linux-kernel - Re: [PATCH v2] iommu: revisit iommu_insert_resv

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Date:   Tue, 6 Aug 2019 00:32:02 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, joro@...tes.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        dwmw2@...radead.org, shameerali.kolothum.thodi@...wei.com,
        alex.williamson@...hat.com, robin.murphy@....com, hch@...radead.org
Subject: Re: [PATCH v2] iommu: revisit iommu_insert_resv_region()
 implementation

A couple nitpicks below:

On Thu, Aug 01, 2019 at 05:59:46PM +0200, Eric Auger wrote:
> - * The new element is sorted by address with respect to the other
> - * regions of the same type. In case it overlaps with another
> - * region of the same type, regions are merged. In case it
> - * overlaps with another region of different type, regions are
> - * not merged.
> + * Elements are sorted by start address and overlapping segments
> + * of the same type are merged.
>   */
> +int iommu_insert_resv_region(struct iommu_resv_region *new,
> +			     struct list_head *regions)
>  {
> +	struct iommu_resv_region *iter, *tmp, *nr, *top;
> +	struct list_head stack;
> +	bool added = false;
>  
> +	INIT_LIST_HEAD(&stack);

Nit: you could just use

	LIST_HEAD(&stack);

to declare and initialize the variable in a single line.

> +	nr = iommu_alloc_resv_region(new->start, new->length,
> +				     new->prot, new->type);
> +	if (!nr)
>  		return -ENOMEM;
>  
> +	/* First add the new elt based on start address sorting */

/elt/element/ ?

> +	list_for_each_entry(iter, regions, list) {
> +		if (nr->start < iter->start) {
> +			list_add_tail(&nr->list, &iter->list);
> +			added = true;
> +			break;
> +		} else if (nr->start == iter->start && nr->type <= iter->type) {
> +			list_add_tail(&nr->list, &iter->list);
> +			added = true;
> +			break;
> +		}

Nit:  no need for an else after a a break.  But then again  both
branches look identical, so why don't you just merge them:

		if (nr->start < iter->start ||
		    (nr->start == iter->start && nr->type <= iter->type)) {
			list_add_tail(&nr->list, &iter->list);
			added = true;
			break;

	}

> +	if (!added)
> +		list_add_tail(&nr->list, regions);

Probably down to preference, but I'd just use a goto to jump past the
list_add and save the added variable.

> +	/* Merge overlapping segments of type nr->type, if any */
> +	list_for_each_entry_safe(iter, tmp, regions, list) {
> +		phys_addr_t top_end, iter_end = iter->start + iter->length - 1;
> +		bool found = false;
> +
> +		/* no merge needed on elements of different types than @nr */
> +		if (iter->type != nr->type) {
> +			list_move_tail(&iter->list, &stack);
> +			continue;
> +		}
> +
> +		/* look for the last stack element of same type as @iter */
> +		list_for_each_entry_reverse(top, &stack, list)
> +			if (top->type == iter->type) {
> +				found = true;
> +				break;
> +			}
> +		if (!found) {

Same here.

> +			list_move_tail(&iter->list, &stack);
> +			continue;
> +		}
> +
> +		top_end = top->start + top->length - 1;
> +
> +		if (iter->start > top_end + 1) {
> +			list_move_tail(&iter->list, &stack);
> +		} else {
> +			top->length = max(top_end, iter_end) - top->start + 1;
> +			list_del(&iter->list);
> +			kfree(iter);
> +		}

I wonder if the body of the outer list_for_each_entry_safe loop would
be a bit nicer in a helper, but again that is probably just down to
personal preference.