Re: [PATCH 1/6] fs: add namei support for doing a non-blocking path lookup

Jeff Layton
2020-01-25T13:15:05Z

On Tue, 2020-01-07 at 10:00 -0700, Jens Axboe wrote:
> If the fast lookup fails, then return -EAGAIN to have the caller retry
> the path lookup. Assume that a dentry having any of:
>
> ->d_revalidate()
> ->d_automount()
> ->d_manage()
>
> could block in those callbacks. Preemptively return -EAGAIN if any of
> these are present.
>
> This is in preparation for supporting non-blocking open.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
> fs/namei.c | 21 ++++++++++++++++++++-
> include/linux/namei.h | 2 ++
> 2 files changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index b367fdb91682..ed108a41634f 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1641,6 +1641,17 @@ static struct dentry *__lookup_hash(const struct qstr *name,
> return dentry;
> }
>
> +static inline bool lookup_could_block(struct dentry *dentry, unsigned int flags)
> +{
> + const struct dentry_operations *ops = dentry->d_op;
> +
> + if (!ops || !(flags & LOOKUP_NONBLOCK))
> + return 0;
> +
> + /* assume these dentry ops may block */
> + return ops->d_revalidate || ops->d_automount || ops->d_manage;
> +}
> +

d_revalidate shouldn't block if LOOKUP_RCU is set.

> static int lookup_fast(struct nameidata *nd,
> struct path *path, struct inode **inode,
> unsigned *seqp)
> @@ -1665,6 +1676,9 @@ static int lookup_fast(struct nameidata *nd,
> return 0;
> }
>
> + if (unlikely(lookup_could_block(dentry, nd->flags)))
> + return -EAGAIN;
> +
> /*
> * This sequence count validates that the inode matches
> * the dentry name information from lookup.
> @@ -1707,7 +1721,10 @@ static int lookup_fast(struct nameidata *nd,
> dentry = __d_lookup(parent, &nd->last);
> if (unlikely(!dentry))
> return 0;
> - status = d_revalidate(dentry, nd->flags);
> + if (unlikely(lookup_could_block(dentry, nd->flags)))
> + status = -EAGAIN;
> + else
> + status = d_revalidate(dentry, nd->flags);
> }
> if (unlikely(status <= 0)) {
> if (!status)
> @@ -1912,6 +1929,8 @@ static int walk_component(struct nameidata *nd, int flags)
> if (unlikely(err <= 0)) {
> if (err < 0)
> return err;
> + if (nd->flags & LOOKUP_NONBLOCK)
> + return -EAGAIN;
> path.dentry = lookup_slow(&nd->last, nd->path.dentry,
> nd->flags);
> if (IS_ERR(path.dentry))
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 4e77068f7a1a..392eb439f88b 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -49,6 +49,8 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
> /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
> #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
>
> +#define LOOKUP_NONBLOCK 0x200000 /* don't block for lookup */
> +
> extern int path_pts(struct path *path);
>
> extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);

--
Jeff Layton <jlayton@kernel.org>