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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	cluster-devel <cluster-devel@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] gfs2: Fix mmap + page fault deadlocks
Date: Thu, 1 Jul 2021 17:30:28 -0700	[thread overview]
Message-ID: <CAHk-=whaRsSuUeVBn_rLECWyG6h9RGuF9idsT_S+==vnsT8org@mail.gmail.com> (raw)
In-Reply-To: <CAHc6FU5XMfTOBW1a0OnMaPSmgWECvO-KYAo8HeVNsCyd-HyV3g@mail.gmail.com>

On Thu, Jul 1, 2021 at 5:20 PM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>
> On Thu, Jul 1, 2021 at 11:41 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > Also, I have to say that I think the direct-IO code is fundamentally
> > mis-designed. Why it is doing the page lookup _during_ the IO is a
> > complete mystery to me. Why wasn't that done ahead of time before the
> > filesystem took the locks it needed?
>
> That would be inconvenient for reads, when the number of bytes read is
> much smaller than the buffer size and we won't need to page in the
> entire buffer.

What?

A file read will READ THE WHOLE BUFFER.

We're not talking pipes or ttys here. If you ask for X bytes, you'll
get X bytes.

Of course, if you ask for more data than the file has, that's another
thing, but who really does that with direct-IO? And if they do, why
should we care about their silly behavior?

Face it, right now direct-IO is *BUGGY* because of this, and you can
deadlock filesystems with it.

So tell me again how it's "inconvenient" to fix this bug, and fix the
bad direct-IO design?

              Linus

WARNING: multiple messages have this Message-ID (diff)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: cluster-devel.redhat.com
Subject: [Cluster-devel] [PATCH] gfs2: Fix mmap + page fault deadlocks
Date: Thu, 1 Jul 2021 17:30:28 -0700	[thread overview]
Message-ID: <CAHk-=whaRsSuUeVBn_rLECWyG6h9RGuF9idsT_S+==vnsT8org@mail.gmail.com> (raw)
In-Reply-To: <CAHc6FU5XMfTOBW1a0OnMaPSmgWECvO-KYAo8HeVNsCyd-HyV3g@mail.gmail.com>

On Thu, Jul 1, 2021 at 5:20 PM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>
> On Thu, Jul 1, 2021 at 11:41 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > Also, I have to say that I think the direct-IO code is fundamentally
> > mis-designed. Why it is doing the page lookup _during_ the IO is a
> > complete mystery to me. Why wasn't that done ahead of time before the
> > filesystem took the locks it needed?
>
> That would be inconvenient for reads, when the number of bytes read is
> much smaller than the buffer size and we won't need to page in the
> entire buffer.

What?

A file read will READ THE WHOLE BUFFER.

We're not talking pipes or ttys here. If you ask for X bytes, you'll
get X bytes.

Of course, if you ask for more data than the file has, that's another
thing, but who really does that with direct-IO? And if they do, why
should we care about their silly behavior?

Face it, right now direct-IO is *BUGGY* because of this, and you can
deadlock filesystems with it.

So tell me again how it's "inconvenient" to fix this bug, and fix the
bad direct-IO design?

              Linus



  reply	other threads:[~2021-07-02  0:30 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-01 20:42 [PATCH] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-07-01 20:42 ` [Cluster-devel] " Andreas Gruenbacher
2021-07-01 21:41 ` Linus Torvalds
2021-07-01 21:41   ` [Cluster-devel] " Linus Torvalds
2021-07-01 21:52   ` Linus Torvalds
2021-07-01 21:52     ` [Cluster-devel] " Linus Torvalds
2021-07-02  0:20   ` Andreas Gruenbacher
2021-07-02  0:20     ` [Cluster-devel] " Andreas Gruenbacher
2021-07-02  0:30     ` Linus Torvalds [this message]
2021-07-02  0:30       ` Linus Torvalds
2021-07-02  0:44       ` Linus Torvalds
2021-07-02  0:44         ` [Cluster-devel] " Linus Torvalds
2021-07-02  1:14         ` Andreas Grünbacher
2021-07-02  1:14           ` [Cluster-devel] " Andreas Grünbacher

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