RE: [RFC PATCH V2 1/10] mm/resource: Move child to new resource when release mem region.

[Michael Kelley <mikelley@microsoft.com>]

From: Tianyu Lan <Tianyu.Lan@microsoft.com> Sent: Tuesday, January 7, 2020 5:10 AM
> 
> When release mem region, old mem region may be splited to
> two regions. Current allocate new struct resource for high
> end mem region but not move child resources whose ranges are
> in the high end range to new resource. When adjust old mem
> region's range, adjust_resource() detects child region's range
> is out of new range and return error. Move child resources to
> high end resource before adjusting old mem range.
> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
>  kernel/resource.c | 38 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/resource.c b/kernel/resource.c
> index 76036a41143b..1c7362825134 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -181,6 +181,38 @@ static struct resource *alloc_resource(gfp_t flags)
>  	return res;
>  }
> 
> +static void move_child_to_newresource(struct resource *old,
> +				      struct resource *new)
> +{
> +	struct resource *tmp, **p, **np;
> +
> +	if (!old->child)
> +		return;

I don't think the above test is needed.  This case is handled by the first
three lines of the "for" loop.

> +
> +	p = &old->child;
> +	np = &new->child;
> +
> +	for (;;) {
> +		tmp = *p;
> +		if (!tmp)
> +			break;
> +
> +		if (tmp->start >= new->start && tmp->end <= new->end) {
> +			tmp->parent = new;
> +			*np = tmp;
> +			np = &tmp->sibling;
> +			*p = tmp->sibling;
> +
> +			if (!tmp->sibling)
> +				*np = NULL;

I don't think the above two lines are right.  They seem tautological.  If the ! were
removed it would be clearing the sibling link for the child as it exists under its new
parent, which should be done.  But the child that is moved to the new parent always
becomes the last entry in the new parent's child list.  So could you just unconditionally
do tmp->sibling = NULL?   That link will get fixed up if another child is moved.

Michael

> +			continue;
> +		}
> +
> +		p = &tmp->sibling;
> +	}
> +}
> +
> +
>  /* Return the conflict entry if you can't request it */
>  static struct resource * __request_resource(struct resource *root, struct resource *new)
>  {
> @@ -1246,9 +1278,6 @@ EXPORT_SYMBOL(__release_region);
>   * Note:
>   * - Additional release conditions, such as overlapping region, can be
>   *   supported after they are confirmed as valid cases.
> - * - When a busy memory resource gets split into two entries, the code
> - *   assumes that all children remain in the lower address entry for
> - *   simplicity.  Enhance this logic when necessary.
>   */
>  int release_mem_region_adjustable(struct resource *parent,
>  				  resource_size_t start, resource_size_t size)
> @@ -1331,11 +1360,12 @@ int release_mem_region_adjustable(struct resource *parent,
>  			new_res->sibling = res->sibling;
>  			new_res->child = NULL;
> 
> +			move_child_to_newresource(res, new_res);
> +			res->sibling = new_res;
>  			ret = __adjust_resource(res, res->start,
>  						start - res->start);
>  			if (ret)
>  				break;
> -			res->sibling = new_res;
>  			new_res = NULL;
>  		}
> 
> --
> 2.14.5