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Subject: Re: [RFC PATCH v3] sparc64: Add support for Application Data
 Integrity (ADI)
To: Khalid Aziz <khalid.aziz@oracle.com>, davem@davemloft.net,
        corbet@lwn.net, arnd@arndb.de, akpm@linux-foundation.org
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        mgorman@techsingularity.net, vdavydov.dev@gmail.com,
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        linux-kernel@vger.kernel.org, sparclinux@vger.kernel.org,
        linux-arch@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org,
        Khalid Aziz <khalid@gonehiking.org>
From: Dave Hansen <dave.hansen@linux.intel.com>
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Date: Wed, 4 Jan 2017 15:27:35 -0800
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On 01/04/2017 02:46 PM, Khalid Aziz wrote:
> This patch extends mprotect to enable ADI (TSTATE.mcde), enable/disable
> MCD (Memory Corruption Detection) on selected memory ranges, enable
> TTE.mcd in PTEs, return ADI parameters to userspace and save/restore ADI
> version tags on page swap out/in. 

I'm a bit confused why we need all the mechanics with set_swp_pte_at().
For pkeys, for instance, all of the PTEs under a given VMA share a pkey.
 When swapping something in, we just get the pkey out of the VMA and
populate the PTE.

ADI doesn't seem to have a similar restriction.  The feature is turned
on or off at a VMA granularity, but we do not (or can enforce that all
pages under a given VMA must share a tag.

But this leads to an interesting question: is the tag associated with
the (populated?) pte, or the virtual address?  Can you have tags
associated with non-present addresses?  What's the mechanism that clears
the tags at munmap() or MADV_FREE time?

Is the tag storage a precious resource?  Can it be exhausted?