linux-kernel - Re: [RFC PATCH v3] sparc64: Add support for Application Data Integrity (ADI)

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Date:   Wed, 4 Jan 2017 17:26:08 -0700
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Rob Gardner <rob.gardner@...cle.com>, davem@...emloft.net,
        corbet@....net, arnd@...db.de, akpm@...ux-foundation.org
Cc:     hpa@...or.com, viro@...iv.linux.org.uk, nitin.m.gupta@...cle.com,
        chris.hyser@...cle.com, tushar.n.dave@...cle.com,
        sowmini.varadhan@...cle.com, mike.kravetz@...cle.com,
        adam.buchbinder@...il.com, minchan@...nel.org, hughd@...gle.com,
        kirill.shutemov@...ux.intel.com, keescook@...omium.org,
        allen.pais@...cle.com, aryabinin@...tuozzo.com,
        atish.patra@...cle.com, joe@...ches.com, pmladek@...e.com,
        jslaby@...e.cz, cmetcalf@...lanox.com,
        paul.gortmaker@...driver.com, mhocko@...e.com, jmarchan@...hat.com,
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        tglx@...utronix.de, mingo@...hat.com, dan.j.williams@...el.com,
        iamjoonsoo.kim@....com, mgorman@...hsingularity.net,
        vdavydov.dev@...il.com, hannes@...xchg.org, namit@...are.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org,
        Khalid Aziz <khalid@...ehiking.org>
Subject: Re: [RFC PATCH v3] sparc64: Add support for Application Data
 Integrity (ADI)

On 01/04/2017 05:14 PM, Dave Hansen wrote:
> On 01/04/2017 04:05 PM, Rob Gardner wrote:
>>> What if two different small pages have different tags and khugepaged
>>> comes along and tries to collapse them?  Will the page be split if a
>>> user attempts to set two different tags inside two different small-page
>>> portions of a single THP?
>>
>> The MCD tags operate at a resolution of cache lines (64 bytes). Page
>> sizes don't matter except that each virtual page must have a bit set in
>> its TTE to allow MCD to be enabled on the page. Any page can have many
>> different tags, one for each cache line.
>
> Is an "MCD tag" the same thing as a "ADI version tag"?
>
> The thing that confused me here is that we're taking an entire page of
> "ADI version tags" and stuffing them into a swap pte (in
> set_swp_pte_at()).  Do we somehow have enough space in a swap pte on
> sparc to fit PAGE_SIZE/64 "ADI version tag"s in there?

No, we do not have space to stuff PAGE_SIZE/64 version tags in swap pte. 
There is enough space for just one tag per page. DaveM had suggested 
doing this since the usual case is for a task to set one tag per page 
even though MMU does not require it. I have implemented this as first 
pass to start a discussion and get feedback on whether rest of the 
swapping implementation and other changes look right, hence the patch is 
"RFC". If this all looks good, I can expand swapping support in a 
subsequent patch or iteration of this patch to allocate space in 
mm_context_t possibly to store per cacheline tags. I am open to any 
other ideas on storing this larger number of version tags.

Thanks,
Khalid