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Message-ID: <20220818131928.6zymceds5ka7hg4u@wittgenstein>
Date: Thu, 18 Aug 2022 15:19:28 +0200
From: Christian Brauner <brauner@...nel.org>
To: Vasily Averin <vvs@...nvz.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, mkoutny@...e.com,
tj@...nel.org, gregkh@...uxfoundation.org, hannes@...xchg.org,
kernel@...nvz.org, linux-kernel@...r.kernel.org, mhocko@...e.com,
shakeelb@...gle.com, songmuchun@...edance.com,
viro@...iv.linux.org.uk
Subject: Re: [RFC PATCH] simple_xattr: switch from list to rb_tree
On Thu, Aug 18, 2022 at 12:12:30PM +0300, Vasily Averin wrote:
> The patch was announced here:
> https://lore.kernel.org/all/62188f37-f816-08e9-cdd5-8df23131746d@openvz.org/
> "5) simple_xattrs: replace list to rb-tree
> This significantly reduces the search time for existing entries."
>
> It was compiled but was not tested yet.
> ---
> Currently simple_xattr uses a list to store existing entries.
> If the list grows, the presence check may be slow and potentially
> lead to problems. Red-black tree should work more efficiently
> in this situation.
>
> This patch replaces list to rb_tree and switches simple_xattr_* calls
> to its using.
>
> Signed-off-by: Vasily Averin <vvs@...nvz.org>
> ---
I think the background for the performance issues in the commit message
would be helpful and I have a few comments. Also, trying to test whether the
lockups are gone due to the rbtree switch would be +1.
This will likely conflict with some acl/xattr changes I have lined up so
if we decide to proceed I wouldn't mind dealing with this series if
there are no objections.
> fs/xattr.c | 109 ++++++++++++++++++++++++++++++++----------
> include/linux/xattr.h | 13 +++--
> 2 files changed, 92 insertions(+), 30 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 6401703707f2..672f2214fcfd 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -1021,6 +1021,60 @@ struct simple_xattr *simple_xattr_alloc(const void *value, size_t size)
> return new_xattr;
> }
>
> +static struct simple_xattr *simple_xattr_rb_search(struct rb_root *root,
> + const char* name)
> +{
> + struct rb_node **new = &(root->rb_node), *parent = NULL;
I'd suggest to not name this "new" but rather just "cur" or "node".
> +
> + /* Figure out where to put new node */
> + while (*new)
> + {
nit: that "{" should be on the same line as the while
> + struct simple_xattr *xattr;
> + int result;
> +
> + xattr = container_of(*new, struct simple_xattr, node);
> + result = strcmp(xattr->name, name);
> +
> + parent = *new;
That variable and assignment seems unnecessary?
> + if (result < 0)
> + new = &((*new)->rb_left);
> + else if (result > 0)
> + new = &((*new)->rb_right);
> + else
> + return xattr;
> + }
> + return NULL;
> +}
> +
> +static bool simple_xattr_rb_insert(struct rb_root *root,
> + struct simple_xattr *new_xattr)
> +{
> + struct rb_node **new = &(root->rb_node), *parent = NULL;
> +
> + /* Figure out where to put new node */
> + while (*new) {
> + struct simple_xattr *xattr;
> + int result;
> +
> + xattr = container_of(*new, struct simple_xattr, node);
> + result = strcmp(xattr->name, new_xattr->name);
> +
> + parent = *new;
> + if (result < 0)
> + new = &((*new)->rb_left);
> + else if (result > 0)
> + new = &((*new)->rb_right);
> + else
> + return false;
> + }
> +
> + /* Add new node and rebalance tree. */
> + rb_link_node(&new_xattr->node, parent, new);
> + rb_insert_color(&new_xattr->node, root);
> +
> + return true;
> +}
> +
> /*
> * xattr GET operation for in-memory/pseudo filesystems
> */
> @@ -1031,10 +1085,8 @@ int simple_xattr_get(struct simple_xattrs *xattrs, const char *name,
> int ret = -ENODATA;
>
> spin_lock(&xattrs->lock);
> - list_for_each_entry(xattr, &xattrs->head, list) {
> - if (strcmp(name, xattr->name))
> - continue;
> -
> + xattr = simple_xattr_rb_search(&xattrs->rb_root, name);
> + if (xattr) {
> ret = xattr->size;
> if (buffer) {
> if (size < xattr->size)
> @@ -1042,7 +1094,6 @@ int simple_xattr_get(struct simple_xattrs *xattrs, const char *name,
> else
> memcpy(buffer, xattr->value, xattr->size);
> }
> - break;
> }
> spin_unlock(&xattrs->lock);
> return ret;
> @@ -1067,7 +1118,7 @@ int simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
> const void *value, size_t size, int flags,
> ssize_t *removed_size)
> {
> - struct simple_xattr *xattr;
> + struct simple_xattr *xattr = NULL;
> struct simple_xattr *new_xattr = NULL;
> int err = 0;
>
> @@ -1088,31 +1139,36 @@ int simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
> }
>
> spin_lock(&xattrs->lock);
> - list_for_each_entry(xattr, &xattrs->head, list) {
> - if (!strcmp(name, xattr->name)) {
> - if (flags & XATTR_CREATE) {
> - xattr = new_xattr;
> - err = -EEXIST;
> - } else if (new_xattr) {
> - list_replace(&xattr->list, &new_xattr->list);
> + if ((flags & XATTR_CREATE) && new_xattr) {
> + /* create new */
> + if (!simple_xattr_rb_insert(&xattrs->rb_root, new_xattr)) {
> + /* already exist */
> + xattr = new_xattr;
> + err = -EEXIST;
> + }
> + } else {
> + /* replace or remove */
> + xattr = simple_xattr_rb_search(&xattrs->rb_root, name);
> + if (xattr) {
> + /* found */
> + if (!new_xattr) {
> + /* remove existing */
> + rb_erase(&xattr->node, &xattrs->rb_root);
> if (removed_size)
> *removed_size = xattr->size;
> } else {
> - list_del(&xattr->list);
> + /* replace existing */
> + rb_replace_node(&xattr->node, &new_xattr->node,
> + &xattrs->rb_root);
> if (removed_size)
> *removed_size = xattr->size;
> }
> - goto out;
> + } else if (new_xattr) {
> + /* not found, incorrect replace */
> + xattr = new_xattr;
> + err = -ENODATA;
> }
> }
> - if (flags & XATTR_REPLACE) {
I think keeping this rather close to the original code might be nicer.
I find the code more difficult to follow afterwards. So how about
(COMPLETELY UNTESTED) sm like:
@@ -1077,30 +1139,40 @@ int simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
}
spin_lock(&xattrs->lock);
- list_for_each_entry(xattr, &xattrs->head, list) {
- if (!strcmp(name, xattr->name)) {
- if (flags & XATTR_CREATE) {
- xattr = new_xattr;
- err = -EEXIST;
- } else if (new_xattr) {
- list_replace(&xattr->list, &new_xattr->list);
- if (removed_size)
- *removed_size = xattr->size;
- } else {
- list_del(&xattr->list);
- if (removed_size)
- *removed_size = xattr->size;
- }
- goto out;
+ /* Find any matching xattr by name. */
+ xattr = simple_xattr_rb_search(&xattrs->rb_root, name);
+ if (xattr) {
+ if (flags & XATTR_CREATE) {
+ /* Creating request but the xattr already existed. */
+ xattr = new_xattr;
+ err = -EEXIST;
+ } else if (new_xattr) {
+ /* Replace the existing xattr. */
+ rb_replace_node(&xattr->node, &new_xattr->node,
+ &xattrs->rb_root);
+ if (removed_size)
+ *removed_size = xattr->size;
+ } else {
+ /* No new xattr specified so wipe the existing xattr. */
+ rb_erase(&xattr->node, &xattrs->rb_root);
+ if (removed_size)
+ *removed_size = xattr->size;
}
+ goto out;
}
+
if (flags & XATTR_REPLACE) {
+ /* There's no matching xattr so fail on replace. */
xattr = new_xattr;
err = -ENODATA;
} else {
- list_add(&new_xattr->list, &xattrs->head);
- xattr = NULL;
+ /*
+ * We're holding the lock and verified that there's no
+ * pre-existing xattr so this should always succeed.
+ */
+ WARN_ON(!simple_xattr_rb_insert(&xattrs->rb_root, new_xattr))
}
+
out:
spin_unlock(&xattrs->lock);
if (xattr) {
> - xattr = new_xattr;
> - err = -ENODATA;
> - } else {
> - list_add(&new_xattr->list, &xattrs->head);
> - xattr = NULL;
> - }
> -out:
> spin_unlock(&xattrs->lock);
> if (xattr) {
> kfree(xattr->name);
> @@ -1149,6 +1205,7 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
> {
> bool trusted = capable(CAP_SYS_ADMIN);
> struct simple_xattr *xattr;
> + struct rb_node *node;
> ssize_t remaining_size = size;
> int err = 0;
>
> @@ -1170,7 +1227,9 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
> #endif
>
> spin_lock(&xattrs->lock);
> - list_for_each_entry(xattr, &xattrs->head, list) {
> + for (node = rb_first(&xattrs->rb_root); node; node = rb_next(node)) {
> + xattr = container_of(node, struct simple_xattr, node);
> +
> /* skip "trusted." attributes for unprivileged callers */
> if (!trusted && xattr_is_trusted(xattr->name))
> continue;
> @@ -1191,6 +1250,6 @@ void simple_xattr_list_add(struct simple_xattrs *xattrs,
> struct simple_xattr *new_xattr)
> {
> spin_lock(&xattrs->lock);
> - list_add(&new_xattr->list, &xattrs->head);
> + simple_xattr_rb_insert(&xattrs->rb_root, new_xattr);
> spin_unlock(&xattrs->lock);
> }
> diff --git a/include/linux/xattr.h b/include/linux/xattr.h
> index 979a9d3e5bfb..bbe81cfb7a4d 100644
> --- a/include/linux/xattr.h
> +++ b/include/linux/xattr.h
> @@ -80,12 +80,12 @@ static inline const char *xattr_prefix(const struct xattr_handler *handler)
> }
>
> struct simple_xattrs {
> - struct list_head head;
> + struct rb_root rb_root;
> spinlock_t lock;
> };
>
> struct simple_xattr {
> - struct list_head list;
> + struct rb_node node;
> char *name;
> size_t size;
> char value[];
> @@ -96,7 +96,7 @@ struct simple_xattr {
> */
> static inline void simple_xattrs_init(struct simple_xattrs *xattrs)
> {
> - INIT_LIST_HEAD(&xattrs->head);
> + xattrs->rb_root = RB_ROOT;
> spin_lock_init(&xattrs->lock);
> }
>
> @@ -105,9 +105,12 @@ static inline void simple_xattrs_init(struct simple_xattrs *xattrs)
> */
> static inline void simple_xattrs_free(struct simple_xattrs *xattrs)
> {
> - struct simple_xattr *xattr, *node;
> + struct simple_xattr *xattr;
> + struct rb_node *node;
>
> - list_for_each_entry_safe(xattr, node, &xattrs->head, list) {
> + while ((node = rb_first(&xattrs->rb_root))) {
> + rb_erase(node, &xattrs->rb_root);
> + xattr = container_of(node, struct simple_xattr, node);
> kfree(xattr->name);
> kvfree(xattr);
> }
> --
> 2.34.1
>
>
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