From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f178.google.com (mail-pd0-f178.google.com [209.85.192.178]) by kanga.kvack.org (Postfix) with ESMTP id B78226B0069 for ; Tue, 14 Oct 2014 09:42:30 -0400 (EDT) Received: by mail-pd0-f178.google.com with SMTP id y10so7408217pdj.23 for ; Tue, 14 Oct 2014 06:42:30 -0700 (PDT) Received: from e23smtp07.au.ibm.com (e23smtp07.au.ibm.com. [202.81.31.140]) by mx.google.com with ESMTPS id mw6si12988392pdb.146.2014.10.14.06.42.28 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 14 Oct 2014 06:42:29 -0700 (PDT) Received: from /spool/local by e23smtp07.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 14 Oct 2014 23:42:24 +1000 Received: from d23relay04.au.ibm.com (d23relay04.au.ibm.com [9.190.234.120]) by d23dlp01.au.ibm.com (Postfix) with ESMTP id DC66A2CE8040 for ; Wed, 15 Oct 2014 00:42:20 +1100 (EST) Received: from d23av01.au.ibm.com (d23av01.au.ibm.com [9.190.234.96]) by d23relay04.au.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id s9EDNhjS26017828 for ; Wed, 15 Oct 2014 00:23:44 +1100 Received: from d23av01.au.ibm.com (localhost [127.0.0.1]) by d23av01.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id s9EDgJnG025163 for ; Wed, 15 Oct 2014 00:42:19 +1100 From: "Aneesh Kumar K.V" Subject: Re: [PATCH 1/2] mm: Introduce a general RCU get_user_pages_fast. In-Reply-To: <20141014122922.GA763@linaro.org> References: <1413284274-13521-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20141014115854.GA32351@linaro.org> <8738aqonuc.fsf@linux.vnet.ibm.com> <20141014122922.GA763@linaro.org> Date: Tue, 14 Oct 2014 19:12:15 +0530 Message-ID: <87zjcyn4y0.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 05:38:43PM +0530, Aneesh Kumar K.V wrote: >> 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 ? >> > > Before calling gup_huge_pmd we check for pmd_none. On ARM we have: > #define pmd_none(pmd) (!pmd_val(pmd)) > #define pmd_present(pmd) (pmd_val(pmd)) > > So we know when we enter gup_huge_pmd that "orig" represents a pmd that > is present. > > On PowerPC I see the definitions are a little different for > pgtable-ppc32.h: > #define pmd_present(pmd) (pmd_val(pmd) & _PMD_PRESENT_MASK) > > A pmd_present() could be added to the beginning of gup_huge_pmd safely > if this is needed for PowerPC. > for the huge pmd case, that is pte in pmd we may really want to check _PAGE_PRESENT bit right ? -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