linux-kernel - Re: [PATCHv3 03/17] mm/ksm: Do not merge pages with different KeyIDs

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Message-ID: <20180613204134.voy7iimrlcywblnx@black.fi.intel.com>
Date:   Wed, 13 Jun 2018 23:41:34 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 03/17] mm/ksm: Do not merge pages with different KeyIDs

On Wed, Jun 13, 2018 at 08:35:46PM +0000, Dave Hansen wrote:
> On 06/13/2018 01:31 PM, Kirill A. Shutemov wrote:
> >> What actually happens without this patch in place?
> > 
> > One of processes would get the page mapped with wrong KeyID and see
> > garbage. 
> 
> OK, but what about two pages with the same KeyID?  It's actually totally
> possible for KSM to determine that two pages have the same plaintext and
> merge them.  Why don't we do that?

That's exactly what we do :)

> > We setup mapping according to KeyID in vma->vm_page_prot.
> 
> Then why do we bother with page_keyid() and the page_ext stuff?

VMA is not always around.

Using KeyID in vma->vm_page_prot we don't need to change anything in PTE
setup functions. It just works.

-- 
 Kirill A. Shutemov