On Tue, Jun 23, 2026 at 4:55 PM Pavel Tikhomirov <[email protected]> wrote: > > > > On 6/23/26 15:48, Johannes Weiner wrote: > > 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 <[email protected]> > >> --- > >> 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? > > Yes, AFAIU in overlay when we use realfile we should always use it > with_ovl_creds(), even though I don't think there is anything cred related > in filemap_cachestat(), I still think we should follow the common pattern > other overlay helpers use (similar to ovl_fadvise() and ovl_flush()). > > note: Actually some places get ovl_real_file() and use it without > with_ovl_creds(), e.g.: ovl_read_iter, ovl_write_iter, ovl_splice_read, > ovl_splice_write. But those look more of an exception than the general > rule. All other instances use with_ovl_creds().
Use with_ovl_creds() is a good practice to keep the mental security model, but it is useless if the security check (can_do_cachestat) is not in the vfs helper (vfs_cachestat), so please move it there. Also it kind of makes more sense to check (flags != 0) in sys_cachestats before checking permissions. > > Also there are simingly no other file_operations which return "realfile" > for further processing, mostly the operation from fsops simply replaces > general operation with its own logic completely. > > Thanks for your review! > > ps: Hope overlay maintainers will correctly if I'm getting this wrong. > I don't think this is wrong per-se, except for can_do_cachestat(). Just be aware that the real file could change from one cachestat call to the next (i.e. due to copy up). Thanks, Amir.
