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 <g...@sphalerite.org>
Reviewed-by: Philippe Mathieu-Daudé <f4...@amsat.org>
Reviewed-by: Greg Kurz <gr...@kaod.org>
Message-Id: <20220908112353.289267-1-...@sphalerite.org>
[CS: - Retain BUG_ON(f->clunked) in get_fid().
- Add TODO comment in clunk_fid(). ]
Signed-off-by: Christian Schoenebeck <qemu_...@crudebyte.com>
---
This squashes the separately submitted patch "9pfs: avoid iterator
invalidation in v9fs_mark_fids_unreclaim"
(20220926124207.1325763-1-...@sphalerite.org) into the previous
version of this change.
I've skipped v4, because the former is arguably a poorly submitted v4.
I've also addressed Christian Schoenebeck's comments on the former:
* (v9fs_mark_fids_unreclaim) switched to g_autoptr for the array
storing the fids intermediately in preparation for reopening
* (v9fs_mark_fids_unreclaim) restored the accidentally removed
FID_NON_RECLAIMABLE mark
* (v9fs_mark_fids_unreclaim) moved fid reference release into a third
loop, which is run even if an error is encountered during a reopen
operation, in order to avoid leaking references to fids.
* (v9fs_reset) implemented logic to avoid the same iterator
invalidation problem
I've also added a comment explaining the exact role of
v9fs_mark_fids_unreclaim, since it's not entirely obvious at a glance.
hw/9pfs/9p.c | 188 ++++++++++++++++++++++++++++-----------------------
hw/9pfs/9p.h | 2 +-
2 files changed, 106 insertions(+), 84 deletions(-)
diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index aebadeaa03..0e485cb631 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -282,33 +282,32 @@ 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 +316,9 @@ 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 +329,7 @@ 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 +420,12 @@ 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 +435,15 @@ 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 +515,82 @@ 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
+ * reopening them happens asynchronously, allowing 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++;
+ 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);
+ fidp->flags |= FID_NON_RECLAIMABLE;
+ /* reopen the file/dir if already closed */
+ err = v9fs_reopen_fid(pdu, fidp);
+ if (err < 0) {
+ goto out;
+ }
}
- return 0;
+ out:
+ 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 +3216,8 @@ 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 +3251,13 @@ 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 +3335,8 @@ 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 +3353,8 @@ 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 +4244,7 @@ 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 +4304,10 @@ 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 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;
--
2.36.2
