9pfs: use GHashTable for fid table

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 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>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
[CS: - Retain BUG_ON(f->clunked) in get_fid().
     - Add TODO comment in clunk_fid(). ]
Message-Id: <20221004104121.713689-1-git@sphalerite.org>
[CS: - Drop unnecessary goto and out: label. ]
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

2 files changed, 112 insertions, 84 deletions

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index aebadeaa03..9bf13133e5 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -256,7 +256,8 @@ static size_t v9fs_string_size(V9fsString *str)
 }
 
 /*
- * returns 0 if fid got re-opened, 1 if not, < 0 on error */
+ * returns 0 if fid got re-opened, 1 if not, < 0 on error
+ */
 static int coroutine_fn v9fs_reopen_fid(V9fsPDU *pdu, V9fsFidState *f)
 {
     int err = 1;
@@ -282,33 +283,32 @@ static V9fsFidState *coroutine_fn get_fid(V9fsPDU *pdu, int32_t fid)
     V9fsFidState *f;
     V9fsState *s = pdu->s;
 
-    QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
+    f = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
+    if (f) {
         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;
+        /*
+         * 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 +317,11 @@ static V9fsFidState *alloc_fid(V9fsState *s, int32_t fid)
 {
     V9fsFidState *f;
 
-    QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
+    f = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
+    if (f) {
         /* 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 +332,7 @@ 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 +423,12 @@ 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;
-        }
+    /* TODO: Use g_hash_table_steal_extended() instead? */
+    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 +438,15 @@ 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, (gpointer *) &f)) {
         /*
          * Unlink fids cannot be reclaimed. Check
          * for them and skip them. Also skip fids
@@ -514,72 +518,85 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
     }
 }
 
+/*
+ * This is used when a path is removed from the directory tree. Any
+ * fids that still reference it must not be closed from then on, since
+ * they cannot be reopened.
+ */
 static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath *path)
 {
-    int err;
+    int err = 0;
     V9fsState *s = pdu->s;
-    V9fsFidState *fidp, *fidp_next;
+    V9fsFidState *fidp;
+    gpointer fid;
+    GHashTableIter iter;
+    /*
+     * The most common case is probably that we have exactly one
+     * fid for the given path, so preallocate exactly one.
+     */
+    g_autoptr(GArray) to_reopen = g_array_sized_new(FALSE, FALSE,
+            sizeof(V9fsFidState *), 1);
+    gint i;
 
-    fidp = QSIMPLEQ_FIRST(&s->fid_list);
-    if (!fidp) {
-        return 0;
-    }
+    g_hash_table_iter_init(&iter, s->fids);
 
     /*
-     * v9fs_reopen_fid() can yield : a reference on the fid must be held
-     * to ensure its pointer remains valid and we can safely pass it to
-     * QSIMPLEQ_NEXT(). The corresponding put_fid() can also yield so
-     * we must keep a reference on the next fid as well. So the logic here
-     * is to get a reference on a fid and only put it back during the next
-     * iteration after we could get a reference on the next fid. Start with
-     * the first one.
+     * We iterate over the fid table looking for the entries we need
+     * to reopen, and store them in to_reopen. This is because
+     * v9fs_reopen_fid() and put_fid() yield. This allows the fid table
+     * to be modified in the meantime, invalidating our iterator.
      */
-    for (fidp->ref++; fidp; fidp = fidp_next) {
+    while (g_hash_table_iter_next(&iter, &fid, (gpointer *) &fidp)) {
         if (fidp->path.size == path->size &&
             !memcmp(fidp->path.data, path->data, path->size)) {
-            /* Mark the fid non reclaimable. */
-            fidp->flags |= FID_NON_RECLAIMABLE;
-
-            /* reopen the file/dir if already closed */
-            err = v9fs_reopen_fid(pdu, fidp);
-            if (err < 0) {
-                put_fid(pdu, fidp);
-                return err;
-            }
-        }
-
-        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.
+             * Ensure the fid survives a potential clunk request during
+             * v9fs_reopen_fid or put_fid.
              */
-            fidp_next->ref++;
+            fidp->ref++;
+            fidp->flags |= FID_NON_RECLAIMABLE;
+            g_array_append_val(to_reopen, fidp);
         }
+    }
 
-        /* We're done with this fid */
-        put_fid(pdu, fidp);
+    for (i = 0; i < to_reopen->len; i++) {
+        fidp = g_array_index(to_reopen, V9fsFidState*, i);
+        /* reopen the file/dir if already closed */
+        err = v9fs_reopen_fid(pdu, fidp);
+        if (err < 0) {
+            break;
+        }
     }
 
-    return 0;
+    for (i = 0; i < to_reopen->len; i++) {
+        put_fid(pdu, g_array_index(to_reopen, V9fsFidState*, i));
+    }
+    return err;
 }
 
 static void coroutine_fn virtfs_reset(V9fsPDU *pdu)
 {
     V9fsState *s = pdu->s;
     V9fsFidState *fidp;
+    GList *freeing;
+    /*
+     * Get a list of all the values (fid states) in the table, which
+     * we then...
+     */
+    g_autoptr(GList) fids = g_hash_table_get_values(s->fids);
 
-    /* Free all fids */
-    while (!QSIMPLEQ_EMPTY(&s->fid_list)) {
-        /* Get fid */
-        fidp = QSIMPLEQ_FIRST(&s->fid_list);
-        fidp->ref++;
+    /* ... remove from the table, taking over ownership. */
+    g_hash_table_steal_all(s->fids);
 
-        /* Clunk fid */
-        QSIMPLEQ_REMOVE(&s->fid_list, fidp, V9fsFidState, next);
+    /*
+     * This allows us to release our references to them asynchronously without
+     * iterating over the hash table and risking iterator invalidation
+     * through concurrent modifications.
+     */
+    for (freeing = fids; freeing; freeing = freeing->next) {
+        fidp = freeing->data;
+        fidp->ref++;
         fidp->clunked = true;
-
         put_fid(pdu, fidp);
     }
 }
@@ -3205,6 +3222,8 @@ static int coroutine_fn v9fs_complete_rename(V9fsPDU *pdu, V9fsFidState *fidp,
     V9fsFidState *tfidp;
     V9fsState *s = pdu->s;
     V9fsFidState *dirfidp = NULL;
+    GHashTableIter iter;
+    gpointer fid;
 
     v9fs_path_init(&new_path);
     if (newdirfid != -1) {
@@ -3238,11 +3257,13 @@ 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, (gpointer *) &tfidp)) {
         if (v9fs_path_is_ancestor(&fidp->path, &tfidp->path)) {
             /* replace the name */
             v9fs_fix_path(&tfidp->path, &new_path, strlen(fidp->path.data));
@@ -3320,6 +3341,8 @@ static int coroutine_fn v9fs_fix_fid_paths(V9fsPDU *pdu, V9fsPath *olddir,
     V9fsPath oldpath, newpath;
     V9fsState *s = pdu->s;
     int err;
+    GHashTableIter iter;
+    gpointer fid;
 
     v9fs_path_init(&oldpath);
     v9fs_path_init(&newpath);
@@ -3336,7 +3359,8 @@ 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, (gpointer *) &tfidp)) {
         if (v9fs_path_is_ancestor(&oldpath, &tfidp->path)) {
             /* replace the name */
             v9fs_fix_path(&tfidp->path, &newpath, strlen(oldpath.data));
@@ -4226,7 +4250,7 @@ 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) {
@@ -4286,6 +4310,10 @@ void v9fs_device_unrealize_common(V9fsState *s)
     if (s->ctx.fst) {
         fsdev_throttle_cleanup(s->ctx.fst);
     }
+    if (s->fids) {
+        g_hash_table_destroy(s->fids);
+        s->fids = NULL;
+    }
     g_free(s->tag);
     qp_table_destroy(&s->qpd_table);
     qp_table_destroy(&s->qpp_table);
diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index a523ac34a9..2fce4140d1 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;