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().
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.
> That would make things
> a bit easier. But short of that, this looks good to me.
>
> Acked-by: Johannes Weiner <[email protected]>
--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.
