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

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Date:	Thu, 13 Jun 2013 14:06:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	torvalds@...ux-foundation.org, roland@...nel.org, mingo@...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, Mike Marciniszyn <infinipath@...el.com>
Subject: Re: [PATCH] mm: Revert pinned_vm braindamage

Let's try to get this wrapped up?

On Thu, 6 Jun 2013 14:43:51 +0200 Peter Zijlstra <peterz@...radead.org> wrote:

> 
> Patch bc3e53f682 ("mm: distinguish between mlocked and pinned pages")
> broke RLIMIT_MEMLOCK.

I rather like what bc3e53f682 did, actually.  RLIMIT_MEMLOCK limits the
amount of memory you can mlock().  Nice and simple.

This pinning thing which infiniband/perf are doing is conceptually
different and if we care at all, perhaps we should be looking at adding
RLIMIT_PINNED.

> Before that patch: mm_struct::locked_vm < RLIMIT_MEMLOCK; after that
> patch we have: mm_struct::locked_vm < RLIMIT_MEMLOCK &&
> mm_struct::pinned_vm < RLIMIT_MEMLOCK.

But this is a policy decision which was implemented in perf_mmap() and
perf can alter that decision.  How bad would it be if perf just ignored
RLIMIT_MEMLOCK?

drivers/infiniband/hw/qib/qib_user_pages.c has issues, btw.  It
compares the amount-to-be-pinned with rlimit(RLIMIT_MEMLOCK), but
forgets to also look at current->mm->pinned_vm.  Duh.

It also does the pinned accounting in __qib_get_user_pages() but in
__qib_release_user_pages(), the caller is supposed to do it, which is
rather awkward.

Longer-term I don't think that inifinband or perf should be dinking
around with rlimit(RLIMIT_MEMLOCK) or ->pinned_vm.  Those policy
decisions should be hoisted into a core mm helper where we can do it
uniformly (and more correctly than infiniband's attempt!).

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