From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave McCracken Subject: Re: [Linux PATCH] Fix to hugepages to work around new PWT handling Date: Thu, 24 Jun 2010 17:38:47 -0500 Message-ID: <201006241738.47925.dcm@mccr.org> References: <20100609140252.18351.48175.sendpatchset@magnum.int.mccr.org> <201006091426.03926.dcm@mccr.org> <4C232DE1.50008@goop.org> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4C232DE1.50008@goop.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Jeremy Fitzhardinge Cc: Xen Developers List List-Id: xen-devel@lists.xenproject.org On Thursday, June 24, 2010, Jeremy Fitzhardinge wrote: > > Um, this is the upper level code. The entire purpose of make_huge_pte is > > to construct a present huge pte from page and pgprot. The problem is > > that the original code makes the pte, then sets the present bit via > > pte_mkhuge(). This means the Xen-specific macro that triggers on > > present is misled and doesn't do the pfn_to_mfn(). Without this patch > > hugepages is handing pfns to the hypervisor to map instead of mfns. > > > > > > In principle, setting present should cause the pte to be converted from > pfn to mfn, but I don't think that ever happens with normal ptes (since > non-present ptes contain swap info). But I don't see where a huge pte > gets present set; pte_mkhuge itself doesn't do anything except set PSE. Wow. I just dug through the code. The landscape has sure changed since the last time I followed this path. It used to be that vma->vm_page_prot only contained the various read/write flags for that vma. At that time pte_mkhuge() did in fact add _PAGE_PRESENT| _PAGE_PSE to the pte. Now it appears that vma->vm_page_prot does include _PAGE_PRESENT in all its various states. So this part of the patch is in fact unnecessary. It's what I get for not rechecking my facts to be sure they haven't changed. Sorry. Dave McCracken Oracle Corp.