Adapt pidfs to use the rhashtable-based xattr path by switching from a
dedicated slab cache to simple_xattrs_alloc().

Previously pidfs used a custom kmem_cache (pidfs_xattr_cachep) that
allocated a struct containing an embedded simple_xattrs plus
simple_xattrs_init(). Replace this with simple_xattrs_alloc() which
combines kzalloc + rhashtable_init, and drop the dedicated slab cache
entirely.

Use simple_xattr_free_rcu() for replaced xattr entries to allow
concurrent RCU readers to finish.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/pidfs.c | 65 +++++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 41 insertions(+), 24 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 1e20e36e0ed5..cb62000681df 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -21,6 +21,7 @@
 #include <linux/utsname.h>
 #include <net/net_namespace.h>
 #include <linux/coredump.h>
+#include <linux/llist.h>
 #include <linux/xattr.h>
 
 #include "internal.h"
@@ -29,7 +30,6 @@
 #define PIDFS_PID_DEAD ERR_PTR(-ESRCH)
 
 static struct kmem_cache *pidfs_attr_cachep __ro_after_init;
-static struct kmem_cache *pidfs_xattr_cachep __ro_after_init;
 
 static struct path pidfs_root_path = {};
 
@@ -44,9 +44,8 @@ enum pidfs_attr_mask_bits {
 	PIDFS_ATTR_BIT_COREDUMP	= 1,
 };
 
-struct pidfs_attr {
+struct pidfs_anon_attr {
 	unsigned long attr_mask;
-	struct simple_xattrs *xattrs;
 	struct /* exit info */ {
 		__u64 cgroupid;
 		__s32 exit_code;
@@ -55,6 +54,14 @@ struct pidfs_attr {
 	__u32 coredump_signal;
 };
 
+struct pidfs_attr {
+	struct simple_xattrs *xattrs;
+	union {
+		struct pidfs_anon_attr;
+		struct llist_node pidfs_llist;
+	};
+};
+
 static struct rb_root pidfs_ino_tree = RB_ROOT;
 
 #if BITS_PER_LONG == 32
@@ -147,10 +154,30 @@ void pidfs_remove_pid(struct pid *pid)
 	write_seqcount_end(&pidmap_lock_seq);
 }
 
+static LLIST_HEAD(pidfs_free_list);
+
+static void pidfs_free_attr_work(struct work_struct *work)
+{
+	struct pidfs_attr *attr, *next;
+	struct llist_node *head;
+
+	head = llist_del_all(&pidfs_free_list);
+	llist_for_each_entry_safe(attr, next, head, pidfs_llist) {
+		struct simple_xattrs *xattrs = attr->xattrs;
+
+		if (xattrs) {
+			simple_xattrs_free(xattrs, NULL);
+			kfree(xattrs);
+		}
+		kfree(attr);
+	}
+}
+
+static DECLARE_WORK(pidfs_free_work, pidfs_free_attr_work);
+
 void pidfs_free_pid(struct pid *pid)
 {
-	struct pidfs_attr *attr __free(kfree) = no_free_ptr(pid->attr);
-	struct simple_xattrs *xattrs __free(kfree) = NULL;
+	struct pidfs_attr *attr = pid->attr;
 
 	/*
 	 * Any dentry must've been wiped from the pid by now.
@@ -169,9 +196,10 @@ void pidfs_free_pid(struct pid *pid)
 	if (IS_ERR(attr))
 		return;
 
-	xattrs = no_free_ptr(attr->xattrs);
-	if (xattrs)
-		simple_xattrs_free(xattrs, NULL);
+	if (likely(!attr->xattrs))
+		kfree(attr);
+	else if (llist_add(&attr->pidfs_llist, &pidfs_free_list))
+		schedule_work(&pidfs_free_work);
 }
 
 #ifdef CONFIG_PROC_FS
@@ -998,7 +1026,7 @@ static int pidfs_xattr_get(const struct xattr_handler *handler,
 
 	xattrs = READ_ONCE(attr->xattrs);
 	if (!xattrs)
-		return 0;
+		return -ENODATA;
 
 	name = xattr_full_name(handler, suffix);
 	return simple_xattr_get(xattrs, name, value, size);
@@ -1018,22 +1046,16 @@ static int pidfs_xattr_set(const struct xattr_handler *handler,
 	/* Ensure we're the only one to set @attr->xattrs. */
 	WARN_ON_ONCE(!inode_is_locked(inode));
 
-	xattrs = READ_ONCE(attr->xattrs);
-	if (!xattrs) {
-		xattrs = kmem_cache_zalloc(pidfs_xattr_cachep, GFP_KERNEL);
-		if (!xattrs)
-			return -ENOMEM;
-
-		simple_xattrs_init(xattrs);
-		smp_store_release(&pid->attr->xattrs, xattrs);
-	}
+	xattrs = simple_xattrs_lazy_alloc(&attr->xattrs, value, flags);
+	if (IS_ERR_OR_NULL(xattrs))
+		return PTR_ERR(xattrs);
 
 	name = xattr_full_name(handler, suffix);
 	old_xattr = simple_xattr_set(xattrs, name, value, size, flags);
 	if (IS_ERR(old_xattr))
 		return PTR_ERR(old_xattr);
 
-	simple_xattr_free(old_xattr);
+	simple_xattr_free_rcu(old_xattr);
 	return 0;
 }
 
@@ -1108,11 +1130,6 @@ void __init pidfs_init(void)
 					 (SLAB_HWCACHE_ALIGN | SLAB_RECLAIM_ACCOUNT |
 					  SLAB_ACCOUNT | SLAB_PANIC), NULL);
 
-	pidfs_xattr_cachep = kmem_cache_create("pidfs_xattr_cache",
-					       sizeof(struct simple_xattrs), 0,
-					       (SLAB_HWCACHE_ALIGN | SLAB_RECLAIM_ACCOUNT |
-						SLAB_ACCOUNT | SLAB_PANIC), NULL);
-
 	pidfs_mnt = kern_mount(&pidfs_type);
 	if (IS_ERR(pidfs_mnt))
 		panic("Failed to mount pidfs pseudo filesystem");

-- 
2.47.3