Re: [PATCH v7 7/7] mm/madvise: allow KSM hints for remote API

From: Michal Hocko <mhocko@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Oleksandr Natalenko <oleksandr@redhat.com>,
	Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	linux-api@vger.kernel.org, Suren Baghdasaryan <surenb@google.com>,
	Tim Murray <timmurray@google.com>,
	Daniel Colascione <dancol@google.com>,
	Sandeep Patil <sspatil@google.com>,
	Sonny Rao <sonnyrao@google.com>,
	Brian Geffon <bgeffon@google.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Shakeel Butt <shakeelb@google.com>,
	John Dias <joaodias@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Jann Horn <jannh@google.com>,
	alexander.h.duyck@linux.intel.com, sj38.park@gmail.com,
	SeongJae Park <sjpark@amazon.de>
Subject: Re: [PATCH v7 7/7] mm/madvise: allow KSM hints for remote API
Date: Mon, 9 Mar 2020 17:03:32 +0100	[thread overview]
Message-ID: <20200309160332.GS8447@dhcp22.suse.cz> (raw)
In-Reply-To: <34f812b8-df54-eaad-5cf0-335f07da55c6@suse.cz>

On Mon 09-03-20 16:42:43, Vlastimil Babka wrote:
> On 3/9/20 4:19 PM, Oleksandr Natalenko wrote:
> > On Mon, Mar 09, 2020 at 04:08:15PM +0100, Michal Hocko wrote:
> >> On Mon 09-03-20 14:11:17, Oleksandr Natalenko wrote:
> >> > On Fri, Mar 06, 2020 at 05:08:18PM +0100, Vlastimil Babka wrote:
> >> [...]
> >> > > Dunno, it's nice to react to signals quickly, for any proces that gets them, no?
> >> > 
> >> > So, do you mean something like this?
> >> > 
> >> > ===
> >> > diff --git a/mm/ksm.c b/mm/ksm.c
> >> > index 363ec8189561..b39c237cfcf4 100644
> >> > --- a/mm/ksm.c
> >> > +++ b/mm/ksm.c
> >> > @@ -849,7 +849,8 @@ static int unmerge_ksm_pages(struct vm_area_struct *vma,
> >> >  	for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
> >> >  		if (ksm_test_exit(vma->vm_mm))
> >> >  			break;
> >> > -		if (signal_pending(current))
> >> > +		if (signal_pending(current) ||
> >> > +		    signal_pending(rcu_dereference(vma->vm_mm->owner)))
> >> >  			err = -ERESTARTSYS;
> >> >  		else
> >> >  			err = break_ksm(vma, addr);
> >> > ===
> >> 
> >> This is broken because mm might be attached to different tasks.
> >> AFAIU this check is meant to allow quick backoff of the _calling_
> >> process so that it doesn't waste time when the context is killed
> >> already. I do not understand why should we care about any other context
> >> here? What is the actual problem this would solve?
> > 
> > I agree with you, but still trying to understand what does Vlastimil mean
> > :).
> 
> Well you wondered if we should stop caring about current, and I said that
> probably wouldn't be nice.
> As for caring about the other task, patch 3/7 does that for
> (MADV_COLD|MADV_PAGEOUT) so I just pointed out that the KSM case doesn't. AFAIU
> if we don't check the signals, we might be blocking the killed task from exiting?

I would have to double check but I do not think this would be a problem
because the remote task should take mmget to prevent address space to
vanish under its feet. That should also rule out the exclusive mmap_sem
usage from the exit path.
-- 
Michal Hocko
SUSE Labs