linux-kernel - Re: [PATCH 11/12] device-dax: Add dis-contiguous resource support

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Date:   Tue, 24 Mar 2020 16:12:21 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, dave.hansen@...ux.intel.com, hch@....de,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/12] device-dax: Add dis-contiguous resource support

On 3/23/20 11:55 PM, Dan Williams wrote:
>  static ssize_t dev_dax_resize(struct dax_region *dax_region,
>  		struct dev_dax *dev_dax, resource_size_t size)
>  {
>  	resource_size_t avail = dax_region_avail_size(dax_region), to_alloc;
> -	resource_size_t dev_size = range_len(&dev_dax->range);
> +	resource_size_t dev_size = dev_dax_size(dev_dax);
>  	struct resource *region_res = &dax_region->res;
>  	struct device *dev = &dev_dax->dev;
> -	const char *name = dev_name(dev);
>  	struct resource *res, *first;
> +	resource_size_t alloc = 0;
> +	int rc;
>  
>  	if (dev->driver)
>  		return -EBUSY;
> @@ -684,38 +766,47 @@ static ssize_t dev_dax_resize(struct dax_region *dax_region,
>  	 * allocating a new resource.
>  	 */
>  	first = region_res->child;
> -	if (!first)
> -		return __alloc_dev_dax_range(dev_dax, dax_region->res.start,
> -				to_alloc);

You probably want to retain the condition above?

Otherwise it removes the ability to create new devices or resizing it , once we
have zero-ed the last one.

> -	for (res = first; to_alloc && res; res = res->sibling) {
> +retry:
> +	rc = -ENOSPC;
> +	for (res = first; res; res = res->sibling) {
>  		struct resource *next = res->sibling;
> -		resource_size_t free;
>  
>  		/* space at the beginning of the region */
> -		free = 0;
> -		if (res == first && res->start > dax_region->res.start)
> -			free = res->start - dax_region->res.start;
> -		if (free >= to_alloc && dev_size == 0)
> -			return __alloc_dev_dax_range(dev_dax,
> -					dax_region->res.start, to_alloc);
> -
> -		free = 0;
> +		if (res == first && res->start > dax_region->res.start) {
> +			alloc = min(res->start - dax_region->res.start,
> +					to_alloc);
> +			rc = __alloc_dev_dax_range(dev_dax,
> +					dax_region->res.start, alloc);
> +			break;
> +		}
> +
> +		alloc = 0;
>  		/* space between allocations */
>  		if (next && next->start > res->end + 1)
> -			free = next->start - res->end + 1;
> +			alloc = min(next->start - (res->end + 1), to_alloc);
>  
>  		/* space at the end of the region */
> -		if (free < to_alloc && !next && res->end < region_res->end)
> -			free = region_res->end - res->end;
> -
> -		if (free >= to_alloc && strcmp(name, res->name) == 0)
> -			return __adjust_dev_dax_range(dev_dax, res,
> -					resource_size(res) + to_alloc);
> -		else if (free >= to_alloc && dev_size == 0)
> -			return __alloc_dev_dax_range(dev_dax, res->end + 1,
> -					to_alloc);
> +		if (!alloc && !next && res->end < region_res->end)
> +			alloc = min(region_res->end - res->end, to_alloc);
> +
> +		if (!alloc)
> +			continue;
> +
> +		if (adjust_ok(dev_dax, res)) {
> +			rc = __adjust_dev_dax_range(dev_dax, res,
> +					resource_size(res) + alloc);
> +			break;
> +		}
> +		rc = __alloc_dev_dax_range(dev_dax, res->end + 1,
> +				alloc);

I am wondering if we should switch to:

	if (adjust_ok(...))
		rc = __adjust_dev_dax_range(...);
	else
		rc = __alloc_dev_dax_range(...);

And then a debug print at the end depicting whether and how did we grabbed
space? Something like:

	dev_dbg(&dev_dax->dev, "%s(%d) %d", action, location, rc);

Assuming we set @location to its values when we allocate space at the end,
beginning or middle; and @action to whether we adjusted up/down or allocated new
range.

Essentially, something similar to namespaces scan_allocate() just to help
troubleshoot?

Regards,
 Joao