Date:	Sat, 20 Sep 2008 22:30:47 -0400
From:	"Theodore Ts'o" <tytso@....edu>
To:	linux-ext4@...r.kernel.org
cc:	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] ext4: Don't use 'struct dentry' for internal lookups

This is a port of a patch from Linus which fixes a 200+ byte stack
usage problem in ext4_get_parent().

It's more efficient to pass down only the actual parts of the dentry
that matter: the parent inode and the name, instead of allocating a
struct dentry on the stack.

Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@....edu>
---

Linus, I plan to push this to you at the next patch window; I'm just
cc'ing you now since you authored the original patch.

It seems that the ext3 version of the patch seems not to have been
picked up by akpm.  Want me to resend it to him?

Also, per your complaints about the stack-heavy ext3 users, one fairly
easy fix for x86_64 (the stack usage isn't all that bad on x86 when
pointers are only 32-bits) is pretty much all of the places where we use
"unsigned long" in fs/ext3 should be safely replaceable by "unsigned
int" or "u32".  Sigh... legacy code back when long==int==32-bits, but no
one has bothered to fix it.  This won't completely fix the worst of the
stack abusers, but it should help a little....  I'll try to put together
a patch to deal with this, in my copious spare time....  hey, at least
our stack usage is not as bad as XFS!  :-P

					   - Ted


diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index a1f72d2..205e740 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -159,7 +159,7 @@ static void dx_set_count(struct dx_entry *entries, unsigned value);
 static void dx_set_limit(struct dx_entry *entries, unsigned value);
 static unsigned dx_root_limit(struct inode *dir, unsigned infosize);
 static unsigned dx_node_limit(struct inode *dir);
-static struct dx_frame *dx_probe(struct dentry *dentry,
+static struct dx_frame *dx_probe(const struct qstr *d_name,
 				 struct inode *dir,
 				 struct dx_hash_info *hinfo,
 				 struct dx_frame *frame,
@@ -177,8 +177,10 @@ static int ext4_htree_next_block(struct inode *dir, __u32 hash,
 				 struct dx_frame *frame,
 				 struct dx_frame *frames,
 				 __u32 *start_hash);
-static struct buffer_head * ext4_dx_find_entry(struct dentry *dentry,
-		       struct ext4_dir_entry_2 **res_dir, int *err);
+static struct buffer_head * ext4_dx_find_entry(struct inode *dir,
+		const struct qstr *d_name,
+		struct ext4_dir_entry_2 **res_dir,
+		int *err);
 static int ext4_dx_add_entry(handle_t *handle, struct dentry *dentry,
 			     struct inode *inode);
 
@@ -345,7 +347,7 @@ struct stats dx_show_entries(struct dx_hash_info *hinfo, struct inode *dir,
  * back to userspace.
  */
 static struct dx_frame *
-dx_probe(struct dentry *dentry, struct inode *dir,
+dx_probe(const struct qstr *d_name, struct inode *dir,
 	 struct dx_hash_info *hinfo, struct dx_frame *frame_in, int *err)
 {
 	unsigned count, indirect;
@@ -356,8 +358,6 @@ dx_probe(struct dentry *dentry, struct inode *dir,
 	u32 hash;
 
 	frame->bh = NULL;
-	if (dentry)
-		dir = dentry->d_parent->d_inode;
 	if (!(bh = ext4_bread (NULL,dir, 0, 0, err)))
 		goto fail;
 	root = (struct dx_root *) bh->b_data;
@@ -373,8 +373,8 @@ dx_probe(struct dentry *dentry, struct inode *dir,
 	}
 	hinfo->hash_version = root->info.hash_version;
 	hinfo->seed = EXT4_SB(dir->i_sb)->s_hash_seed;
-	if (dentry)
-		ext4fs_dirhash(dentry->d_name.name, dentry->d_name.len, hinfo);
+	if (d_name)
+		ext4fs_dirhash(d_name->name, d_name->len, hinfo);
 	hash = hinfo->hash;
 
 	if (root->info.unused_flags & 1) {
@@ -649,7 +649,7 @@ int ext4_htree_fill_tree(struct file *dir_file, __u32 start_hash,
 	}
 	hinfo.hash = start_hash;
 	hinfo.minor_hash = 0;
-	frame = dx_probe(NULL, dir_file->f_path.dentry->d_inode, &hinfo, frames, &err);
+	frame = dx_probe(NULL, dir, &hinfo, frames, &err);
 	if (!frame)
 		return err;
 
@@ -805,15 +805,15 @@ static inline int ext4_match (int len, const char * const name,
  */
 static inline int search_dirblock(struct buffer_head *bh,
 				  struct inode *dir,
-				  struct dentry *dentry,
+				  const struct qstr *d_name,
 				  unsigned long offset,
 				  struct ext4_dir_entry_2 ** res_dir)
 {
 	struct ext4_dir_entry_2 * de;
 	char * dlimit;
 	int de_len;
-	const char *name = dentry->d_name.name;
-	int namelen = dentry->d_name.len;
+	const char *name = d_name->name;
+	int namelen = d_name->len;
 
 	de = (struct ext4_dir_entry_2 *) bh->b_data;
 	dlimit = bh->b_data + dir->i_sb->s_blocksize;
@@ -852,7 +852,8 @@ static inline int search_dirblock(struct buffer_head *bh,
  * The returned buffer_head has ->b_count elevated.  The caller is expected
  * to brelse() it when appropriate.
  */
-static struct buffer_head * ext4_find_entry (struct dentry *dentry,
+static struct buffer_head * ext4_find_entry (struct inode *dir,
+					const struct qstr *d_name,
 					struct ext4_dir_entry_2 ** res_dir)
 {
 	struct super_block *sb;
@@ -866,16 +867,15 @@ static struct buffer_head * ext4_find_entry (struct dentry *dentry,
 	int num = 0;
 	ext4_lblk_t  nblocks;
 	int i, err;
-	struct inode *dir = dentry->d_parent->d_inode;
 	int namelen;
 
 	*res_dir = NULL;
 	sb = dir->i_sb;
-	namelen = dentry->d_name.len;
+	namelen = d_name.len;
 	if (namelen > EXT4_NAME_LEN)
 		return NULL;
 	if (is_dx(dir)) {
-		bh = ext4_dx_find_entry(dentry, res_dir, &err);
+		bh = ext4_dx_find_entry(dir, d_name, res_dir, &err);
 		/*
 		 * On success, or if the error was file not found,
 		 * return.  Otherwise, fall back to doing a search the
@@ -928,7 +928,7 @@ restart:
 			brelse(bh);
 			goto next;
 		}
-		i = search_dirblock(bh, dir, dentry,
+		i = search_dirblock(bh, dir, d_name,
 			    block << EXT4_BLOCK_SIZE_BITS(sb), res_dir);
 		if (i == 1) {
 			EXT4_I(dir)->i_dir_start_lookup = block;
@@ -962,7 +962,7 @@ cleanup_and_exit:
 	return ret;
 }
 
-static struct buffer_head * ext4_dx_find_entry(struct dentry *dentry,
+static struct buffer_head * ext4_dx_find_entry(struct inode *dir, const struct qstr *d_name,
 		       struct ext4_dir_entry_2 **res_dir, int *err)
 {
 	struct super_block * sb;
@@ -973,14 +973,13 @@ static struct buffer_head * ext4_dx_find_entry(struct dentry *dentry,
 	struct buffer_head *bh;
 	ext4_lblk_t block;
 	int retval;
-	int namelen = dentry->d_name.len;
-	const u8 *name = dentry->d_name.name;
-	struct inode *dir = dentry->d_parent->d_inode;
+	int namelen = d_name->len;
+	const u8 *name = d_name->name;
 
 	sb = dir->i_sb;
 	/* NFS may look up ".." - look at dx_root directory block */
 	if (namelen > 2 || name[0] != '.'||(name[1] != '.' && name[1] != '\0')){
-		if (!(frame = dx_probe(dentry, NULL, &hinfo, frames, err)))
+		if (!(frame = dx_probe(d_name, dir, &hinfo, frames, err)))
 			return NULL;
 	} else {
 		frame = frames;
@@ -1041,7 +1040,7 @@ static struct dentry *ext4_lookup(struct inode *dir, struct dentry *dentry, stru
 	if (dentry->d_name.len > EXT4_NAME_LEN)
 		return ERR_PTR(-ENAMETOOLONG);
 
-	bh = ext4_find_entry(dentry, &de);
+	bh = ext4_find_entry(dir, &dentry->d_name, &de);
 	inode = NULL;
 	if (bh) {
 		unsigned long ino = le32_to_cpu(de->inode);
@@ -1064,15 +1063,14 @@ struct dentry *ext4_get_parent(struct dentry *child)
 	unsigned long ino;
 	struct dentry *parent;
 	struct inode *inode;
-	struct dentry dotdot;
+	static const struct qstr dotdot = {
+		.name = "..",
+		.len = 2,
+	};
 	struct ext4_dir_entry_2 * de;
 	struct buffer_head *bh;
 
-	dotdot.d_name.name = "..";
-	dotdot.d_name.len = 2;
-	dotdot.d_parent = child; /* confusing, isn't it! */
-
-	bh = ext4_find_entry(&dotdot, &de);
+	bh = ext4_find_entry(child->d_inode, &dotdot, &de);
 	inode = NULL;
 	if (!bh)
 		return ERR_PTR(-ENOENT);
@@ -1508,7 +1506,7 @@ static int ext4_dx_add_entry(handle_t *handle, struct dentry *dentry,
 	struct ext4_dir_entry_2 *de;
 	int err;
 
-	frame = dx_probe(dentry, NULL, &hinfo, frames, &err);
+	frame = dx_probe(&dentry->d_name, dir, &hinfo, frames, &err);
 	if (!frame)
 		return err;
 	entries = frame->entries;
@@ -2089,7 +2087,7 @@ static int ext4_rmdir(struct inode *dir, struct dentry *dentry)
 		return PTR_ERR(handle);
 
 	retval = -ENOENT;
-	bh = ext4_find_entry(dentry, &de);
+	bh = ext4_find_entry(dir, &dentry->d_name, &de);
 	if (!bh)
 		goto end_rmdir;
 
@@ -2151,7 +2149,7 @@ static int ext4_unlink(struct inode *dir, struct dentry *dentry)
 		handle->h_sync = 1;
 
 	retval = -ENOENT;
-	bh = ext4_find_entry(dentry, &de);
+	bh = ext4_find_entry(dir, &dentry->d_name, &de);
 	if (!bh)
 		goto end_unlink;
 
@@ -2312,7 +2310,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 	if (IS_DIRSYNC(old_dir) || IS_DIRSYNC(new_dir))
 		handle->h_sync = 1;
 
-	old_bh = ext4_find_entry(old_dentry, &old_de);
+	old_bh = ext4_find_entry(old_dir, &old_dentry->d_name, &old_de);
 	/*
 	 *  Check for inode number is _not_ due to possible IO errors.
 	 *  We might rmdir the source, keep it as pwd of some process
@@ -2325,7 +2323,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 		goto end_rename;
 
 	new_inode = new_dentry->d_inode;
-	new_bh = ext4_find_entry(new_dentry, &new_de);
+	new_bh = ext4_find_entry(new_dir, &new_dentry->d_name, &new_de);
 	if (new_bh) {
 		if (!new_inode) {
 			brelse(new_bh);
@@ -2392,7 +2390,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 		struct buffer_head *old_bh2;
 		struct ext4_dir_entry_2 *old_de2;
 
-		old_bh2 = ext4_find_entry(old_dentry, &old_de2);
+		old_bh2 = ext4_find_entry(old_dir, &old_dentry->d_name, &old_de2);
 		if (old_bh2) {
 			retval = ext4_delete_entry(handle, old_dir,
 						   old_de2, old_bh2);
--
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