On 1/17/19 2:02 PM, Jeff Moyer wrote:

Jens Axboe [off-list ref] writes:

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It seems that sq_entries, cq_entries are not limited at all.  Can nasty
app consume a lot of kernel pages calling io_setup_uring() from a loop
passing random entries number? (or even better: decreasing entries 
number,
in order to consume all pages orders with min number of loops).

...

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One concern here is that, at least looking at my boxes, the default
setting for RLIMIT_MEMLOCK is really low. I'd hate for everyone to run
into issues using io_uring just because it seems to require root,
because the memlock limit is so low.

That's much less of a concern with the fixed buffers, since it's a more
esoteric part of it. But everyone should be able to setup a few io_uring
queues and use them without having to worry about failing due to an
absurdly low RLIMIT_MEMLOCK.

Comments?

Yeah, the default is 64k here.  We should probably up that.  I'd say we
either tackle the ridiculously low rlimits, or I guess we just go the
aio route and add a sysctl.  :-\  I'll see what's involved in the
former.

After giving it a bit of thought, let's go the rlimit route. It is cleaner,
and I don't want a sysctl knob for this either. 64k will enable anyone to
set up at least one decently sized ring.

OK.  Note that the MLOCK_LIMIT size has been dictated by gpg's
requirements:

commit f947ff8af30f75cb9cf0e966caf8f4809ad1b92e
Author: Rik van Riel [off-list ref]
Date:   Sun Aug 22 23:06:58 2004 -0700

    [PATCH] increase per-user mlock limit default to 32k

    Since various gnupg users have indicated that gpg wants to mlock 32kB of
    memory, I created the patch below that increases the default mlock ulimit
    to 32kB.

and then

commit 0833422274ff00729a603b020fac297e69a03e40
Author: Kurt Garloff [off-list ref]
Date:   Wed Oct 29 14:00:48 2008 -0700

    mm: increase the default mlock limit from 32k to 64k

...
    However, newer gpg2 needs 64k in various circumstances and otherwise
    fails miserably, see bnc#329675.

So all we need to do is modify gpg2 so that is requires more locked
memory, and we're golden!  ;-)

Haha, that's some nice digging there!

Yes, we could bump it, but with the default, we can get a 512 sized
ring per user, that's 13 pages (rounded up). Probably good enough to
get things off the ground?

-- 
Jens Axboe

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