linux-kernel - [PATCH 6/7] lookup_fast(): don't bother with inode

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Date:   Tue, 5 Jul 2022 00:16:30 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Vitaly Buka <vitalybuka@...gle.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: [PATCH 6/7] lookup_fast(): don't bother with inode

Note that validation of ->d_seq after ->d_inode fetch is gone, along
with fetching of ->d_inode itself.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 fs/namei.c | 24 +++++-------------------
 1 file changed, 5 insertions(+), 19 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index fe95fe39634c..cdb61d09df79 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1617,8 +1617,7 @@ static struct dentry *__lookup_hash(const struct qstr *name,
 	return dentry;
 }
 
-static struct dentry *lookup_fast(struct nameidata *nd,
-				  struct inode **inode)
+static struct dentry *lookup_fast(struct nameidata *nd)
 {
 	struct dentry *dentry, *parent = nd->path.dentry;
 	int status = 1;
@@ -1636,22 +1635,11 @@ static struct dentry *lookup_fast(struct nameidata *nd,
 			return NULL;
 		}
 
-		/*
-		 * This sequence count validates that the inode matches
-		 * the dentry name information from lookup.
-		 */
-		*inode = d_backing_inode(dentry);
-		if (unlikely(read_seqcount_retry(&dentry->d_seq, nd->next_seq)))
-			return ERR_PTR(-ECHILD);
-
-		/*
+	        /*
 		 * This sequence count validates that the parent had no
 		 * changes while we did the lookup of the dentry above.
-		 *
-		 * The memory barrier in read_seqcount_begin of child is
-		 *  enough, we can use __read_seqcount_retry here.
 		 */
-		if (unlikely(__read_seqcount_retry(&parent->d_seq, nd->seq)))
+		if (unlikely(read_seqcount_retry(&parent->d_seq, nd->seq)))
 			return ERR_PTR(-ECHILD);
 
 		status = d_revalidate(dentry, nd->flags);
@@ -1993,7 +1981,6 @@ static const char *handle_dots(struct nameidata *nd, int type)
 static const char *walk_component(struct nameidata *nd, int flags)
 {
 	struct dentry *dentry;
-	struct inode *inode;
 	/*
 	 * "." and ".." are special - ".." especially so because it has
 	 * to be able to know about the current root directory and
@@ -2004,7 +1991,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
 			put_link(nd);
 		return handle_dots(nd, nd->last_type);
 	}
-	dentry = lookup_fast(nd, &inode);
+	dentry = lookup_fast(nd);
 	if (IS_ERR(dentry))
 		return ERR_CAST(dentry);
 	if (unlikely(!dentry)) {
@@ -3392,7 +3379,6 @@ static const char *open_last_lookups(struct nameidata *nd,
 	struct dentry *dir = nd->path.dentry;
 	int open_flag = op->open_flag;
 	bool got_write = false;
-	struct inode *inode;
 	struct dentry *dentry;
 	const char *res;
 
@@ -3408,7 +3394,7 @@ static const char *open_last_lookups(struct nameidata *nd,
 		if (nd->last.name[nd->last.len])
 			nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
 		/* we _can_ be in RCU mode here */
-		dentry = lookup_fast(nd, &inode);
+		dentry = lookup_fast(nd);
 		if (IS_ERR(dentry))
 			return ERR_CAST(dentry);
 		if (likely(dentry))
-- 
2.30.2