Message-ID: <a3ad70a2-77a8-d50e-f372-731a8e27c03b@redhat.com>
Date:   Fri, 21 Aug 2020 12:06:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>, akpm@...ux-foundation.org
Cc:     Vishal Verma <vishal.l.verma@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        peterz@...radead.org, ard.biesheuvel@...aro.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        joao.m.martins@...cle.com, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 11/23] device-dax: Kill dax_kmem_res

On 03.08.20 07:03, Dan Williams wrote:
> Several related issues around this unneeded attribute:
> 
> - The dax_kmem_res property allows the kmem driver to stash the adjusted
>   resource range that was used for the hotplug operation, but that can be
>   recalculated from the original base range.
> 
> - kmem is using an open coded release_resource() + kfree() when an
>   idiomatic release_mem_region() is sufficient.
> 
> - The driver managed resource need only manage the busy flag. Other flags
>   are of no concern to the kmem driver. In fact if kmem inherits some
>   memory range that add_memory_driver_managed() rejects that is a
>   memory-hotplug-core policy that the driver is in no position to
>   override.
> 
> - The implementation trusts that failed remove_memory() results in the
>   entire resource range remaining pinned busy. The driver need not make
>   that layering violation assumption and just maintain the busy state in
>   its local resource.
> 
> - The "Hot-remove not yet implemented." comment is stale since hotremove
>   support is now included.

I think some of these changes could have been nicely split out to
simplify reviewing. E.g., comment update, release_mem_region(),  &=
~IORESOURCE_BUSY ...

[...]

> +
>  int dev_dax_kmem_probe(struct device *dev)
>  {
>  	struct dev_dax *dev_dax = to_dev_dax(dev);
> -	struct range *range = &dev_dax->range;
> -	resource_size_t kmem_start;
> -	resource_size_t kmem_size;
> -	resource_size_t kmem_end;
> -	struct resource *new_res;
> -	const char *new_res_name;
> -	int numa_node;
> +	struct range range = dax_kmem_range(dev_dax);
> +	int numa_node = dev_dax->target_node;
> +	struct resource *res;
> +	char *res_name;
>  	int rc;
>  
>  	/*
> @@ -37,109 +45,94 @@ int dev_dax_kmem_probe(struct device *dev)
>  	 * could be mixed in a node with faster memory, causing
>  	 * unavoidable performance issues.
>  	 */
> -	numa_node = dev_dax->target_node;
>  	if (numa_node < 0) {
>  		dev_warn(dev, "rejecting DAX region with invalid node: %d\n",
>  				numa_node);
>  		return -EINVAL;
>  	}
>  
> -	/* Hotplug starting at the beginning of the next block: */
> -	kmem_start = ALIGN(range->start, memory_block_size_bytes());
> -
> -	kmem_size = range_len(range);
> -	/* Adjust the size down to compensate for moving up kmem_start: */
> -	kmem_size -= kmem_start - range->start;
> -	/* Align the size down to cover only complete blocks: */
> -	kmem_size &= ~(memory_block_size_bytes() - 1);
> -	kmem_end = kmem_start + kmem_size;
> -
> -	new_res_name = kstrdup(dev_name(dev), GFP_KERNEL);
> -	if (!new_res_name)
> +	res_name = kstrdup(dev_name(dev), GFP_KERNEL);
> +	if (!res_name)
>  		return -ENOMEM;
>  
> -	/* Region is permanently reserved if hotremove fails. */
> -	new_res = request_mem_region(kmem_start, kmem_size, new_res_name);
> -	if (!new_res) {
> -		dev_warn(dev, "could not reserve region [%pa-%pa]\n",
> -			 &kmem_start, &kmem_end);
> -		kfree(new_res_name);
> +	res = request_mem_region(range.start, range_len(&range), res_name);

I think our range could be empty after aligning. I assume
request_mem_region() would check that, but maybe we could report a
better error/warning in that case.

> +	if (!res) {
> +		dev_warn(dev, "could not reserve region [%#llx-%#llx]\n",
> +				range.start, range.end);
> +		kfree(res_name);
>  		return -EBUSY;
>  	}
>  
>  	/*
> -	 * Set flags appropriate for System RAM.  Leave ..._BUSY clear
> -	 * so that add_memory() can add a child resource.  Do not
> -	 * inherit flags from the parent since it may set new flags
> -	 * unknown to us that will break add_memory() below.
> +	 * Temporarily clear busy to allow add_memory_driver_managed()
> +	 * to claim it.
>  	 */
> -	new_res->flags = IORESOURCE_SYSTEM_RAM;
> +	res->flags &= ~IORESOURCE_BUSY;

Right, same approach is taken by virtio-mem.

>  
>  	/*
>  	 * Ensure that future kexec'd kernels will not treat this as RAM
>  	 * automatically.
>  	 */
> -	rc = add_memory_driver_managed(numa_node, new_res->start,
> -				       resource_size(new_res), kmem_name);
> +	rc = add_memory_driver_managed(numa_node, res->start,
> +				       resource_size(res), kmem_name);
> +
> +	res->flags |= IORESOURCE_BUSY;

Hm, I don't think that's correct. Any specific reason why to mark the
not-added, unaligned parts BUSY? E.g., walk_system_ram_range() could
suddenly stumble over it - and e.g., similarly kexec code when trying to
find memory for placing kexec images. I think we should leave this
!BUSY, just as it is right now.

>  	if (rc) {
> -		release_resource(new_res);
> -		kfree(new_res);
> -		kfree(new_res_name);
> +		release_mem_region(range.start, range_len(&range));
> +		kfree(res_name);
>  		return rc;
>  	}
> -	dev_dax->dax_kmem_res = new_res;
> +
> +	dev_set_drvdata(dev, res_name);
>  
>  	return 0;
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTREMOVE
> -static int dev_dax_kmem_remove(struct device *dev)
> +static void dax_kmem_release(struct dev_dax *dev_dax)
>  {
> -	struct dev_dax *dev_dax = to_dev_dax(dev);
> -	struct resource *res = dev_dax->dax_kmem_res;
> -	resource_size_t kmem_start = res->start;
> -	resource_size_t kmem_size = resource_size(res);
> -	const char *res_name = res->name;
>  	int rc;
> +	struct device *dev = &dev_dax->dev;
> +	const char *res_name = dev_get_drvdata(dev);
> +	struct range range = dax_kmem_range(dev_dax);
>  
>  	/*
>  	 * We have one shot for removing memory, if some memory blocks were not
>  	 * offline prior to calling this function remove_memory() will fail, and
>  	 * there is no way to hotremove this memory until reboot because device
> -	 * unbind will succeed even if we return failure.
> +	 * unbind will proceed regardless of the remove_memory result.
>  	 */
> -	rc = remove_memory(dev_dax->target_node, kmem_start, kmem_size);
> -	if (rc) {
> -		any_hotremove_failed = true;
> -		dev_err(dev,
> -			"DAX region %pR cannot be hotremoved until the next reboot\n",
> -			res);
> -		return rc;
> +	rc = remove_memory(dev_dax->target_node, range.start, range_len(&range));
> +	if (rc == 0) {

if (!rc) ?

> +		release_mem_region(range.start, range_len(&range));

remove_memory() does a release_mem_region_adjustable(). Don't you
actually want to release the *unaligned* region you requested?

> +		dev_set_drvdata(dev, NULL);
> +		kfree(res_name);
> +		return;
>  	}

Not sure if inverting the error handling improved the code / review here.
>  
> -	/* Release and free dax resources */
> -	release_resource(res);
> -	kfree(res);
> -	kfree(res_name);
> -	dev_dax->dax_kmem_res = NULL;
> -
> -	return 0;
> +	any_hotremove_failed = true;
> +	dev_err(dev, "%#llx-%#llx cannot be hotremoved until the next reboot\n",
> +			range.start, range.end);
>  }
>  #else
> -static int dev_dax_kmem_remove(struct device *dev)
> +static void dax_kmem_release(struct dev_dax *dev_dax)
>  {
>  	/*
> -	 * Without hotremove purposely leak the request_mem_region() for the
> -	 * device-dax range and return '0' to ->remove() attempts. The removal
> -	 * of the device from the driver always succeeds, but the region is
> -	 * permanently pinned as reserved by the unreleased
> -	 * request_mem_region().
> +	 * Without hotremove purposely leak the request_mem_region() for
> +	 * the device-dax range attempts. The removal of the device from
> +	 * the driver always succeeds, but the region is permanently
> +	 * pinned as reserved by the unreleased request_mem_region().
>  	 */
>  	any_hotremove_failed = true;
> -	return 0;
>  }
>  #endif /* CONFIG_MEMORY_HOTREMOVE */
>  
> +static int dev_dax_kmem_remove(struct device *dev)
> +{
> +	dax_kmem_release(to_dev_dax(dev));
> +	return 0;
> +}
> +
>  static struct dax_device_driver device_dax_kmem_driver = {
>  	.drv = {
>  		.probe = dev_dax_kmem_probe,
> 

Maybe split some of these changes out. Would at least help me to review ;)

-- 
Thanks,

David / dhildenb

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