linux-kernel - Re: [PATCH 4/4] dax: "Hotplug" persistent memory for use like normal RAM

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Date:   Wed, 16 Jan 2019 13:40:48 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     dave@...1.net, Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, zwisler@...nel.org,
        vishal.l.verma@...el.com, thomas.lendacky@....com,
        Andrew Morton <akpm@...ux-foundation.org>, mhocko@...e.com,
        linux-nvdimm@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Huang Ying <ying.huang@...el.com>,
        Wu Fengguang <fengguang.wu@...el.com>,
        Borislav Petkov <bp@...e.de>, baiyaowei@...s.chinamobile.com,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 4/4] dax: "Hotplug" persistent memory for use like normal
 RAM

On 1/16/19 1:16 PM, Bjorn Helgaas wrote:
> On Wed, Jan 16, 2019 at 12:25 PM Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>> Currently, a persistent memory region is "owned" by a device driver,
>> either the "Direct DAX" or "Filesystem DAX" drivers.  These drivers
>> allow applications to explicitly use persistent memory, generally
>> by being modified to use special, new libraries.
> 
> Is there any documentation about exactly what persistent memory is?
> In Documentation/, I see references to pstore and pmem, which sound
> sort of similar, but maybe not quite the same?

One instance of persistent memory is nonvolatile DIMMS.  They're
described in great detail here: Documentation/nvdimm/nvdimm.txt

>> +config DEV_DAX_KMEM
>> +       def_bool y
> 
> Is "y" the right default here?  I periodically see Linus complain
> about new things defaulting to "on", but I admit I haven't paid enough
> attention to know whether that would apply here.
> 
>> +       depends on DEV_DAX_PMEM   # Needs DEV_DAX_PMEM infrastructure
>> +       depends on MEMORY_HOTPLUG # for add_memory() and friends

Well, it doesn't default to "on for everyone".  It inherits the state of
DEV_DAX_PMEM so it's only foisted on folks who have already opted in to
generic pmem support.

>> +int dev_dax_kmem_probe(struct device *dev)
>> +{
>> +       struct dev_dax *dev_dax = to_dev_dax(dev);
>> +       struct resource *res = &dev_dax->region->res;
>> +       resource_size_t kmem_start;
>> +       resource_size_t kmem_size;
>> +       struct resource *new_res;
>> +       int numa_node;
>> +       int rc;
>> +
>> +       /* Hotplug starting at the beginning of the next block: */
>> +       kmem_start = ALIGN(res->start, memory_block_size_bytes());
>> +
>> +       kmem_size = resource_size(res);
>> +       /* Adjust the size down to compensate for moving up kmem_start: */
>> +        kmem_size -= kmem_start - res->start;
>> +       /* Align the size down to cover only complete blocks: */
>> +       kmem_size &= ~(memory_block_size_bytes() - 1);
>> +
>> +       new_res = devm_request_mem_region(dev, kmem_start, kmem_size,
>> +                                         dev_name(dev));
>> +
>> +       if (!new_res) {
>> +               printk("could not reserve region %016llx -> %016llx\n",
>> +                               kmem_start, kmem_start+kmem_size);
> 
> 1) It'd be nice to have some sort of module tag in the output that
> ties it to this driver.

Good point.  That should probably be a dev_printk().

> 2) It might be nice to print the range in the same format as %pR,
> i.e., "[mem %#010x-%#010x]" with the end included (start + size -1 ).

Sure, that sounds like a sane thing to do as well.

>> +               return -EBUSY;
>> +       }
>> +
>> +       /*
>> +        * Set flags appropriate for System RAM.  Leave ..._BUSY clear
>> +        * so that add_memory() can add a child resource.
>> +        */
>> +       new_res->flags = IORESOURCE_SYSTEM_RAM;
> 
> IIUC, new_res->flags was set to "IORESOURCE_MEM | ..." in the
> devm_request_mem_region() path.  I think you should keep at least
> IORESOURCE_MEM so the iomem_resource tree stays consistent.
> 
>> +       new_res->name = dev_name(dev);
>> +
>> +       numa_node = dev_dax->target_node;
>> +       if (numa_node < 0) {
>> +               pr_warn_once("bad numa_node: %d, forcing to 0\n", numa_node);
> 
> It'd be nice to again have a module tag and an indication of what
> range is affected, e.g., %pR of new_res.
> 
> You don't save the new_res pointer anywhere, which I guess you intend
> for now since there's no remove or anything else to do with this
> resource?  I thought maybe devm_request_mem_region() would implicitly
> save it, but it doesn't; it only saves the parent (iomem_resource, the
> start (kmem_start), and the size (kmem_size)).

Yeah, that's the intention: removal is currently not supported.  I'll
add a comment to clarify.

>> +               numa_node = 0;
>> +       }
>> +
>> +       rc = add_memory(numa_node, new_res->start, resource_size(new_res));
>> +       if (rc)
>> +               return rc;
>> +
>> +       return 0;
> 
> Doesn't this mean "return rc" or even just "return add_memory(...)"?

Yeah, all of those are equivalent.  I guess I just prefer the explicit
error handling path.