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Subject: Re: [PATCH v4 11/23] device-dax: Kill dax_kmem_res
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Joao Martins <joao.m.martins@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Linux MM <linux-mm@kvack.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <d160c05b-9caa-1ffb-9c01-5bb261c744b5@redhat.com>
Date:   Thu, 24 Sep 2020 09:25:57 +0200
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On 23.09.20 23:41, Dan Williams wrote:
> On Wed, Sep 23, 2020 at 1:04 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> 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);
> 
> Another thought... I notice that you've taught
> register_memory_resource() a IORESOURCE_MEM_DRIVER_MANAGED special
> case. Lets just make the assumption of add_memory_driver_managed()
> that it is the driver's responsibility to mark the range busy before
> calling, and the driver's responsibility to release the region. I.e.
> validate (rather than request) that the range is busy in
> register_memory_resource(), and teach release_memory_resource() to
> skip releasing the region when the memory is marked driver managed.
> That would let dax_kmem drop its manipulation of the 'busy' flag which
> is a layering violation no matter how many comments we put around it.

IIUC, that won't work for virtio-mem, whereby the parent resource spans
multiple possible (future) add_memory_driver_managed() calls and is
(just like for kmem) a pure indication to which device memory ranges belong.

For example, when exposing 2GB via a virtio-mem device with max 4GB:

(/proc/iomem)
240000000-33fffffff : virtio0
  240000000-2bfffffff : System RAM (virtio_mem)

And after hotplugging additional 2GB:

240000000-33fffffff : virtio0
  240000000-33fffffff : System RAM (virtio_mem)

So marking "virtio0" always BUSY (especially right from the start) would
be wrong. The assumption is that anything that's IORESOURCE_SYSTEM_RAM
and IORESOUCE_BUSY is currently added to the system as system RAM (e.g.,
after add_memory() and friends, or during boot).

I do agree that manually clearing the busy flag is ugly. What we most
probably want is request_mem_region() that performs similar checks (no
overlaps with existing BUSY resources), but doesn't set the region busy.

-- 
Thanks,

David / dhildenb