linux-kernel - Re: [PATCH v4] mm: per-thread vma caching

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Message-ID: <1393902810.30648.36.camel@buesod1.americas.hpqcorp.net>
Date:	Mon, 03 Mar 2014 19:13:30 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>, aswin@...com,
	scott.norton@...com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm: per-thread vma caching

On Mon, 2014-03-03 at 19:12 -0800, Andrew Morton wrote:
> On Mon, 03 Mar 2014 18:42:33 -0800 Davidlohr Bueso <davidlohr@...com> wrote:
> 
> > On Mon, 2014-03-03 at 17:23 -0800, Andrew Morton wrote:
> > > On Mon, 03 Mar 2014 16:59:38 -0800 Davidlohr Bueso <davidlohr@...com> wrote:
> > > 
> > > > > >...
> > > > > >
> > > > > > +static bool vmacache_valid(struct mm_struct *mm)
> > > > > > +{
> > > > > > +	struct task_struct *curr = current;
> > > > > > +
> > > > > > +	if (mm != curr->mm)
> > > > > > +		return false;
> > > > > 
> > > > > What's going on here?  Handling a task poking around in someone else's
> > > > > mm?  I'm thinking "__access_remote_vm", but I don't know what you were
> > > > > thinking ;) An explanatory comment would be revealing.
> > > > 
> > > > I don't understand the doubt here. Seems like a pretty obvious thing to
> > > > check -- yes it's probably unlikely but we certainly don't want to be
> > > > validating the cache on an mm that's not ours... or are you saying it's
> > > > redundant??
> > > 
> > > Well it has to be here for a reason and I'm wondering that that reason
> > > is.  If nobody comes here with a foreign mm then let's remove it.
> > 
> > find_vma() can be called by concurrent threads sharing the mm->mmap_sem
> > for reading, thus this check needs to be there.
> 
> Confused.  If the threads share mm->mmap_sem then they share mm and the
> test will always be false?

Yes, I shortly realized that was silly... but I can say for sure it can
happen and a quick qemu run confirms it. So I see your point as to
asking why we need it, so now I'm looking for an explanation in the
code.

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