struct erofs_inode_extended {
>    * for read-only fs, no need to introduce h_refcount
>    */
>   struct erofs_xattr_ibody_header {
> -	__le32 h_reserved;
> +	__le32 h_map;	/* bloom filter, bit value 1 indicates not-present */

`map` here is too ambiguous, could we rename it as "h_name_filter"?

>   	__u8   h_shared_count;
>   	__u8   h_reserved2[7];
>   	__le32 h_shared_xattrs[];       /* shared xattr id array */
> @@ -221,6 +222,11 @@ struct erofs_xattr_ibody_header {
>   #define EROFS_XATTR_LONG_PREFIX		0x80
>   #define EROFS_XATTR_LONG_PREFIX_MASK	0x7f
>   
> +#define EROFS_XATTR_BLOOM_BITS		32
> +#define EROFS_XATTR_BLOOM_MASK		(EROFS_XATTR_BLOOM_BITS - 1)
> +#define EROFS_XATTR_BLOOM_DEFAULT	UINT32_MAX
> +#define EROFS_XATTR_BLOOM_COUNTS	2

could we rename them as EROFS_XATTR_NAME_FILTER_xxx?

Thanks,
Gao Xiang