[PATCH 2/5] fs: add support for LOOKUP_NONBLOCK

[Jens Axboe <axboe@kernel.dk>]

io_uring always punts opens to async context, since there's no control
over whether the lookup blocks or not. Add LOOKUP_NONBLOCK 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_NONBLOCK attempt as well.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/namei.c            | 9 +++++++++
 include/linux/namei.h | 1 +
 2 files changed, 10 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 7eb7830da298..07a1aa874f65 100644
--- a/fs/namei.c
+++ b/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_NONBLOCK)
+		goto out1;
 	if (unlikely(!legitimize_links(nd)))
 		goto out1;
 	if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
@@ -722,6 +724,8 @@ static int unlazy_child(struct nameidata *nd, struct dentry *dentry, unsigned se
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
 	nd->flags &= ~LOOKUP_RCU;
+	if (nd->flags & LOOKUP_NONBLOCK)
+		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_NONBLOCK;
 		if (!try_to_unlazy(nd))
 			return -ECHILD;
 	}
@@ -2202,6 +2207,10 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	int error;
 	const char *s = nd->name->name;
 
+	/* LOOKUP_NONBLOCK requires RCU, ask caller to retry */
+	if ((flags & (LOOKUP_RCU | LOOKUP_NONBLOCK)) == LOOKUP_NONBLOCK)
+		return ERR_PTR(-EAGAIN);
+
 	if (!*s)
 		flags &= ~LOOKUP_RCU;
 	if (flags & LOOKUP_RCU)
diff --git a/include/linux/namei.h b/include/linux/namei.h
index a4bb992623c4..c36c4e0805fc 100644
--- a/include/linux/namei.h
+++ b/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_NONBLOCK		0x200000 /* don't block for lookup */
 /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
 #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
 
-- 
2.29.2