On Thu, May 7, 2020 at 4:21 AM Chengguang Xu <cgxu519@xxxxxxxxxxxx> wrote:
>
> When a file is only in a lower layer, after lookup a negative
Or in no layer at all...
> dentry will be generated in the upper layer or even worse many
> negetive dentries will be generated in upper/lower layers. These
> negative dentries will be useless after construction of overlayfs'
> own dentry and may keep in the memory long time even after unmount
> of overlayfs instance. This patch tries to kill unnecessary negative
> dentry during lookup.
>
> Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
> ---
> fs/overlayfs/namei.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 723d17744758..cf0ec4d7bcec 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -200,7 +200,7 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
> int err;
> bool last_element = !post[0];
>
> - this = lookup_positive_unlocked(name, base, namelen);
> + this = lookup_one_len_unlocked(name, base, namelen);
> if (IS_ERR(this)) {
> err = PTR_ERR(this);
> this = NULL;
> @@ -209,6 +209,15 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
> goto out_err;
> }
>
> + /* Borrow the check from lookup_positive_unlocked */
> + if (d_flags_negative(smp_load_acquire(&this->d_flags))) {
> + d_drop(this);
> + dput(this);
> + this = NULL;
> + err = -ENOENT;
> + goto out;
> + }
> +
This is a nice improvement, but my feeling is that this low level code
belongs in a vfs helper with well documented semantics.
Thanks,
Amir.
