mmap_cache: Use two chains for hash collision.

struct sss_mc_rec had two hash members (hash1 and hash2) but only one next
member. This was a big problem in case of higher probability of hash collision.

structure sss_mc_rec will have two next members (next1, next2) with this patch.
next1 is related to hash1 and next2 is related to hash1.

Iterating over chains is changed, because we need to choose right next pointer.
Right next pointer will be chosen after comparing record hashes.
This behaviour is wrapped in function sss_mc_next_slot_with_hash.

Adding new record to chain is also changed. The situation is very similar to
iterating. We need to choose right next pointer (next1 or next2).
Right next pointer will be chosen after comparing record hashes.
Adding reference to next slot is wrapped in function
sss_mc_chain_slot_to_record_with_hash

Size of structure sss_mc_rec was increased from 32 bytes to 40 bytes.

Resolves:
https://fedorahosted.org/sssd/ticket/2049

1 files changed, 12 insertions, 8 deletions

diff --git a/src/util/mmap_cache.h b/src/util/mmap_cache.h
index 7c6693ac8..81269fe7e 100644
--- a/src/util/mmap_cache.h
+++ b/src/util/mmap_cache.h
@@ -53,15 +53,15 @@ typedef uint32_t rel_ptr_t;
 #define MC_INVALID_VAL MC_INVALID_VAL32
 
 /*
- * 32 seem a good compromise for slot size
+ * 40 seem a good compromise for slot size
  * 4 blocks are enough for the average passwd entry of 42 bytes
- * passwd records have 84 bytes of overhead, 128 - 82 = 46 bytes
- * 3 blocks can contain a very minimal entry, 96 - 82 = 14 bytes
+ * passwd records have 84 bytes of overhead, 160 - 82 = 78 bytes
+ * 3 blocks can contain a very minimal entry, 120 - 82 = 38 bytes
  *
  * 3 blocks are enough for groups w/o users (private user groups)
- * group records have 68 bytes of overhead, 96 - 66 = 30 bytes
+ * group records have 68 bytes of overhead, 120 - 66 = 54 bytes
  */
-#define MC_SLOT_SIZE 32
+#define MC_SLOT_SIZE 40
 #define MC_SIZE_TO_SLOTS(len) (((len) + (MC_SLOT_SIZE - 1)) / MC_SLOT_SIZE)
 #define MC_PTR_TO_SLOT(base, ptr) (MC_PTR_DIFF(ptr, base) / MC_SLOT_SIZE)
 #define MC_SLOT_TO_PTR(base, slot, type) \
@@ -78,8 +78,8 @@ typedef uint32_t rel_ptr_t;
                             - MC_PTR_DIFF(rec, (mc_ctx)->data_table))))
 
 
-#define SSS_MC_MAJOR_VNO    0
-#define SSS_MC_MINOR_VNO    4
+#define SSS_MC_MAJOR_VNO    1
+#define SSS_MC_MINOR_VNO    0
 
 #define SSS_MC_HEADER_UNINIT    0   /* after ftruncate or before reset */
 #define SSS_MC_HEADER_ALIVE     1   /* current and in use */
@@ -106,9 +106,13 @@ struct sss_mc_rec {
     uint32_t b1;            /* barrier 1 */
     uint32_t len;           /* total record length including record data */
     uint64_t expire;        /* record expiration time (cast to time_t) */
-    rel_ptr_t next;         /* ptr of next record rel to data_table */
+    rel_ptr_t next1;        /* ptr of next record rel to data_table */
+                            /* next1 is related to hash1 */
+    rel_ptr_t next2;        /* ptr of next record rel to data_table */
+                            /* next2 is related to hash2 */
     uint32_t hash1;         /* val of first hash (usually name of record) */
     uint32_t hash2;         /* val of second hash (usually id of record) */
+    uint32_t padding;       /* padding & reserved for future changes */
     uint32_t b2;            /* barrier 2 - 32 bytes mark, fits a slot */
     char data[0];
 };