linux-kernel - Re: io_pgetevents & aio fsync V2

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Message-ID: <x497epklcfc.fsf@segfault.boston.devel.redhat.com>
Date:   Fri, 06 Apr 2018 08:57:59 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Al Viro <viro@...IV.linux.org.uk>, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: io_pgetevents & aio fsync V2

Christoph Hellwig <hch@....de> writes:

> On Fri, Apr 06, 2018 at 04:16:30AM +0100, Al Viro wrote:
>> BTW, this is only tangentially related, but... does *anything* call
>> io_submit() for huge amounts of iocb?

I don't know.  If an application did that, as many I/Os as could fit
into the ring buffer would be submitted, and that's what gets returned
from the system call (the number of submitted iocbs).

>> Check in do_io_submit() is insane - "no more than MAX_LONG total of
>> _pointers_".  Compat variant goes for "no more than a page worth of
>> pointers" and there's a hard limit in ioctx_alloc() - we can't ever
>> get more than 8M slots in ring buffer...
>
> Logical upper bound for io_submit is nr_events passed to io_setup(),
> which is bound by aio_max_nr.  Except that we never actually check
> against nr_events (or max_reqs as it is known in kernel) in io_submit.
> Sigh..

io_submit_one calls aio_get_req which calls get_reqs_available, which is
what does the checking for an available ring buffer entry.

-Jeff