Re: [PATCH v6 12/16] mm: list_lru: replace linear array with xarray

["NeilBrown" <neilb@suse.de>]

On Thu, 31 Mar 2022, Muchun Song wrote:
> 
> Thanks for your report.  I knew the reason. It is because the following
> patch in this series was missed upstream.  Could you help me test if it
> works properly?
> 
> [v6,06/16] nfs42: use a specific kmem_cache to allocate nfs4_xattr_entry
> 

Thanks for the quick response!  That patch helps, but has a bug.  My
testing resulted in refcount underflow errors.

Problem is that kref_init() is called in nfs4_xattr_entry_init_once().
This means that it is initialised to '1' the first time an entry is
allocated, but it is left as zero the second time.
I applied:
--- a/fs/nfs/nfs42xattr.c
+++ b/fs/nfs/nfs42xattr.c
@@ -191,6 +191,7 @@ nfs4_xattr_alloc_entry(const char *name, const void *value,
 	entry = kmem_cache_alloc_lru(nfs4_xattr_entry_cachep, lru, gfp);
 	if (!entry)
 		return NULL;
+	kref_init(&entry->ref);
 	namep = kstrdup_const(name, gfp);
 	if (!namep && name)
 		goto free_buf;
@@ -974,7 +975,6 @@ static void nfs4_xattr_entry_init_once(void *p)
 {
 	struct nfs4_xattr_entry *entry = p;
 
-	kref_init(&entry->ref);
 	entry->bucket = NULL;
 	INIT_LIST_HEAD(&entry->lru);
 	INIT_LIST_HEAD(&entry->dispose);

and now it seems to work.

The complete patch that I applied is below.  I haven't reviewed it, just
tested it.
  Tested-by: NeilBrown <neilb@suse.de>

Thanks,
NeilBrown


From: Muchun Song <songmuchun@bytedance.com>
Date: Mon, 28 Feb 2022 20:21:16 +0800
Subject: [PATCH] nfs42: use a specific kmem_cache to allocate nfs4_xattr_entry

If we want to add the allocated objects to its list_lru, we should use
kmem_cache_alloc_lru() to allocate objects. So intruduce
nfs4_xattr_entry_cachep which is used to allocate nfs4_xattr_entry.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>

diff --git a/fs/nfs/nfs42xattr.c b/fs/nfs/nfs42xattr.c
index ad3405c64b9e..d4163c46acf1 100644
--- a/fs/nfs/nfs42xattr.c
+++ b/fs/nfs/nfs42xattr.c
@@ -81,7 +81,7 @@ struct nfs4_xattr_entry {
 	struct hlist_node hnode;
 	struct list_head lru;
 	struct list_head dispose;
-	char *xattr_name;
+	const char *xattr_name;
 	void *xattr_value;
 	size_t xattr_size;
 	struct nfs4_xattr_bucket *bucket;
@@ -98,6 +98,7 @@ static struct list_lru nfs4_xattr_entry_lru;
 static struct list_lru nfs4_xattr_large_entry_lru;
 
 static struct kmem_cache *nfs4_xattr_cache_cachep;
+static struct kmem_cache *nfs4_xattr_entry_cachep;
 
 /*
  * Hashing helper functions.
@@ -177,49 +178,28 @@ nfs4_xattr_alloc_entry(const char *name, const void *value,
 {
 	struct nfs4_xattr_entry *entry;
 	void *valp;
-	char *namep;
-	size_t alloclen, slen;
-	char *buf;
-	uint32_t flags;
+	const char *namep;
+	uint32_t flags = len > PAGE_SIZE ? NFS4_XATTR_ENTRY_EXTVAL : 0;
+	gfp_t gfp = GFP_KERNEL;
+	struct list_lru *lru;
 
 	BUILD_BUG_ON(sizeof(struct nfs4_xattr_entry) +
 	    XATTR_NAME_MAX + 1 > PAGE_SIZE);
 
-	alloclen = sizeof(struct nfs4_xattr_entry);
-	if (name != NULL) {
-		slen = strlen(name) + 1;
-		alloclen += slen;
-	} else
-		slen = 0;
-
-	if (alloclen + len <= PAGE_SIZE) {
-		alloclen += len;
-		flags = 0;
-	} else {
-		flags = NFS4_XATTR_ENTRY_EXTVAL;
-	}
-
-	buf = kmalloc(alloclen, GFP_KERNEL);
-	if (buf == NULL)
+	lru = flags & NFS4_XATTR_ENTRY_EXTVAL ? &nfs4_xattr_large_entry_lru :
+	      &nfs4_xattr_entry_lru;
+	entry = kmem_cache_alloc_lru(nfs4_xattr_entry_cachep, lru, gfp);
+	if (!entry)
 		return NULL;
-	entry = (struct nfs4_xattr_entry *)buf;
-
-	if (name != NULL) {
-		namep = buf + sizeof(struct nfs4_xattr_entry);
-		memcpy(namep, name, slen);
-	} else {
-		namep = NULL;
-	}
-
-
-	if (flags & NFS4_XATTR_ENTRY_EXTVAL) {
-		valp = kvmalloc(len, GFP_KERNEL);
-		if (valp == NULL) {
-			kfree(buf);
-			return NULL;
-		}
-	} else if (len != 0) {
-		valp = buf + sizeof(struct nfs4_xattr_entry) + slen;
+	kref_init(&entry->ref);
+	namep = kstrdup_const(name, gfp);
+	if (!namep && name)
+		goto free_buf;
+
+	if (len != 0) {
+		valp = kvmalloc(len, gfp);
+		if (!valp)
+			goto free_name;
 	} else
 		valp = NULL;
 
@@ -232,23 +212,23 @@ nfs4_xattr_alloc_entry(const char *name, const void *value,
 
 	entry->flags = flags;
 	entry->xattr_value = valp;
-	kref_init(&entry->ref);
 	entry->xattr_name = namep;
 	entry->xattr_size = len;
-	entry->bucket = NULL;
-	INIT_LIST_HEAD(&entry->lru);
-	INIT_LIST_HEAD(&entry->dispose);
-	INIT_HLIST_NODE(&entry->hnode);
 
 	return entry;
+free_name:
+	kfree_const(namep);
+free_buf:
+	kmem_cache_free(nfs4_xattr_entry_cachep, entry);
+	return NULL;
 }
 
 static void
 nfs4_xattr_free_entry(struct nfs4_xattr_entry *entry)
 {
-	if (entry->flags & NFS4_XATTR_ENTRY_EXTVAL)
-		kvfree(entry->xattr_value);
-	kfree(entry);
+	kvfree(entry->xattr_value);
+	kfree_const(entry->xattr_name);
+	kmem_cache_free(nfs4_xattr_entry_cachep, entry);
 }
 
 static void
@@ -289,7 +269,7 @@ nfs4_xattr_alloc_cache(void)
 {
 	struct nfs4_xattr_cache *cache;
 
-	cache = kmem_cache_alloc(nfs4_xattr_cache_cachep, GFP_KERNEL);
+	cache = kmem_cache_alloc_lru(nfs4_xattr_cache_cachep, &nfs4_xattr_cache_lru, GFP_KERNEL);
 	if (cache == NULL)
 		return NULL;
 
@@ -991,6 +971,16 @@ static void nfs4_xattr_cache_init_once(void *p)
 	INIT_LIST_HEAD(&cache->dispose);
 }
 
+static void nfs4_xattr_entry_init_once(void *p)
+{
+	struct nfs4_xattr_entry *entry = p;
+
+	entry->bucket = NULL;
+	INIT_LIST_HEAD(&entry->lru);
+	INIT_LIST_HEAD(&entry->dispose);
+	INIT_HLIST_NODE(&entry->hnode);
+}
+
 int __init nfs4_xattr_cache_init(void)
 {
 	int ret = 0;
@@ -1002,6 +992,13 @@ int __init nfs4_xattr_cache_init(void)
 	if (nfs4_xattr_cache_cachep == NULL)
 		return -ENOMEM;
 
+	nfs4_xattr_entry_cachep = kmem_cache_create("nfs4_xattr_entry",
+			sizeof(struct nfs4_xattr_entry), 0,
+			(SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD | SLAB_ACCOUNT),
+			nfs4_xattr_entry_init_once);
+	if (!nfs4_xattr_entry_cachep)
+		goto out5;
+
 	ret = list_lru_init_memcg(&nfs4_xattr_large_entry_lru,
 	    &nfs4_xattr_large_entry_shrinker);
 	if (ret)
@@ -1039,6 +1036,8 @@ int __init nfs4_xattr_cache_init(void)
 out3:
 	list_lru_destroy(&nfs4_xattr_large_entry_lru);
 out4:
+	kmem_cache_destroy(nfs4_xattr_entry_cachep);
+out5:
 	kmem_cache_destroy(nfs4_xattr_cache_cachep);
 
 	return ret;