Re: io_pgetevents & aio fsync V2

Jeff Moyer
2018-04-06T12:58:02Z

Christoph Hellwig <hch@lst.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