Re: [PATCH v4 11/23] device-dax: Kill dax_kmem_res

[David Hildenbrand <david@redhat.com>]

On 08.09.20 17:33, Joao Martins wrote:
> [Sorry for the late response]
> 
> On 8/21/20 11:06 AM, David Hildenbrand wrote:
>> On 03.08.20 07:03, Dan Williams wrote:
>>> @@ -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.
>>
> dax_kmem_range() already returns a memory-block-aligned @range but
> IIUC request_mem_region() isn't checking for that. Having said that
> the returned @res wouldn't be different from the passed range.start.
> 
>>>  	/*
>>>  	 * 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.
>>
> Agreed.
> 
>>>  	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) ?
>>
> Better off would be to keep the old order:
> 
> 	if (rc) {
> 		any_hotremove_failed = true;
> 		dev_err(dev, "%#llx-%#llx cannot be hotremoved until the next reboot\n",
> 				range.start, range.end);
> 	        return;
> 	}
> 
> 	release_mem_region(range.start, range_len(&range));
> 	dev_set_drvdata(dev, NULL);
> 	kfree(res_name);
> 	return;
> 
> 
>>> +		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?
>>
> Isn't it what we're doing here?
> (The release_mem_region_adjustable() is using the same
> dax_kmem-aligned range and there's no split/adjust)
> 
> Meaning right now (+ parent marked as !BUSY), and if I am understanding
> this correctly:
> 
> request_mem_region(range.start, range_len)
>    __request_region(iomem_res, range.start, range_len) -> alloc @parent
> add_memory_driver_managed(parent.start, resource_size(parent))
>    __request_region(parent.start, resource_size(parent)) -> alloc @child
> 
> [...]
> 
> remove_memory(range.start, range_len)
>  request_mem_region_adjustable(range.start, range_len)
>   __release_region(range.start, range_len) -> remove @child
> 
> release_mem_region(range.start, range_len)
>   __release_region(range.start, range_len) -> doesn't remove @parent because !BUSY?
> 
> The add/removal of this relies on !BUSY. But now I am wondering if the parent remaining
> unreleased is deliberate even on CONFIG_MEMORY_HOTREMOVE=y.
> 
> 	Joao
> 

Thinking about it, if we don't set the parent resource BUSY (which is
what I think is the right way of doing things), and don't want to store
the parent resource pointer, we could add something like
lookup_resource() - e.g., lookup_mem_resource() - , however, searching
properly in the whole hierarchy (instead of only the first level), and
traversing down to the last hierarchy. Then it would be as simple as

remove_memory(range.start, range_len)
res = lookup_mem_resource(range.start);
release_resource(res);

-- 
Thanks,

David / dhildenb