Re: [PATCH] Documentation: document the design of iomap and how to port

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Jun 12, 2024 at 12:07:40PM +0530, Ritesh Harjani wrote:
> 
> "Darrick J. Wong" <djwong@kernel.org> writes:
> 
> > On Tue, Jun 11, 2024 at 04:15:02PM +0530, Ritesh Harjani wrote:
> >> 
> >> Hi Darrick,
> >> 
> >> Resuming my review from where I left off yesterday.
> >
> 
> <snip>
> >> > +Writes
> >> > +~~~~~~
> >> > +
> >> > +The ``iomap_file_buffered_write`` function writes an ``iocb`` to the
> >> > +pagecache.
> >> > +``IOMAP_WRITE`` or ``IOMAP_WRITE`` | ``IOMAP_NOWAIT`` will be passed as
> >> > +the ``flags`` argument to ``->iomap_begin``.
> >> > +Callers commonly take ``i_rwsem`` in either shared or exclusive mode.
> >> 
> >> shared(e.g. aligned overwrites) 
> >
> 
> Ok, I see we were in buffered I/O section (Sorry, I misunderstood
> thinking this was for direct-io)

Aha.  I'll change these headings to "Buffered Readahead and Reads" and
"Buffered Writes".

> > That's a matter of debate -- xfs locks out concurrent reads by taking
> > i_rwsem in exclusive mode, whereas (I think?) ext4 and most other
> > filesystems take it in shared mode and synchronizes readers and writers
> > with folio locks.
> 
> Ext4 too takes inode lock in exclusive mode in case of
> buffered-write. It's the DIO writes/overwrites in ext4 which has special
> casing for shared/exclusive mode locking.
> 
> But ext4 buffered-read does not take any inode lock (it uses
> generic_file_read_iter()). So the synchronization must happen via folio
> lock w.r.t buffered-writes.
> 
> However, I am not sure if we have any filesystem taking VFS inode lock in
> shared more for buffered-writes.

In theory you could if no other metadata needed updating, such as a dumb
filesystem with fixed size files where timestamps don't matter.

> BTW -
> I really like all of the other updates that you made w.r.t the review
> comments. All of those looks more clear to me. (so not commenting on them
> individually).
> 
> Thanks!

No, thank /you/ and everyone else for reading all the way through it.
I'll finish cleaning things up and put out a v2.

--D

> -ritesh
>