From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f43.google.com (mail-wm0-f43.google.com [74.125.82.43]) by kanga.kvack.org (Postfix) with ESMTP id EC7BF828F3 for ; Sun, 10 Jan 2016 08:59:26 -0500 (EST) Received: by mail-wm0-f43.google.com with SMTP id u188so186128712wmu.1 for ; Sun, 10 Jan 2016 05:59:26 -0800 (PST) Received: from mail-wm0-x231.google.com (mail-wm0-x231.google.com. [2a00:1450:400c:c09::231]) by mx.google.com with ESMTPS id k206si14978201wmf.37.2016.01.10.05.59.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 10 Jan 2016 05:59:25 -0800 (PST) Received: by mail-wm0-x231.google.com with SMTP id f206so183475277wmf.0 for ; Sun, 10 Jan 2016 05:59:25 -0800 (PST) Message-ID: <569263BA.5060503@plexistor.com> Date: Sun, 10 Jan 2016 15:59:22 +0200 From: Boaz Harrosh MIME-Version: 1.0 Subject: [PATCHSET 0/2] Allow single pagefault in write access of a VM_MIXEDMAP mapping Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" , Dan Williams , Andrew Morton , Matthew Wilcox , "linux-mm@kvack.org" Cc: Ross Zwisler , Oleg Nesterov , Mel Gorman , Johannes Weiner Hi Today any VM_MIXEDMAP or VM_PFN mapping when enabling a write access to their mapping, will have a double pagefault for every write access. This is because vma->vm_page_prot defines how a page/pfn is inserted into the page table (see vma_wants_writenotify in mm/mmap.c). Which means that it is always inserted with read-only under the assumption that we want to be notified when write access occurs. But this is not always true and adds an unnecessary page-fault on every new mmap-write access This patchset is trying to give the fault handler more choice by passing an pgprot_t to vm_insert_mixed() via a new vm_insert_mixed_prot() API. If the mm guys feel that the pgprot_t and its helpers and flags are private to mm/memory.c I can easily do a new: vm_insert_mixed_rw() instead. of the above vm_insert_mixed_prot() which enables any control not only write. Following is a patch to DAX to optimize out the extra page-fault. TODO: I only did 4k mapping perhaps 2M mapping can enjoy the same single fault on write access. If interesting to anyone I can attempt a fix. Dan Andrew who needs to pick this up please? list of patches: [PATCH 1/2] mm: Allow single pagefault on mmap-write with VM_MIXEDMAP [PATCH 2/2] dax: Only fault once on mmap write access Thank you Boaz -- 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