linux-kernel - Re: [LSF/MM/BPF TOPIC] untorn buffered writes

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Date: Wed, 22 May 2024 14:56:57 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: John Garry <john.g.garry@...cle.com>, David Bueso <dave@...olabs.net>
Cc: Theodore Ts'o <tytso@....edu>, lsf-pc@...ts.linux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org
Subject: Re: [LSF/MM/BPF TOPIC] untorn buffered writes

On Wed, May 15, 2024 at 01:54:39PM -0600, John Garry wrote:
> On 27/02/2024 23:12, Theodore Ts'o wrote:
> > Last year, I talked about an interest to provide database such as
> > MySQL with the ability to issue writes that would not be torn as they
> > write 16k database pages[1].
> > 
> > [1] https://urldefense.com/v3/__https://lwn.net/Articles/932900/__;!!ACWV5N9M2RV99hQ!Ij_ZeSZrJ4uPL94Im73udLMjqpkcZwHmuNnznogL68ehu6TDTXqbMsC4xLUqh18hq2Ib77p1D8_4mV5Q$
> > 
> 
> After discussing this topic earlier this week, I would like to know if there
> are still objections or concerns with the untorn-writes userspace API
> proposed in https://lore.kernel.org/linux-block/20240326133813.3224593-1-john.g.garry@oracle.com/
> 
> I feel that the series for supporting direct-IO only, above, is stuck
> because of this topic of buffered IO.

I think it was good we had the discussions at LSFMM over it, however
I personally don't percieve it as stuck, however without any consensus
being obviated or written down anywhere it would not be clear to anyone
that we did reach any consensus at all. Hope is that lwn captures any
consensus if any was indeed reached as you're not making it clear any
was.

In case it helps, as we did with the LBS effort it may also be useful to
put together bi-monthly cabals to follow up progress, and divide and
conquer any pending work items.

> So I sent an RFC for buffered untorn-writes last month in https://lore.kernel.org/linux-fsdevel/20240422143923.3927601-1-john.g.garry@oracle.com/,
> which did leverage the bs > ps effort. Maybe it did not get noticed due to
> being an RFC. It works on the following principles:
> 
> - A buffered atomic write requires RWF_ATOMIC flag be set, same as
>   direct IO. The same other atomic writes rules apply.
> - For an inode, only a single size of buffered write is allowed. So for
>   statx, atomic_write_unit_min = atomic_write_unit_max always for
>   buffered atomic writes.
> - A single folio maps to an atomic write in the pagecache. So inode
>   address_space folio min order = max order = atomic_write_unit_min/max
> - A folio is tagged as "atomic" when atomically written and written back
>   to storage "atomically", same as direct-IO method would do for an
>   atomic write.
> - If userspace wants to guarantee a buffered atomic write is written to
>   storage atomically after the write syscall returns, it must use
>   RWF_SYNC or similar (along with RWF_ATOMIC).

>From my perspective the above just needs the IOCB atomic support, and
the pending long term work item there is the near-write-through buffered
IO support. We could just wait for buffered-IO support until we have
support for that. I can't think of anying blocking DIO support though,
now that we at least have a mental model of how buffered IO *should*
work.

What about testing? Are you extending fstests, blktests?

  Luis