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