Re: [RFC v2 0/5] Non-blockling buffered fs read (page cache only)

From: Jonathan Corbet
Date: Fri Sep 19 2014 - 10:42:40 EST


On Wed, 17 Sep 2014 22:20:45 +0000
Milosz Tanski <milosz@xxxxxxxxx> wrote:

> This patcheset introduces an ability to perform a non-blocking read from
> regular files in buffered IO mode. This works by only for those filesystems
> that have data in the page cache.
>
> It does this by introducing new syscalls new syscalls readv2/writev2 and
> preadv2/pwritev2. These new syscalls behave like the network sendmsg, recvmsg
> syscalls that accept an extra flag argument (O_NONBLOCK).

So I'm trying to understand the reasoning behind this approach so I can
explain it to others. When you decided to add these syscalls, you
ruled out some other approaches that have been out there for a while.
I assume that, before these syscalls can be merged, people will want to
understand why you did that. So I'll ask the dumb questions:

- Non-blocking I/O has long been supported with a well-understood set
of operations - O_NONBLOCK and fcntl(). Why do we need a different
mechanism here - one that's only understood in the context of
buffered file I/O? I assume you didn't want to implement support
for poll() and all that, but is that a good enough reason to add a
new Linux-specific non-blocking I/O technique?

- Patches adding fincore() have been around since at least 2010; see,
for example, https://lwn.net/Articles/371538/ or
https://lwn.net/Articles/604640/. It seems this could be used in
favor of four new read() syscalls; is there a reason it's not
suitable for your use case?

- Patches adding buffered support for AIO have been around since at
least 2003 - https://lwn.net/Articles/24422/, for example. I guess
I don't really have to ask why you don't want to take that
approach! :)

Apologies for my ignorance here; that's what I get for hanging around
with the MM folks at LSFMM, I guess. Anyway, I suspect I'm not the
only one who would appreciate any background you could give here.

Thanks,

jon
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