linux-kernel - Re: [RFC, PATCH] Extensible AIO interface

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Message-ID: <20121001231222.GB14533@lenny.home.zabbo.net>
Date:	Mon, 1 Oct 2012 16:12:22 -0700
From:	Zach Brown <zab@...bo.net>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tytso@...gle.com, tj@...nel.org,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	"Maxim V. Patlasov" <mpatlasov@...allels.com>,
	michael.mesnier@...el.com, jeffrey.d.skirvin@...el.com,
	Martin Petersen <martin.petersen@...cle.com>
Subject: Re: [RFC, PATCH] Extensible AIO interface

On Mon, Oct 01, 2012 at 03:23:41PM -0700, Kent Overstreet wrote:
> So, I and other people keep running into things where we really need to
> add an interface to pass some auxiliary... stuff along with a pread() or
> pwrite().

Sure.  Martin (cc:ed) will sympathize.

> A few examples:
> 
> * IO scheduler hints...
> * Cache hints...
> 
> * Passing checksums out to userspace. We've got bio integrity, which is
> a (somewhat) generic interface for passing data checksums between the
> filesystem and the hardware.

Hmm, careful here.  I think that in DIF/DIX the checksums are
per-sector, not per IO, right?  That'd mean that the PAGE_SIZE attr
limit in this patch would be magically creating different max IO size
limits on different architectures.  That doesn't seem great.

> Hence, AIO attributes.

I have to be honest: I really don't like tying the interface to AIO, but
I guess it's the only per-io facility we have today.  It'd be nice to
include sync O_DIRECT when designing the interface to make sure that it
is possible to use generic syscalls in the future without running up
against unexpected problems. 

> An iocb_attr has an id field, and a size field - and some amount of data
> specific to that attribute.

I'd hope that we can come up with a less fragile interface.  The kernel
would have to scan the attributes to make sure that there aren't
malicious sizes.  I only quickly glanced at the loops, but it seemed
like you could have a 0 size attribute in there and _next() would spin
forever.

- z
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