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    SubjectRe: Word-at-a-time dcache name accesses (was Re: .. anybody know ofany filesystems that depend on the exact VFS 'namehash' implementation?)
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    A passer-by's mumble.

    We are discussing about systems with huge "struct dentry" instances where
    selecting a hash list based on "struct qstr"->hash cannot effectively narrow
    down candidates to compare, aren't we?

    Then, I thought we can reconstruct candidates in a hash list.

    For example, sort "struct dentry" based on return value of
    strcmp(dentry1->d_name.name, dentry2->d_name.name).

    For another example, embed another hash value (calculated using some algorithm
    that is different from the one used for "struct qstr"->hash) into the leading 1
    or 2 or 4 bytes of "struct dentry"->d_name.name (e.g.

    dentry1->d_name.name == { 0x12, 0x56, 0x34, 0x78, 'n', 'a', 'm', 'e', '1' }
    dentry2->d_name.name == { 0xab, 0xcd, 0xef, 0x01, 'n', 'a', 'm', 'e', '2' }
    for 32 bits hash case, where 0x12563478 is hash value for name1 and 0xabcdef01
    is hash value for name2) and compare the embedded hash (e.g.

    * (u32 *) dentry1->d_name.name == * (u32 *) dentry2->d_name.name

    ) before starting comparison of char array (e.g.

    memcmp(dentry1->d_name.name + 4, dentry2->d_name.name + 4, len)

    ).


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