[PATCHv9 3/7] block: allow ability to limit partition write hints

[Christoph Hellwig]

On Fri, Oct 25, 2024 at 02:36:41PM -0700, Keith Busch wrote:
> From: Keith Busch <kbusch at kernel.org>
> 
> When multiple partitions are used, you may want to enforce different
> subsets of the available write hints for each partition. Provide a
> bitmap attribute of the available write hints, and allow an admin to
> write a different mask to set the partition's allowed write hints.

Trying my best Greg impersonator voice: This needs to be documented
in Documentation/ABI/stable/sysfs-block.

That would have also helped me understanding it.  AFAIK the split here
is an opt-in, which means the use case I explained in the previous
case would still not work out of the box, right?

> +	max_write_hints = bdev_max_write_hints(bdev);
> +	if (max_write_hints) {
> +		int size = BITS_TO_LONGS(max_write_hints) * sizeof(long);
> +
> +		bdev->write_hint_mask = kmalloc(size, GFP_KERNEL);
> +		if (!bdev->write_hint_mask) {
> +			free_percpu(bdev->bd_stats);
> +			iput(inode);
> +			return NULL;
> +		}
> +		memset(bdev->write_hint_mask, 0xff, size);
> +	}

This could simply use bitmap_alloc().  Similarly the other uses
would probably benefit from using the bitmap API.

> +	struct block_device *bdev = dev_to_bdev(dev);
> +	unsigned short max_write_hints = bdev_max_write_hints(bdev);
> +
> +	if (max_write_hints)
> +		return sprintf(buf, "%*pb\n", max_write_hints, bdev->write_hint_mask);
> +	else
> +		return sprintf(buf, "0");

No need for the else.  And if you write this as:

	if (!max_write_hints)
		return sprintf(buf, "0");
	return sprintf(buf, "%*pb\n", max_write_hints, bdev->write_hint_mask);

you'd also avoid the overly long line.

> +
> +static ssize_t part_write_hint_mask_store(struct device *dev,
> +					  struct device_attribute *attr,
> +					  const char *buf, size_t count)
> +{
> +	struct block_device *bdev = dev_to_bdev(dev);
> +	unsigned short max_write_hints = bdev_max_write_hints(bdev);
> +	unsigned long *new_mask;
> +	int size;
> +
> +	if (!max_write_hints)
> +		return count;
> +
> +	size = BITS_TO_LONGS(max_write_hints) * sizeof(long);
> +	new_mask = kzalloc(size, GFP_KERNEL);
> +	if (!new_mask)
> +		return -ENOMEM;
> +
> +	bitmap_parse(buf, count, new_mask, max_write_hints);
> +	bitmap_copy(bdev->write_hint_mask, new_mask, max_write_hints);

What protects access to bdev->write_hint_mask?