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

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Date:   Mon, 5 Mar 2018 13:50:26 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Khalid Aziz <khalid.aziz@...cle.com>, davem@...emloft.net,
        akpm@...ux-foundation.org
Cc:     corbet@....net, bob.picco@...cle.com, steven.sistare@...cle.com,
        pasha.tatashin@...cle.com, mike.kravetz@...cle.com,
        rob.gardner@...cle.com, mingo@...nel.org, nitin.m.gupta@...cle.com,
        anthony.yznaga@...cle.com, kirill.shutemov@...ux.intel.com,
        tom.hromatka@...cle.com, allen.pais@...cle.com,
        tklauser@...tanz.ch, shannon.nelson@...cle.com,
        vijay.ac.kumar@...cle.com, mhocko@...e.com, jack@...e.cz,
        punit.agrawal@....com, hughd@...gle.com, thomas.tai@...cle.com,
        ross.zwisler@...ux.intel.com, dave.jiang@...el.com,
        willy@...radead.org, minchan@...nel.org,
        imbrenda@...ux.vnet.ibm.com, aarcange@...hat.com,
        kstewart@...uxfoundation.org, pombredanne@...b.com,
        tglx@...utronix.de, gregkh@...uxfoundation.org,
        nagarathnam.muthusamy@...cle.com, linux@...ck-us.net,
        jane.chu@...cle.com, dan.j.williams@...el.com, jglisse@...hat.com,
        ktkhai@...tuozzo.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        sparclinux@...r.kernel.org, Khalid Aziz <khalid@...ehiking.org>
Subject: Re: [PATCH v12 10/11] sparc64: Add support for ADI (Application Data
 Integrity)

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);