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Subject: Re: [PATCH] xen/gntdev: fix up blockable calls to mn_invl_range_start
To:     Michal Hocko <mhocko@kernel.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        xen-devel@lists.xenproject.org,
        LKML <linux-kernel@vger.kernel.org>,
        Juergen Gross <jgross@suse.com>
References: <20180823120707.10998-1-mhocko@kernel.org>
 <07c7ead4-334d-9b25-f588-25e9b46bbea0@i-love.sakura.ne.jp>
 <20180823135151.GM29735@dhcp22.suse.cz>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <9d2d11eb-7fe1-b836-056c-7886d6fc56e5@oracle.com>
Date:   Thu, 23 Aug 2018 10:06:53 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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On 08/23/2018 09:51 AM, Michal Hocko wrote:
> On Thu 23-08-18 22:44:07, Tetsuo Handa wrote:
>> On 2018/08/23 21:07, Michal Hocko wrote:
>>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>>> index 57390c7666e5..e7d8bb1bee2a 100644
>>> --- a/drivers/xen/gntdev.c
>>> +++ b/drivers/xen/gntdev.c
>>> @@ -519,21 +519,20 @@ static int mn_invl_range_start(struct mmu_notifier *mn,
>>>  	struct gntdev_grant_map *map;
>>>  	int ret = 0;
>>>  
>>> -	/* TODO do we really need a mutex here? */
>>>  	if (blockable)
>>>  		mutex_lock(&priv->lock);
>>>  	else if (!mutex_trylock(&priv->lock))
>>>  		return -EAGAIN;
>>>  
>>>  	list_for_each_entry(map, &priv->maps, next) {
>>> -		if (in_range(map, start, end)) {
>>> +		if (!blockable && in_range(map, start, end)) {
>> This still looks strange. Prior to 93065ac753e4, in_range() test was
>> inside unmap_if_in_range(). But this patch removes in_range() test
>> if blockable == true. That is, unmap_if_in_range() will unconditionally
>> unmap if blockable == true, which seems to be an unexpected change.
> You are right. I completely forgot I've removed in_range there. Does
> this look any better?
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index e7d8bb1bee2a..30f81004ea63 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -525,14 +525,20 @@ static int mn_invl_range_start(struct mmu_notifier *mn,
>  		return -EAGAIN;
>  
>  	list_for_each_entry(map, &priv->maps, next) {
> -		if (!blockable && in_range(map, start, end)) {
> +		if (in_range(map, start, end)) {
> +			if (blockable)
> +				continue;
> +
>  			ret = -EAGAIN;
>  			goto out_unlock;
>  		}
>  		unmap_if_in_range(map, start, end);


(I obviously missed that too with my R-b).

This will never get anything done either. How about

    if (in_range()) {
        if (!blockable) {
            ret = -EGAIN;
            goto out_unlock;
        }
        unmap_range(); // new name since unmap_if_in_range() doesn't
perform any checks now
    }


-boris


>  	}
>  	list_for_each_entry(map, &priv->freeable_maps, next) {
> -		if (!blockable && in_range(map, start, end)) {
> +		if (in_range(map, start, end)) {
> +			if (blockable)
> +				continue;
> +			
>  			ret = -EAGAIN;
>  			goto out_unlock;
>  		}