From: Linus Torvalds <torvalds@linux-foundation.org>
To: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Matthew Wilcox <willy@infradead.org>,
Luis Chamberlain <mcgrof@kernel.org>,
lsf-pc@lists.linux-foundation.org,
linux-fsdevel@vger.kernel.org, linux-mm <linux-mm@kvack.org>,
Daniel Gomez <da.gomez@samsung.com>,
Pankaj Raghav <p.raghav@samsung.com>,
Jens Axboe <axboe@kernel.dk>, Dave Chinner <david@fromorbit.com>,
Christoph Hellwig <hch@lst.de>, Chris Mason <clm@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO
Date: Sun, 25 Feb 2024 18:34:26 -0800 [thread overview]
Message-ID: <CAHk-=wjmUQN4vZJAotcHHPcoEMV7OutKWJc1dtrDXErdxTMgcA@mail.gmail.com> (raw)
In-Reply-To: <sfh6bvpihpbtwb7tgdkrhfd333qcxrqmvl52s5v5gbdpd2hvwl@p7aoxxndqk75>
On Sun, 25 Feb 2024 at 17:58, Kent Overstreet <kent.overstreet@linux.dev> wrote:
>
> According to my reading just now, ext4 and btrfs (as well as bcachefs)
> also don't take the inode lock in the read path - xfs is the only one
> that does.
Yeah, I should have remembered that detail - DaveC has pointed it out
at some point how other filesystems don't actually honor the whole
"all or nothing visible to read".
And I was actually wrong about the common cases like ext2 - they use
generic_file_write_iter(), which does take that inode lock, and I was
confused with generic_perform_write() (which does not).
It was always the read side that didn't care, as you point out. It's
been some time since I looked at that.
But as mentioned, nobody has actually ever shown any real interest in
caring about the lack of POSIX technicality.
> I think write vs. write consistency is the more interesting case; the
> question there is does falling back to the inode lock when we can't lock
> all the folios simultaneously work.
I really don't think the write-write consistency is all that
interesting either, and it really does hurt.
If you're some toy database that would love to use buffered writes on
just a DB file, that "no concurrent writes" can hurt a lot. So then
people say "use DIO", but that has its own issues...
There is one obvious special case, and I think it's the primary one
why we end up having that inode_lock: O_APPEND or any other write
extending the size of the file.
THAT one obviously has to work right, and that's the case when
multiple writers actually do want to get write-write consistency, and
where it makes total sense to serialize them all.
That's the one case that even DIO cares about.
In the other cases, it's hard to argue that "one or the other wins the
whole range" is seriously hugely better than "one or the other wins at
some granularity". What the hell are you doing overlapping write
ranges for if you have a "one or the other" mentality?
Of course, maybe in practice it would be fine to do the "lock all the
folios, with the fallback being the inode lock" - and we could even
start with "all" being a pretty small number (perhaps starting with
"one" ;^).
Linus
next prev parent reply other threads:[~2024-02-26 2:34 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-23 23:59 [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO Luis Chamberlain
2024-02-24 4:12 ` Matthew Wilcox
2024-02-24 17:31 ` Linus Torvalds
2024-02-24 18:13 ` Matthew Wilcox
2024-02-24 18:24 ` Linus Torvalds
2024-02-24 18:20 ` Linus Torvalds
2024-02-24 19:11 ` Linus Torvalds
2024-02-24 21:42 ` Theodore Ts'o
2024-02-24 22:57 ` Chris Mason
2024-02-24 23:40 ` Linus Torvalds
2024-05-10 23:57 ` Luis Chamberlain
2024-02-25 5:18 ` Kent Overstreet
2024-02-25 6:04 ` Kent Overstreet
2024-02-25 13:10 ` Matthew Wilcox
2024-02-25 17:03 ` Linus Torvalds
2024-02-25 21:14 ` Matthew Wilcox
2024-02-25 23:45 ` Linus Torvalds
2024-02-26 1:02 ` Kent Overstreet
2024-02-26 1:32 ` Linus Torvalds
2024-02-26 1:58 ` Kent Overstreet
2024-02-26 2:06 ` Kent Overstreet
2024-02-26 2:34 ` Linus Torvalds [this message]
2024-02-26 2:50 ` Al Viro
2024-02-26 17:17 ` Linus Torvalds
2024-02-26 21:07 ` Matthew Wilcox
2024-02-26 21:17 ` Kent Overstreet
2024-02-26 21:19 ` Kent Overstreet
2024-02-26 21:55 ` Paul E. McKenney
2024-02-26 23:29 ` Kent Overstreet
2024-02-27 0:05 ` Paul E. McKenney
2024-02-27 0:29 ` Kent Overstreet
2024-02-27 0:55 ` Paul E. McKenney
2024-02-27 1:08 ` Kent Overstreet
2024-02-27 5:17 ` Paul E. McKenney
2024-02-27 6:21 ` Kent Overstreet
2024-02-27 15:32 ` Paul E. McKenney
2024-02-27 15:52 ` Kent Overstreet
2024-02-27 16:06 ` Paul E. McKenney
2024-02-27 15:54 ` Matthew Wilcox
2024-02-27 16:21 ` Paul E. McKenney
2024-02-27 16:34 ` Kent Overstreet
2024-02-27 17:58 ` Paul E. McKenney
2024-02-28 23:55 ` Kent Overstreet
2024-02-29 19:42 ` Paul E. McKenney
2024-02-29 20:51 ` Kent Overstreet
2024-03-05 2:19 ` Paul E. McKenney
2024-02-27 0:43 ` Dave Chinner
2024-02-26 22:46 ` Linus Torvalds
2024-02-26 23:48 ` Linus Torvalds
2024-02-27 7:21 ` Kent Overstreet
2024-02-27 15:39 ` Matthew Wilcox
2024-02-27 15:54 ` Kent Overstreet
2024-02-27 16:34 ` Linus Torvalds
2024-02-27 16:47 ` Kent Overstreet
2024-02-27 17:07 ` Linus Torvalds
2024-02-27 17:20 ` Kent Overstreet
2024-02-27 18:02 ` Linus Torvalds
2024-05-14 11:52 ` Luis Chamberlain
2024-05-14 16:04 ` Linus Torvalds
2024-02-25 21:29 ` Kent Overstreet
2024-02-25 17:32 ` Kent Overstreet
2024-02-24 17:55 ` Luis Chamberlain
2024-02-25 5:24 ` Kent Overstreet
2024-02-26 12:22 ` Dave Chinner
2024-02-27 10:07 ` Kent Overstreet
2024-02-27 14:08 ` Luis Chamberlain
2024-02-27 14:57 ` Kent Overstreet
2024-02-27 22:13 ` Dave Chinner
2024-02-27 22:21 ` Kent Overstreet
2024-02-27 22:42 ` Dave Chinner
2024-02-28 7:48 ` [Lsf-pc] " Amir Goldstein
2024-02-28 14:01 ` Chris Mason
2024-02-29 0:25 ` Dave Chinner
2024-02-29 0:57 ` Kent Overstreet
2024-03-04 0:46 ` Dave Chinner
2024-02-27 22:46 ` Linus Torvalds
2024-02-27 23:00 ` Linus Torvalds
2024-02-28 2:22 ` Kent Overstreet
2024-02-28 3:00 ` Matthew Wilcox
2024-02-28 4:22 ` Matthew Wilcox
2024-02-28 17:34 ` Kent Overstreet
2024-02-28 18:04 ` Matthew Wilcox
2024-02-28 18:18 ` Kent Overstreet
2024-02-28 19:09 ` Linus Torvalds
2024-02-28 19:29 ` Kent Overstreet
2024-02-28 20:17 ` Linus Torvalds
2024-02-28 23:21 ` Kent Overstreet
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAHk-=wjmUQN4vZJAotcHHPcoEMV7OutKWJc1dtrDXErdxTMgcA@mail.gmail.com' \
--to=torvalds@linux-foundation.org \
--cc=axboe@kernel.dk \
--cc=clm@fb.com \
--cc=da.gomez@samsung.com \
--cc=david@fromorbit.com \
--cc=hannes@cmpxchg.org \
--cc=hch@lst.de \
--cc=kent.overstreet@linux.dev \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=lsf-pc@lists.linux-foundation.org \
--cc=mcgrof@kernel.org \
--cc=p.raghav@samsung.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).