linux-kernel - Re: [PATCH] mm: Revert pinned

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Date:	Thu, 20 Jun 2013 14:48:56 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, roland@...nel.org,
	tglx@...utronix.de, kosaki.motohiro@...il.com, penberg@...nel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-rdma@...r.kernel.org
Subject: Re: [PATCH] mm: Revert pinned_vm braindamage

On Thu, 20 Jun 2013, Ingo Molnar wrote:

> Peter clearly pointed it out that in the perf case it's user-space that
> initiates the pinned memory mapping which is resource-controlled via
> RLIMIT_MEMLOCK - and this was implemented that way before your commit
> broke the code.

There is no way that user space can initiate a page pin right now. Perf is
pinning the page from the kernel. Similarly the IB subsystem pins memory
meeded for device I/O.

> You seem to be hell bent on defining 'memory pinning' only as "the thing
> done via the mlock*() system calls", but that is a nonsensical distinction
> that actively and incorrectly ignores other system calls that can and do
> pin memory legitimately.

Nope. I have said that Memory pinning is done by increasing the refcount
which is different from mlock which sets a page flag.

I have consistently argued that these are two different things. And I am
a
bit surprised that this point has not been understood after all these
repetitions.

Memory pinning these days is done as a side effect of kernel / driver
needs. I.e. the memory registration done through the IB subsystem and
elsewhere.

> int can_do_mlock(void)
> {
>         if (capable(CAP_IPC_LOCK))
>                 return 1;
>         if (rlimit(RLIMIT_MEMLOCK) != 0)
>                 return 1;
>         return 0;
> }
> EXPORT_SYMBOL(can_do_mlock);
>
> Q.E.D.

Argh. Just checked the apps. True. They did set the rlimit to 0 at some
point in order to make this work. Then they monitor the number of locked
pages and create alerts so that action can be taking if a system uses too
many mlocked pages.
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