linux-kernel - Re: [PATCH] zswap: Same-filled pages handling

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Message-ID: <20171019132427.GA14440@bombadil.infradead.org>
Date:   Thu, 19 Oct 2017 06:24:27 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Srividya Desireddy <srividya.dr@...sung.com>,
        "sjenning@...hat.com" <sjenning@...hat.com>,
        "ddstreet@...e.org" <ddstreet@...e.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "penberg@...nel.org" <penberg@...nel.org>,
        Dinakar Reddy Pathireddy <dinakar.p@...sung.com>,
        SHARAN ALLUR <sharan.allur@...sung.com>,
        RAJIB BASU <rajib.basu@...sung.com>,
        JUHUN KIM <juhunkim@...sung.com>,
        "srividya.desireddy@...il.com" <srividya.desireddy@...il.com>
Subject: Re: [PATCH] zswap: Same-filled pages handling

On Wed, Oct 18, 2017 at 09:30:32PM -0700, Andi Kleen wrote:
> > Yes.  Every 64-bit repeating pattern is also a 32-bit repeating pattern.
> > Supporting a 64-bit pattern on a 32-bit kernel is painful, but it makes
> > no sense to *not* support a 64-bit pattern on a 64-bit kernel.  
> 
> But a 32bit repeating pattern is not necessarily a 64bit pattern.

Oops, I said it backwards.  What I mean is that if you have the repeating
pattern:

0x12345678 12345678 12345678 12345678 12345678 12345678

that's the same as the repeating pattern:

0x1234567812345678 1234567812345678 1234567812345678

so the 64-bit kernel is able to find all patterns that the 32-bit kernel is,
and more.