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

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Message-ID: <20180326145431.GC1840639@devbig577.frc2.facebook.com>
Date:   Mon, 26 Mar 2018 07:54:31 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     security@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-mm@...ck.org,
        torvalds@...ux-foundation.org, jannh@...gle.com,
        paulmck@...ux.vnet.ibm.com, bcrl@...ck.org,
        viro@...iv.linux.org.uk, kent.overstreet@...il.com
Subject: Re: [PATCH 4/8] HMM: Remove superflous RCU protection around radix
 tree lookup

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.

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@...nel.org>
> Reviewed-by: Jérôme Glisse <jglisse@...hat.com>
> Cc: linux-mm@...ck.org
> Cc: Linus Torvalds <torvalds@...ux-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