libfs: Attempt exact-match comparison first during casefolded lookup
Casefolded comparisons are (obviously) way more costly than a simple memcmp. Try the case-sensitive comparison first, falling-back to the case-insensitive lookup only when needed. This allows any exact-match lookup to complete without having to walk the utf8 trie. Note that, for strict mode, generic_ci_d_compare used to reject an invalid UTF-8 string, which would now be considered valid if it exact-matches the disk-name. But, if that is the case, the filesystem is corrupt. More than that, it really doesn't matter in practice, because the name-under-lookup will have already been rejected by generic_ci_d_hash and we won't even get here. The memcmp is safe under RCU because we are operating on str/len instead of dentry->d_name directly, and the caller guarantees their consistency between each other in __d_lookup_rcu_op_compare. Link: https://lore.kernel.org/r/87ttn2sip7.fsf_-_@mailhost.krisman.be Suggested-by:Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by:
Gabriel Krisman Bertazi <krisman@suse.de>
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