Re: [PATCH 1/3] xen/gntdev: fix up blockable calls to mn_invl_range_start

Boris Ostrovsky
2018-08-27T20:03:28Z

On 08/27/2018 07:26 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.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@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Signed-off-by: Michal Hocko <mhocko@suse.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);
> }
>