linux-kernel - Re: [PATCH 1/3] xen/gntdev: fix up blockable calls to mn_invl_range

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Date:   Mon, 27 Aug 2018 16:04:33 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>, Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 1/3] xen/gntdev: fix up blockable calls to
 mn_invl_range_start

On 08/27/2018 07:26 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
>
> 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers")
> has introduced blockable parameter to all mmu_notifiers and the notifier
> has to back off when called in !blockable case and it could block down
> the road.
>
> The above commit implemented that for mn_invl_range_start but both
> in_range checks are done unconditionally regardless of the blockable
> mode and as such they would fail all the time for regular calls.
> Fix this by checking blockable parameter as well.
>
> Once we are there we can remove the stale TODO. The lock has to be
> sleepable because we wait for completion down in gnttab_unmap_refs_sync.
>
> Changes since v1
> - pull in_range check into mn_invl_range_start - Juergen
>
> Fixes: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers")
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: Juergen Gross <jgross@...e.com>
> Signed-off-by: Michal Hocko <mhocko@...e.com>


LGTM, although in_range() has a single call site so we really don't need it.

I'll wait for Juergen to bless this since he asked for this approach.

-boris


> ---
>  drivers/xen/gntdev.c | 26 +++++++++++++++-----------
>  1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 57390c7666e5..b0b02a501167 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -492,12 +492,19 @@ static bool in_range(struct gntdev_grant_map *map,
>  	return true;
>  }
>  
> -static void unmap_if_in_range(struct gntdev_grant_map *map,
> -			      unsigned long start, unsigned long end)
> +static int unmap_if_in_range(struct gntdev_grant_map *map,
> +			      unsigned long start, unsigned long end,
> +			      bool blockable)
>  {
>  	unsigned long mstart, mend;
>  	int err;
>  
> +	if (!in_range(map, start, end))
> +		return 0;
> +
> +	if (!blockable)
> +		return -EAGAIN;
> +
>  	mstart = max(start, map->vma->vm_start);
>  	mend   = min(end,   map->vma->vm_end);
>  	pr_debug("map %d+%d (%lx %lx), range %lx %lx, mrange %lx %lx\n",
> @@ -508,6 +515,8 @@ static void unmap_if_in_range(struct gntdev_grant_map *map,
>  				(mstart - map->vma->vm_start) >> PAGE_SHIFT,
>  				(mend - mstart) >> PAGE_SHIFT);
>  	WARN_ON(err);
> +
> +	return 0;
>  }
>  
>  static int mn_invl_range_start(struct mmu_notifier *mn,
> @@ -519,25 +528,20 @@ 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)) {
> -			ret = -EAGAIN;
> +		ret = unmap_if_in_range(map, start, end, blockable);
> +		if (ret)
>  			goto out_unlock;
> -		}
> -		unmap_if_in_range(map, start, end);
>  	}
>  	list_for_each_entry(map, &priv->freeable_maps, next) {
> -		if (in_range(map, start, end)) {
> -			ret = -EAGAIN;
> +		ret = unmap_if_in_range(map, start, end, blockable);
> +		if (ret)
>  			goto out_unlock;
> -		}
> -		unmap_if_in_range(map, start, end);
>  	}
>  
>  out_unlock: