linux-kernel - Re: [PATCH v12 02/11] mm, swap: Add infrastructure for saving page metadata on swap

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Date:   Mon, 5 Mar 2018 12:29:51 -0700
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        akpm@...ux-foundation.org, davem@...emloft.net, arnd@...db.de
Cc:     kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        ross.zwisler@...ux.intel.com, dave.jiang@...el.com,
        mgorman@...hsingularity.net, willy@...radead.org, hughd@...gle.com,
        minchan@...nel.org, hannes@...xchg.org, shli@...com,
        mingo@...nel.org, jglisse@...hat.com, me@...in.cc,
        anthony.yznaga@...cle.com, linux-arch@...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 02/11] mm, swap: Add infrastructure for saving page
 metadata on swap

On 03/05/2018 12:20 PM, Dave Hansen wrote:
> On 02/21/2018 09:15 AM, Khalid Aziz wrote:
>> If a processor supports special metadata for a page, for example ADI
>> version tags on SPARC M7, this metadata must be saved when the page is
>> swapped out. The same metadata must be restored when the page is swapped
>> back in. This patch adds two new architecture specific functions -
>> arch_do_swap_page() to be called when a page is swapped in, and
>> arch_unmap_one() to be called when a page is being unmapped for swap
>> out. These architecture hooks allow page metadata to be saved if the
>> architecture supports it.
> 
> I still think silently squishing cacheline-level hardware data into
> page-level software data structures is dangerous.
> 
> But, you seem rather determined to do it this way.  I don't think this
> will _hurt_ anyone else, though other than needlessly cluttering up the
> code.

Hello Dave,

Thanks for taking the time to look at this patch and providing feedback.

ADI data is per page data and is held in the spare bits in the RAM. It 
is loaded into the cache when data is loaded from RAM and flushed out to 
spare bits in the RAM when data is flushed from cache. Sparc allows one 
tag for each ADI block size of data and ADI block size is same as 
cacheline size. When a page is loaded into RAM from swap space, all of 
the associated ADI data for the page must also be loaded into the RAM, 
so it looks like page level data and storing it in page level software 
data structure makes sense. I am open to other suggestions though.

Thanks,
Khalid