Re: [PATCH RFC 00/11] mm/filemap: split out folio wait and VFS code

On 5/28/26 8:49 AM, Christoph Hellwig wrote:
> On Thu, May 28, 2026 at 11:22:37AM +0200, Jan Kara wrote:
>> Overall this makes sense to me. In particular I agree it makes sense to
>> move the file read/write helpers into fs.
> 
> I disagree very strongly.  Mixing default implementations with the
> higher level APIs is a really bad idea and leads to people taking
> stupid shortcuts and other layering violations.

fs/read_write.c already contains some of these "generic" function
implementations, including generic_write_checks(), which is called by
generic_file_write_iter() in mm/filemap.c. Right now the two files are
unnecessarily interdependent. I do think fs/read_write.c is the natural home
for these functions.

> Splitting up filemap.c makes sense, but I'd rather keep the generic copy
> into and out of the pagecache code with the MM infrastructure for it,
> as it is not VFS code, and making that clear to anyone touching the code
> is important.

About half the code moved is implementing direct I/O or multiplexing between
page cache I/O and direct I/O. It definitely shouldn't be in the page cache,
and I do think it is VFS code. The one exception I see is
generic_perform_write(), which is analogous to filemap_read() and should stay
in filemap.c (and probably be renamed to something like filemap_write()).