Re: [PATCH v12 10/11] sparc64: Add support for ADI (Application Data Integrity)

[Dave Hansen]

On 03/05/2018 01:37 PM, Khalid Aziz wrote:
>> How big can this storage get, btw? Superficially it seems like it might
>> be able to be gigantic for a large, sparse VMA.
>>
> Tags are stored only for the pages being swapped out, not for the pages
> in entire vma. Each tag storage page can hold tags for 128 pages (each
> page has 128 4-bit tags, hence 64 bytes are needed to store tags for an
> entire page allowing each page to store tags for 128 pages). Sparse VMA
> does not cause any problems since holes do not have corresponding pages
> that will be swapped out. Tag storage pages are freed once all the pages
> they store tags for have been swapped back in, except for a small number
> of pages (maximum of 8) marked for emergency tag storage.

With a linear scan holding a process-wide spinlock?  If you have a fast
swap device, does this become the bottleneck when swapping ADI-tagged
memory?

FWIW, this tag storage is complex and subtle enough code that it
deserves to be in its own well-documented patch, not buried in a
thousand-line patch.

> +tag_storage_desc_t *find_tag_store(struct mm_struct *mm,
> +				   struct vm_area_struct *vma,
> +				   unsigned long addr)
> +{
> +	tag_storage_desc_t *tag_desc = NULL;
> +	unsigned long i, max_desc, flags;
> +
> +	/* Check if this vma already has tag storage descriptor
> +	 * allocated for it.
> +	 */
> +	max_desc = PAGE_SIZE/sizeof(tag_storage_desc_t);
> +	if (mm->context.tag_store) {
> +		tag_desc = mm->context.tag_store;
> +		spin_lock_irqsave(&mm->context.tag_lock, flags);
> +		for (i = 0; i < max_desc; i++) {
> +			if ((addr >= tag_desc->start) &&
> +			    ((addr + PAGE_SIZE - 1) <= tag_desc->end))
> +				break;
> +			tag_desc++;
> +		}
> +		spin_unlock_irqrestore(&mm->context.tag_lock, flags);