fs: add support for LOOKUP_CACHED

io_uring always punts opens to async context, since there's no control
over whether the lookup blocks or not. Add LOOKUP_CACHED to support
just doing the fast RCU based lookups, which we know will not block. If
we can do a cached path resolution of the filename, then we don't have
to always punt lookups for a worker.

During path resolution, we always do LOOKUP_RCU first. If that fails and
we terminate LOOKUP_RCU, then fail a LOOKUP_CACHED attempt as well.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -686,6 +686,8 @@ static bool try_to_unlazy(struct nameidata *nd)
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
 	nd->flags &= ~LOOKUP_RCU;
+	if (nd->flags & LOOKUP_CACHED)
+		goto out1;
 	if (unlikely(!legitimize_links(nd)))
 		goto out1;
 	if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
@@ -722,6 +724,8 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry, unsi
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
 	nd->flags &= ~LOOKUP_RCU;
+	if (nd->flags & LOOKUP_CACHED)
+		goto out2;
 	if (unlikely(!legitimize_links(nd)))
 		goto out2;
 	if (unlikely(!legitimize_mnt(nd->path.mnt, nd->m_seq)))
@@ -792,6 +796,7 @@ static int complete_walk(struct nameidata *nd)
 		 */
 		if (!(nd->flags & (LOOKUP_ROOT | LOOKUP_IS_SCOPED)))
 			nd->root.mnt = NULL;
+		nd->flags &= ~LOOKUP_CACHED;
 		if (!try_to_unlazy(nd))
 			return -ECHILD;
 	}
@@ -2204,6 +2209,10 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	int error;
 	const char *s = nd->name->name;
 
+	/* LOOKUP_CACHED requires RCU, ask caller to retry */
+	if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
+		return ERR_PTR(-EAGAIN);
+
 	if (!*s)
 		flags &= ~LOOKUP_RCU;
 	if (flags & LOOKUP_RCU)

include/linux/namei.h

@@ -46,6 +46,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
 #define LOOKUP_NO_XDEV		0x040000 /* No mountpoint crossing. */
 #define LOOKUP_BENEATH		0x080000 /* No escaping from starting point. */
 #define LOOKUP_IN_ROOT		0x100000 /* Treat dirfd as fs root. */
+#define LOOKUP_CACHED		0x200000 /* Only do cached lookup */
 /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
 #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)