linux-kernel - Re: [PATCH 2/3] fork: reset mm->pinned

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Date:	Fri, 20 Jun 2014 11:38:38 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<oleg@...hat.com>, <rientjes@...gle.com>, <cl@...ux.com>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] fork: reset mm->pinned_vm

On Thu, Jun 19, 2014 at 01:58:20PM -0700, Andrew Morton wrote:
> On Thu, 19 Jun 2014 13:07:47 +0400 Vladimir Davydov <vdavydov@...allels.com> wrote:
> 
> > mm->pinned_vm counts pages of mm's address space that were permanently
> > pinned in memory by increasing their reference counter. The counter was
> > introduced by commit bc3e53f682d9 ("mm: distinguish between mlocked and
> > pinned pages"), while before it locked_vm had been used for such pages.
> > 
> > Obviously, we should reset the counter on fork if !CLONE_VM, just like
> > we do with locked_vm, but currently we don't. Let's fix it.
> > 
> > ...
> >
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -534,6 +534,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
> >  	atomic_long_set(&mm->nr_ptes, 0);
> >  	mm->map_count = 0;
> >  	mm->locked_vm = 0;
> > +	mm->pinned_vm = 0;
> >  	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
> >  	spin_lock_init(&mm->page_table_lock);
> >  	mm_init_cpumask(mm);
> 
> What are the runtime effects of this?  I think it is only
> "/proc/pid/status:VmPin is screwed up", because we don't use vm_pinned
> in rlimit checks.  Yes?

Hmm, ib_umem_get[infiniband] and perf_mmap still check pinned_vm against
RLIMIT_MEMLOCK. It's left from the times when pinned pages were
accounted under locked_vm, but today it looks wrong. It isn't clear to
me how we should deal with it.

And BTW, we still have some drivers accounting pinned pages under
mm->locked_vm - this is what commit bc3e53f682d9 was fighting against.
It's infiniband/usnic and vfio.

Thanks.
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