[PATCH] 9pfs: use GHashMap for fid table

[Linus Heckemann]

The previous implementation would iterate over the fid table for
lookup operations, resulting in an operation with O(n) complexity on
the number of open files and poor cache locality -- for nearly every
open, stat, read, write, etc operation.

This change uses a hashtable for this instead, significantly improving
the performance of the 9p filesystem. The runtime of NixOS's simple
installer test, which copies ~122k files totalling ~1.8GiB from 9p,
decreased by a factor of about 10.

Signed-off-by: Linus Heckemann <git@sphalerite.org>
---
 hw/9pfs/9p.c | 130 +++++++++++++++++++++++++++------------------------
 hw/9pfs/9p.h |   2 +-
 2 files changed, 69 insertions(+), 63 deletions(-)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index aebadeaa03..ff466afe39 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -282,33 +282,31 @@ static V9fsFidState *coroutine_fn get_fid(V9fsPDU *pdu, 
int32_t fid)
     V9fsFidState *f;
     V9fsState *s = pdu->s;
 
-    QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
-        BUG_ON(f->clunked);
-        if (f->fid == fid) {
-            /*
-             * Update the fid ref upfront so that
-             * we don't get reclaimed when we yield
-             * in open later.
-             */
-            f->ref++;
-            /*
-             * check whether we need to reopen the
-             * file. We might have closed the fd
-             * while trying to free up some file
-             * descriptors.
-             */
-            err = v9fs_reopen_fid(pdu, f);
-            if (err < 0) {
-                f->ref--;
-                return NULL;
-            }
-            /*
-             * Mark the fid as referenced so that the LRU
-             * reclaim won't close the file descriptor
-             */
-            f->flags |= FID_REFERENCED;
-            return f;
+    f = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
+    if (f) {
+        /*
+         * Update the fid ref upfront so that
+         * we don't get reclaimed when we yield
+         * in open later.
+         */
+        f->ref++;
+        /*
+         * check whether we need to reopen the
+         * file. We might have closed the fd
+         * while trying to free up some file
+         * descriptors.
+         */
+        err = v9fs_reopen_fid(pdu, f);
+        if (err < 0) {
+            f->ref--;
+            return NULL;
         }
+        /*
+         * Mark the fid as referenced so that the LRU
+         * reclaim won't close the file descriptor
+         */
+        f->flags |= FID_REFERENCED;
+        return f;
     }
     return NULL;
 }
@@ -317,12 +315,9 @@ static V9fsFidState *alloc_fid(V9fsState *s, int32_t fid)
 {
     V9fsFidState *f;
 
-    QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
+    if (g_hash_table_contains(s->fids, GINT_TO_POINTER(fid))) {
         /* If fid is already there return NULL */
-        BUG_ON(f->clunked);
-        if (f->fid == fid) {
-            return NULL;
-        }
+        return NULL;
     }
     f = g_new0(V9fsFidState, 1);
     f->fid = fid;
@@ -333,7 +328,7 @@ static V9fsFidState *alloc_fid(V9fsState *s, int32_t fid)
      * reclaim won't close the file descriptor
      */
     f->flags |= FID_REFERENCED;
-    QSIMPLEQ_INSERT_TAIL(&s->fid_list, f, next);
+    g_hash_table_insert(s->fids, GINT_TO_POINTER(fid), f);
 
     v9fs_readdir_init(s->proto_version, &f->fs.dir);
     v9fs_readdir_init(s->proto_version, &f->fs_reclaim.dir);
@@ -424,12 +419,11 @@ static V9fsFidState *clunk_fid(V9fsState *s, int32_t fid)
 {
     V9fsFidState *fidp;
 
-    QSIMPLEQ_FOREACH(fidp, &s->fid_list, next) {
-        if (fidp->fid == fid) {
-            QSIMPLEQ_REMOVE(&s->fid_list, fidp, V9fsFidState, next);
-            fidp->clunked = true;
-            return fidp;
-        }
+    fidp = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
+    if (fidp) {
+        g_hash_table_remove(s->fids, GINT_TO_POINTER(fid));
+        fidp->clunked = true;
+        return fidp;
     }
     return NULL;
 }
@@ -439,10 +433,15 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
     int reclaim_count = 0;
     V9fsState *s = pdu->s;
     V9fsFidState *f;
+
+    GHashTableIter iter;
+    gpointer fid;
+    g_hash_table_iter_init(&iter, s->fids);
+
     QSLIST_HEAD(, V9fsFidState) reclaim_list =
         QSLIST_HEAD_INITIALIZER(reclaim_list);
 
-    QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
+    while (g_hash_table_iter_next(&iter, &fid, (void **) &f)) {
         /*
          * Unlink fids cannot be reclaimed. Check
          * for them and skip them. Also skip fids
@@ -518,12 +517,12 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU 
*pdu, V9fsPath *path)
 {
     int err;
     V9fsState *s = pdu->s;
-    V9fsFidState *fidp, *fidp_next;
+    V9fsFidState *fidp;
+    gpointer fid;
+
+    GHashTableIter iter;
+    g_hash_table_iter_init(&iter, s->fids);
 
-    fidp = QSIMPLEQ_FIRST(&s->fid_list);
-    if (!fidp) {
-        return 0;
-    }
 
     /*
      * v9fs_reopen_fid() can yield : a reference on the fid must be held
@@ -534,7 +533,13 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU 
*pdu, V9fsPath *path)
      * iteration after we could get a reference on the next fid. Start with
      * the first one.
      */
-    for (fidp->ref++; fidp; fidp = fidp_next) {
+    while (g_hash_table_iter_next(&iter, &fid, (void **) &fidp)) {
+        /*
+         * Ensure the fid survives a potential clunk request during
+         * put_fid() below and v9fs_reopen_fid() in the next iteration.
+         */
+        fidp->ref++;
+
         if (fidp->path.size == path->size &&
             !memcmp(fidp->path.data, path->data, path->size)) {
             /* Mark the fid non reclaimable. */
@@ -548,16 +553,6 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU 
*pdu, V9fsPath *path)
             }
         }
 
-        fidp_next = QSIMPLEQ_NEXT(fidp, next);
-
-        if (fidp_next) {
-            /*
-             * Ensure the next fid survives a potential clunk request during
-             * put_fid() below and v9fs_reopen_fid() in the next iteration.
-             */
-            fidp_next->ref++;
-        }
-
         /* We're done with this fid */
         put_fid(pdu, fidp);
     }
@@ -570,18 +565,20 @@ static void coroutine_fn virtfs_reset(V9fsPDU *pdu)
     V9fsState *s = pdu->s;
     V9fsFidState *fidp;
 
+    gpointer fid;
+    GHashTableIter iter;
+    g_hash_table_iter_init(&iter, s->fids);
+
     /* Free all fids */
-    while (!QSIMPLEQ_EMPTY(&s->fid_list)) {
-        /* Get fid */
-        fidp = QSIMPLEQ_FIRST(&s->fid_list);
+    while (g_hash_table_iter_next(&iter, &fid, (void **) &fidp)) {
         fidp->ref++;
 
         /* Clunk fid */
-        QSIMPLEQ_REMOVE(&s->fid_list, fidp, V9fsFidState, next);
         fidp->clunked = true;
 
         put_fid(pdu, fidp);
     }
+    g_hash_table_remove_all(s->fids);
 }
 
 #define P9_QID_TYPE_DIR         0x80
@@ -3206,6 +3203,9 @@ static int coroutine_fn v9fs_complete_rename(V9fsPDU 
*pdu, V9fsFidState *fidp,
     V9fsState *s = pdu->s;
     V9fsFidState *dirfidp = NULL;
 
+    GHashTableIter iter;
+    gpointer fid;
+
     v9fs_path_init(&new_path);
     if (newdirfid != -1) {
         dirfidp = get_fid(pdu, newdirfid);
@@ -3238,11 +3238,13 @@ static int coroutine_fn v9fs_complete_rename(V9fsPDU 
*pdu, V9fsFidState *fidp,
     if (err < 0) {
         goto out;
     }
+
     /*
      * Fixup fid's pointing to the old name to
      * start pointing to the new name
      */
-    QSIMPLEQ_FOREACH(tfidp, &s->fid_list, next) {
+    g_hash_table_iter_init(&iter, s->fids);
+    while (g_hash_table_iter_next(&iter, &fid, (void **) &tfidp)) {
         if (v9fs_path_is_ancestor(&fidp->path, &tfidp->path)) {
             /* replace the name */
             v9fs_fix_path(&tfidp->path, &new_path, strlen(fidp->path.data));
@@ -3321,6 +3323,9 @@ static int coroutine_fn v9fs_fix_fid_paths(V9fsPDU *pdu, 
V9fsPath *olddir,
     V9fsState *s = pdu->s;
     int err;
 
+    GHashTableIter iter;
+    gpointer fid;
+
     v9fs_path_init(&oldpath);
     v9fs_path_init(&newpath);
     err = v9fs_co_name_to_path(pdu, olddir, old_name->data, &oldpath);
@@ -3336,7 +3341,8 @@ static int coroutine_fn v9fs_fix_fid_paths(V9fsPDU *pdu, 
V9fsPath *olddir,
      * Fixup fid's pointing to the old name to
      * start pointing to the new name
      */
-    QSIMPLEQ_FOREACH(tfidp, &s->fid_list, next) {
+    g_hash_table_iter_init(&iter, s->fids);
+    while (g_hash_table_iter_next(&iter, &fid, (void **) &tfidp)) {
         if (v9fs_path_is_ancestor(&oldpath, &tfidp->path)) {
             /* replace the name */
             v9fs_fix_path(&tfidp->path, &newpath, strlen(oldpath.data));
@@ -4226,7 +4232,7 @@ int v9fs_device_realize_common(V9fsState *s, const 
V9fsTransport *t,
     s->ctx.fmode = fse->fmode;
     s->ctx.dmode = fse->dmode;
 
-    QSIMPLEQ_INIT(&s->fid_list);
+    s->fids = g_hash_table_new(NULL, NULL);
     qemu_co_rwlock_init(&s->rename_lock);
 
     if (s->ops->init(&s->ctx, errp) < 0) {
diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index 994f952600..10fd2076c2 100644
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -339,7 +339,7 @@ typedef struct {
 struct V9fsState {
     QLIST_HEAD(, V9fsPDU) free_list;
     QLIST_HEAD(, V9fsPDU) active_list;
-    QSIMPLEQ_HEAD(, V9fsFidState) fid_list;
+    GHashTable *fids;
     FileOperations *ops;
     FsContext ctx;
     char *tag;
-- 
2.36.0