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

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

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 <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?

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 <hannes@cmpxchg.org>

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.