From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f169.google.com (mail-ig0-f169.google.com [209.85.213.169]) by kanga.kvack.org (Postfix) with ESMTP id DC4896B006C for ; Tue, 14 Oct 2014 08:08:57 -0400 (EDT) Received: by mail-ig0-f169.google.com with SMTP id uq10so17034651igb.2 for ; Tue, 14 Oct 2014 05:08:57 -0700 (PDT) Received: from e28smtp08.in.ibm.com (e28smtp08.in.ibm.com. [122.248.162.8]) by mx.google.com with ESMTPS id lo8si1888234igb.18.2014.10.14.05.08.54 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 14 Oct 2014 05:08:57 -0700 (PDT) Received: from /spool/local by e28smtp08.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 14 Oct 2014 17:38:50 +0530 Received: from d28relay02.in.ibm.com (d28relay02.in.ibm.com [9.184.220.59]) by d28dlp02.in.ibm.com (Postfix) with ESMTP id E043C3940043 for ; Tue, 14 Oct 2014 17:38:46 +0530 (IST) Received: from d28av03.in.ibm.com (d28av03.in.ibm.com [9.184.220.65]) by d28relay02.in.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id s9EC96sY55443468 for ; Tue, 14 Oct 2014 17:39:07 +0530 Received: from d28av03.in.ibm.com (localhost [127.0.0.1]) by d28av03.in.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id s9EC8jjg022054 for ; Tue, 14 Oct 2014 17:38:45 +0530 From: "Aneesh Kumar K.V" Subject: Re: [PATCH 1/2] mm: Introduce a general RCU get_user_pages_fast. In-Reply-To: <20141014115854.GA32351@linaro.org> References: <1413284274-13521-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20141014115854.GA32351@linaro.org> Date: Tue, 14 Oct 2014 17:38:43 +0530 Message-ID: <8738aqonuc.fsf@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain Sender: owner-linux-mm@kvack.org List-ID: To: Steve Capper Cc: linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Steve Capper writes: > On Tue, Oct 14, 2014 at 04:27:53PM +0530, Aneesh Kumar K.V wrote: >> get_user_pages_fast attempts to pin user pages by walking the page >> tables directly and avoids taking locks. Thus the walker needs to be >> protected from page table pages being freed from under it, and needs >> to block any THP splits. >> >> One way to achieve this is to have the walker disable interrupts, and >> rely on IPIs from the TLB flushing code blocking before the page table >> pages are freed. >> >> On some platforms we have hardware broadcast of TLB invalidations, thus >> the TLB flushing code doesn't necessarily need to broadcast IPIs; and >> spuriously broadcasting IPIs can hurt system performance if done too >> often. >> >> This problem has been solved on PowerPC and Sparc by batching up page >> table pages belonging to more than one mm_user, then scheduling an >> rcu_sched callback to free the pages. This RCU page table free logic >> has been promoted to core code and is activated when one enables >> HAVE_RCU_TABLE_FREE. Unfortunately, these architectures implement >> their own get_user_pages_fast routines. >> >> The RCU page table free logic coupled with a an IPI broadcast on THP >> split (which is a rare event), allows one to protect a page table >> walker by merely disabling the interrupts during the walk. >> >> This patch provides a general RCU implementation of get_user_pages_fast >> that can be used by architectures that perform hardware broadcast of >> TLB invalidations. >> >> It is based heavily on the PowerPC implementation. >> >> Signed-off-by: Aneesh Kumar K.V >> --- >> >> NOTE: I kept the description of patch as it is and also retained the documentation. >> I also dropped the tested-by and other SOB, because i was not sure whether they want >> to be blamed for the bugs here. Please feel free to update. > > Hi Aneesh, > Thank you for coding this up. > > I've compiled and briefly tested this on arm (with and without LPAE), > and arm64. I ran a custom futex on THP tail test, and this passed. > I'll test this a little more aggressively with ltp. > > I think Linus has already pulled in the RCU gup I posted, could you > please instead write a patch against? Will do that. > 2667f50 mm: introduce a general RCU get_user_pages_fast() In that patch don't you need to check the _PAGE_PRESENT details in case of gup_huge_pmd ? -aneesh -- 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