Re: [PATCH 1/3] fs, mm: add ->cachestat() file operation

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jun 23, 2026 at 01:14:48PM +0200, Pavel Tikhomirov wrote:
> The cachestat() syscall reads page cache statistics straight from the
> file's f_mapping. Stackable filesystems such as overlayfs keep the data
> pages in an underlying inode's mapping rather than in the overlay
> inode's, so cachestat() reports all zeroes for them.
> 
> Add a ->cachestat() file operation and route the syscall through a new
> vfs_cachestat() helper that calls it when present, falling back to
> file's f_mapping otherwise. This lets stackable filesystems forward the
> query to the file that actually owns the page cache. No behaviour change
> for regular files.
> 
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
> ---
> Note: Memset change might be a bit tricky, I moved it to no
> ->cachestat() path to avoid multiple memset on nested overlayfs, that
> means that ->cachestat() is expected to be able to handle unitialized
> cs.
> ---
>  include/linux/fs.h | 10 ++++++++++
>  mm/filemap.c       | 43 +++++++++++++++++++++++++++++++++++--------
>  2 files changed, 45 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6da44573ce450..966b6564707e4 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -53,6 +53,8 @@
>  
>  struct bdi_writeback;
>  struct bio;
> +struct cachestat_range;
> +struct cachestat;
>  struct io_comp_batch;
>  struct fiemap_extent_info;
>  struct kiocb;
> @@ -1963,6 +1965,8 @@ struct file_operations {
>  				   struct file *file_out, loff_t pos_out,
>  				   loff_t len, unsigned int remap_flags);
>  	int (*fadvise)(struct file *, loff_t, loff_t, int);
> +	int (*cachestat)(struct file *file, struct cachestat_range *csr,
> +			 struct cachestat *cs);

I suppose you can't just have it return the real file because of the
with_ovl_creds() scope you need during access? That would make things
a bit easier. But short of that, this looks good to me.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>