From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yk0-f180.google.com (mail-yk0-f180.google.com [209.85.160.180]) by kanga.kvack.org (Postfix) with ESMTP id 9F3716B0038 for ; Thu, 8 Jan 2015 12:50:30 -0500 (EST) Received: by mail-yk0-f180.google.com with SMTP id 9so1880116ykp.11 for ; Thu, 08 Jan 2015 09:50:30 -0800 (PST) Received: from SMTP.CITRIX.COM (smtp.citrix.com. [66.165.176.89]) by mx.google.com with ESMTPS id z1si3409262ykb.29.2015.01.08.09.50.28 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 08 Jan 2015 09:50:29 -0800 (PST) Message-ID: <54AEC358.9000001@citrix.com> Date: Thu, 8 Jan 2015 17:50:16 +0000 From: David Vrabel MIME-Version: 1.0 Subject: Re: [PATCH 1/2] mm: allow for an alternate set of pages for userspace mappings References: <1420730924-22811-1-git-send-email-david.vrabel@citrix.com> <1420730924-22811-2-git-send-email-david.vrabel@citrix.com> <20150108172007.GB32079@phnom.home.cmpxchg.org> In-Reply-To: <20150108172007.GB32079@phnom.home.cmpxchg.org> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, xen-devel@lists.xenproject.org, Konrad Rzeszutek Wilk , Boris Ostrovsky On 08/01/15 17:20, Johannes Weiner wrote: > On Thu, Jan 08, 2015 at 03:28:43PM +0000, David Vrabel wrote: >> Add an optional array of pages to struct vm_area_struct that can be >> used find the page backing a VMA. This is useful in cases where the >> normal mechanisms for finding the page don't work. This array is only >> inspected if the PTE is special. >> >> Splitting a VMA with such an array of pages is trivially done by >> adjusting vma->pages. The original creator of the VMA must only free >> the page array once all sub-VMAs are closed (e.g., by ref-counting in >> vm_ops->open and vm_ops->close). >> >> One use case is a Xen PV guest mapping foreign pages into userspace. >> >> In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for >> example) must do an MFN to PFN (M2P) lookup before it can get the >> page. For foreign pages (those owned by another guest) the M2P lookup >> returns the PFN as seen by the foreign guest (which would be >> completely the wrong page for the local guest). >> >> This cannot be fixed up improving the M2P lookup since one MFN may be >> mapped onto two or more pages so getting the right page is impossible >> given just the MFN. [...] >> --- a/include/linux/mm_types.h >> +++ b/include/linux/mm_types.h >> @@ -309,6 +309,14 @@ struct vm_area_struct { >> #ifdef CONFIG_NUMA >> struct mempolicy *vm_policy; /* NUMA policy for the VMA */ >> #endif >> + /* >> + * Array of pages to override the default vm_normal_page() >> + * result iff the PTE is special. >> + * >> + * The memory for this should be refcounted in vm_ops->open >> + * and vm_ops->close. >> + */ >> + struct page **pages; > > Please make this configuration-dependent, not every Linux user should > have to pay for a Xen optimization. If the additional field in struct vm_area_struct is a concern, I would prefer to use a vm_flag bit and union pages with an existing field. Perhaps using VM_PFNMAP and reusing vm_file? David -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org