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

Ritesh Harjani
2024-06-12T06:55:19Z

"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)

> 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.

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!
-ritesh