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

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

On 6/23/26 18:01, Nhat Pham wrote:
> On Tue, Jun 23, 2026 at 4:15 AM Pavel Tikhomirov
> <ptikhomirov@virtuozzo.com> 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.
> 
> nit: maybe a comment somewhere to document this?

Sure, will do in v2.

> 
>> ---
>>  include/linux/fs.h | 10 ++++++++++
>>  mm/filemap.c       | 43 +++++++++++++++++++++++++++++++++++--------
>>  2 files changed, 45 insertions(+), 8 deletions(-)
>>
> 
> LGTM otherwise. With Amir's suggestions incorporated:
> 
> Acked-by: Nhat Pham <nphamcs@gmail.com>

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