9p: stat_to_qid: implement slow path

stat_to_qid attempts via qid_path_prefixmap to map unique files (which are
identified by 64 bit inode nr and 32 bit device id) to a 64 QID path value.
However this implementation makes some assumptions about inode number
generation on the host.

If qid_path_prefixmap fails, we still have 48 bits available in the QID
path to fall back to a less memory efficient full mapping.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
[CS: - Rebased to https://github.com/gkurz/qemu/commits/9p-next
       (SHA1 7fc4c49e91).
     - Updated hash calls to new xxhash API.
     - Removed unnecessary parantheses in qpf_lookup_func().
     - Removed unnecessary g_malloc0() result checks.
     - Log error message when running out of prefixes in
       qid_path_fullmap().
     - Log warning message about potential degraded performance in
       qid_path_prefixmap().
     - Wrapped qpf_table initialization to dedicated qpf_table_init()
       function.
     - Fixed typo in comment. ]
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Signed-off-by: Greg Kurz <groug@kaod.org>

hw/9pfs/9p.c

@@ -579,23 +579,34 @@ static uint32_t qpp_hash(QppEntry e)
     return qemu_xxhash7(e.ino_prefix, e.dev, 0, 0, 0);
 }
 
+static uint32_t qpf_hash(QpfEntry e)
+{
+    return qemu_xxhash7(e.ino, e.dev, 0, 0, 0);
+}
+
 static bool qpp_lookup_func(const void *obj, const void *userp)
 {
     const QppEntry *e1 = obj, *e2 = userp;
     return e1->dev == e2->dev && e1->ino_prefix == e2->ino_prefix;
 }
 
-static void qpp_table_remove(void *p, uint32_t h, void *up)
+static bool qpf_lookup_func(const void *obj, const void *userp)
+{
+    const QpfEntry *e1 = obj, *e2 = userp;
+    return e1->dev == e2->dev && e1->ino == e2->ino;
+}
+
+static void qp_table_remove(void *p, uint32_t h, void *up)
 {
     g_free(p);
 }
 
-static void qpp_table_destroy(struct qht *ht)
+static void qp_table_destroy(struct qht *ht)
 {
     if (!ht || !ht->map) {
         return;
     }
-    qht_iter(ht, qpp_table_remove, NULL);
+    qht_iter(ht, qp_table_remove, NULL);
     qht_destroy(ht);
 }
 
@@ -604,6 +615,50 @@ static void qpp_table_init(struct qht *ht)
     qht_init(ht, qpp_lookup_func, 1, QHT_MODE_AUTO_RESIZE);
 }
 
+static void qpf_table_init(struct qht *ht)
+{
+    qht_init(ht, qpf_lookup_func, 1 << 16, QHT_MODE_AUTO_RESIZE);
+}
+
+static int qid_path_fullmap(V9fsPDU *pdu, const struct stat *stbuf,
+                            uint64_t *path)
+{
+    QpfEntry lookup = {
+        .dev = stbuf->st_dev,
+        .ino = stbuf->st_ino
+    }, *val;
+    uint32_t hash = qpf_hash(lookup);
+
+    /* most users won't need the fullmap, so init the table lazily */
+    if (!pdu->s->qpf_table.map) {
+        qpf_table_init(&pdu->s->qpf_table);
+    }
+
+    val = qht_lookup(&pdu->s->qpf_table, &lookup, hash);
+
+    if (!val) {
+        if (pdu->s->qp_fullpath_next == 0) {
+            /* no more files can be mapped :'( */
+            error_report_once(
+                "9p: No more prefixes available for remapping inodes from "
+                "host to guest."
+            );
+            return -ENFILE;
+        }
+
+        val = g_malloc0(sizeof(QppEntry));
+        *val = lookup;
+
+        /* new unique inode and device combo */
+        val->path = pdu->s->qp_fullpath_next++;
+        pdu->s->qp_fullpath_next &= QPATH_INO_MASK;
+        qht_insert(&pdu->s->qpf_table, val, hash, NULL);
+    }
+
+    *path = val->path;
+    return 0;
+}
+
 /*
  * stat_to_qid needs to map inode number (64 bits) and device id (32 bits)
  * to a unique QID path (64 bits). To avoid having to map and keep track
@@ -629,9 +684,8 @@ static int qid_path_prefixmap(V9fsPDU *pdu, const struct stat *stbuf,
     if (!val) {
         if (pdu->s->qp_prefix_next == 0) {
             /* we ran out of prefixes */
-            error_report_once(
-                "9p: No more prefixes available for remapping inodes from "
-                "host to guest."
+            warn_report_once(
+                "9p: Potential degraded performance of inode remapping"
             );
             return -ENFILE;
         }
@@ -656,6 +710,10 @@ static int stat_to_qid(V9fsPDU *pdu, const struct stat *stbuf, V9fsQID *qidp)
     if (pdu->s->ctx.export_flags & V9FS_REMAP_INODES) {
         /* map inode+device to qid path (fast path) */
         err = qid_path_prefixmap(pdu, stbuf, &qidp->path);
+        if (err == -ENFILE) {
+            /* fast path didn't work, fall back to full map */
+            err = qid_path_fullmap(pdu, stbuf, &qidp->path);
+        }
         if (err) {
             return err;
         }
@@ -3820,6 +3878,7 @@ int v9fs_device_realize_common(V9fsState *s, const V9fsTransport *t,
 
     qpp_table_init(&s->qpp_table);
     s->qp_prefix_next = 1; /* reserve 0 to detect overflow */
+    s->qp_fullpath_next = 1;
 
     s->ctx.fst = &fse->fst;
     fsdev_throttle_init(s->ctx.fst);
@@ -3842,7 +3901,8 @@ void v9fs_device_unrealize_common(V9fsState *s, Error **errp)
         fsdev_throttle_cleanup(s->ctx.fst);
     }
     g_free(s->tag);
-    qpp_table_destroy(&s->qpp_table);
+    qp_table_destroy(&s->qpp_table);
+    qp_table_destroy(&s->qpf_table);
     g_free(s->ctx.fs_root);
 }
 

hw/9pfs/9p.h

@@ -245,6 +245,13 @@ typedef struct {
     uint16_t qp_prefix;
 } QppEntry;
 
+/* QID path full entry, as above */
+typedef struct {
+    dev_t dev;
+    ino_t ino;
+    uint64_t path;
+} QpfEntry;
+
 struct V9fsState
 {
     QLIST_HEAD(, V9fsPDU) free_list;
@@ -268,7 +275,9 @@ struct V9fsState
     V9fsQID root_qid;
     dev_t dev_id;
     struct qht qpp_table;
+    struct qht qpf_table;
     uint16_t qp_prefix_next;
+    uint64_t qp_fullpath_next;
 };
 
 /* 9p2000.L open flags */