Re: [PATCH v2 00/13] Support sync buffered writes for io-uring

[Dave Chinner <david@fromorbit.com>]

On Fri, Feb 18, 2022 at 11:57:26AM -0800, Stefan Roesch wrote:
> This patch series adds support for async buffered writes. Currently
> io-uring only supports buffered writes in the slow path, by processing
> them in the io workers. With this patch series it is now possible to
> support buffered writes in the fast path. To be able to use the fast
> path the required pages must be in the page cache or they can be loaded
> with noio. Otherwise they still get punted to the slow path.

Where's the filesystem support? You need to plumb in ext4 to this
bufferhead support, and add iomap/xfs support as well so we can
shake out all the problems with APIs and fallback paths that are
needed for full support of buffered writes via io_uring.

> If a buffered write request requires more than one page, it is possible
> that only part of the request can use the fast path, the resst will be
> completed by the io workers.

That's ugly, especially at the filesystem/iomap layer where we are
doing delayed allocation and so partial writes like this could have
significant extra impact. It opens up the possibility of things like
ENOSPC/EDQUOT mid-way through the write instead of being an up-front
error, and so there's lots more complexity in the failure/fallback
paths that the io_uring infrastructure will have to handle
correctly...

Also, it breaks the "atomic buffered write" design of iomap/XFS
where other readers and writers will only see whole completed writes
and not intermediate partial writes. This is where a lot of the bugs
in the DIO io_uring support were found (deadlocks, data corruptions,
etc), so there's a bunch of semantic and API issues that filesystems
require from io_uring that need to be sorted out before we think
about merge buffered write support...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com