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

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Date:   Wed, 18 Oct 2017 07:11:16 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Timofey Titovets <nefelim4ag@...il.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 04:33:43PM +0300, Timofey Titovets wrote:
> 2017-10-18 15:34 GMT+03:00 Matthew Wilcox <willy@...radead.org>:
> > On Wed, Oct 18, 2017 at 10:48:32AM +0000, Srividya Desireddy wrote:
> >> +static void zswap_fill_page(void *ptr, unsigned long value)
> >> +{
> >> +     unsigned int pos;
> >> +     unsigned long *page;
> >> +
> >> +     page = (unsigned long *)ptr;
> >> +     if (value == 0)
> >> +             memset(page, 0, PAGE_SIZE);
> >> +     else {
> >> +             for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++)
> >> +                     page[pos] = value;
> >> +     }
> >> +}
> >
> > I think you meant:
> >
> > static void zswap_fill_page(void *ptr, unsigned long value)
> > {
> >         memset_l(ptr, value, PAGE_SIZE / sizeof(unsigned long));
> > }
> 
> IIRC kernel have special zero page, and if i understand correctly.
> You can map all zero pages to that zero page and not touch zswap completely.
> (Your situation look like some KSM case (i.e. KSM can handle pages
> with same content), but i'm not sure if that applicable there)

You're confused by the word "same".  What Srividya meant was that the
page is filled with a pattern, eg 0xfffefffefffefffe..., not that it is
the same as any other page.