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