From df149f8640200d7bbc9e104ce600a88bb405bef9 Mon Sep 17 00:00:00 2001 From: Alexander Motin Date: Wed, 24 Apr 2024 16:47:36 -0400 Subject: [PATCH] Slightly improve dnode hash As I understand just for being less predictable dnode hash includes 8 bits of objset pointer, starting at 6. But since objset_t is more than 1KB in size, its allocations are likely aligned to 2KB, that means 11 lower bits provide no entropy. Just take the 8 bits starting from 11. Signed-off-by: Alexander Motin Sponsored by: iXsystems, Inc. --- module/zfs/dmu_objset.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/module/zfs/dmu_objset.c b/module/zfs/dmu_objset.c index 2ba26f68e398..5ea99f742810 100644 --- a/module/zfs/dmu_objset.c +++ b/module/zfs/dmu_objset.c @@ -400,10 +400,10 @@ dnode_hash(const objset_t *os, uint64_t obj) ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY); /* - * The low 6 bits of the pointer don't have much entropy, because - * the objset_t is larger than 2^6 bytes long. + * The lower 11 bits of the pointer don't have much entropy, because + * the objset_t is more than 1KB long and so likely aligned to 2KB. */ - crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (osv >> 6)) & 0xFF]; + crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (osv >> 11)) & 0xFF]; crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 0)) & 0xFF]; crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 8)) & 0xFF]; crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 16)) & 0xFF];