Re: [PATCH 2/3] ovl: support cachestat() syscall on overlayfs files

[Nhat Pham]

On Tue, Jun 23, 2026 at 4:15 AM Pavel Tikhomirov
<ptikhomirov@xxxxxxxxxxxxx> wrote:
>
> Overlayfs forwards data I/O to the real (upper/lower) file, so the page
> cache lives in the real inode's mapping and cachestat() on an overlay
> fd returned all zeroes.
>
> Implement the ->cachestat() file operation by forwarding to the real
> file via vfs_cachestat(), the same way ovl_fadvise() forwards
> for fadvise.
>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
> ---
>  fs/overlayfs/file.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 27cc07738f33b..a7e252a91ea43 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -518,6 +518,21 @@ static int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
>                 return vfs_fadvise(realfile, offset, len, advice);
>  }
>
> +#ifdef CONFIG_CACHESTAT_SYSCALL
> +static int ovl_cachestat(struct file *file, struct cachestat_range *csr,
> +                        struct cachestat *cs)
> +{
> +       struct file *realfile;
> +
> +       realfile = ovl_real_file(file);
> +       if (IS_ERR(realfile))
> +               return PTR_ERR(realfile);

We're propagating the error of ovl_real_file() all the way to
userspace right? I think we need to handle this.

For example, we might get -EIO here, which is unexpected and
undocumented from cachestat's POV.

Maybe handle it and just return -EBADF or sth like that (with some
updated documentations, etc.)

The rest LGTM, but I'll let overlayfs maintainers check the
overlayfs-specific bits :)

> +
> +       with_ovl_creds(file_inode(file)->i_sb)
> +               return vfs_cachestat(realfile, csr, cs);
> +}