linux-kernel - Re: [PATCH] xen/gntdev: fix up blockable calls to mn_invl_range

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Date:   Thu, 23 Aug 2018 10:06:53 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org,
        LKML <linux-kernel@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] xen/gntdev: fix up blockable calls to mn_invl_range_start

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;
>  		}