Re: [PATCH 4/8] HMM: Remove superflous RCU protection around radix tree lookup

From: Jerome Glisse <jglisse@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	security@kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, linux-mm@kvack.org,
	torvalds@linux-foundation.org, jannh@google.com,
	paulmck@linux.vnet.ibm.com, bcrl@kvack.org,
	viro@zeniv.linux.org.uk, kent.overstreet@gmail.com
Subject: Re: [PATCH 4/8] HMM: Remove superflous RCU protection around radix tree lookup
Date: Tue, 27 Mar 2018 12:12:45 -0400	[thread overview]
Message-ID: <20180327161244.GA4251@redhat.com> (raw)
In-Reply-To: <20180326145431.GC1840639@devbig577.frc2.facebook.com>

On Mon, Mar 26, 2018 at 07:54:31AM -0700, Tejun Heo wrote:
> Hello, Andrew.
> 
> Do you mind picking up the following patch?  I can't find a good tree
> to route this through.  The raw patch can be found at
> 
>   https://marc.info/?l=linux-mm&m=152105674112496&q=raw
> 
> Thank you very much.

I am fine with which ever route there is low probability of conflict
when merging HMM through different tree.

> 
> On Wed, Mar 14, 2018 at 12:45:11PM -0700, Tejun Heo wrote:
> > hmm_devmem_find() requires rcu_read_lock_held() but there's nothing
> > which actually uses the RCU protection.  The only caller is
> > hmm_devmem_pages_create() which already grabs the mutex and does
> > superflous rcu_read_lock/unlock() around the function.
> > 
> > This doesn't add anything and just adds to confusion.  Remove the RCU
> > protection and open-code the radix tree lookup.  If this needs to
> > become more sophisticated in the future, let's add them back when
> > necessary.
> > 
> > Signed-off-by: Tejun Heo <tj@kernel.org>
> > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> > Cc: linux-mm@kvack.org
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > ---
> > Hello,
> > 
> > Jérôme, how do you want to route this patch?  If you prefer, I can
> > route it together with other patches.
> > 
> > Thanks.
> > 
> >  mm/hmm.c | 12 ++----------
> >  1 file changed, 2 insertions(+), 10 deletions(-)
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 320545b98..d4627c5 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -845,13 +845,6 @@ static void hmm_devmem_release(struct device *dev, void *data)
> >  	hmm_devmem_radix_release(resource);
> >  }
> >  
> > -static struct hmm_devmem *hmm_devmem_find(resource_size_t phys)
> > -{
> > -	WARN_ON_ONCE(!rcu_read_lock_held());
> > -
> > -	return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT);
> > -}
> > -
> >  static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
> >  {
> >  	resource_size_t key, align_start, align_size, align_end;
> > @@ -892,9 +885,8 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
> >  	for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) {
> >  		struct hmm_devmem *dup;
> >  
> > -		rcu_read_lock();
> > -		dup = hmm_devmem_find(key);
> > -		rcu_read_unlock();
> > +		dup = radix_tree_lookup(&hmm_devmem_radix,
> > +					key >> PA_SECTION_SHIFT);
> >  		if (dup) {
> >  			dev_err(device, "%s: collides with mapping for %s\n",
> >  				__func__, dev_name(dup->device));
> > -- 
> > 2.9.5
> > 
> 
> -- 
> tejun