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Sat, 5 Oct 2019 08:35:52 +0000 (GMT) Received: from b03ledav005.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 86E3BBE056; Sat, 5 Oct 2019 08:35:31 +0000 (GMT) Received: from [9.199.38.126] (unknown [9.199.38.126]) by b03ledav005.gho.boulder.ibm.com (Postfix) with ESMTP; Sat, 5 Oct 2019 08:35:31 +0000 (GMT) Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks To: Peter Zijlstra , Leonardo Bras References: <20191003013325.2614-1-leonardo@linux.ibm.com> <20191003013325.2614-2-leonardo@linux.ibm.com> <20191003071145.GM4536@hirez.programming.kicks-ass.net> <20191003115141.GJ4581@hirez.programming.kicks-ass.net> From: "Aneesh Kumar K.V" Message-ID: <651856af-30de-15ad-2312-158f3ad5292e@linux.ibm.com> Date: Sat, 5 Oct 2019 14:05:29 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <20191003115141.GJ4581@hirez.programming.kicks-ass.net> Content-Type: text/plain; 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Levin" , Keith Busch , linux-mm@kvack.org, Paul Mackerras , Christoph Lameter , Ira Weiny , Thomas Gleixner , Elena Reshetova , linux-arch@vger.kernel.org, Santosh Sivaraj , Davidlohr Bueso , Bartlomiej Zolnierkiewicz , Mike Rapoport , Jason Gunthorpe , Vlastimil Babka , Mahesh Salgaonkar , Andrey Ryabinin , Alexey Dobriyan , Ingo Molnar , Andrea Arcangeli , Ralph Campbell , Arnd Bergmann , Jann Horn , John Hubbard , Jesper Dangaard Brouer , kvm-ppc@vger.kernel.org, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Mathieu Desnoyers , Nicholas Piggin , Dan Williams , Reza Arbab , Allison Randal , Christian Brauner , Greg Kroah-Hartman , linux-kernel@vger.kernel.org, Logan Gunthorpe , Souptick Joarder , Andrew Morton , linuxppc-dev@lists.ozlabs.org, Roman Gushchin , "Kirill A. Shutemov" , Al Viro Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 10/3/19 5:21 PM, Peter Zijlstra wrote: > On Thu, Oct 03, 2019 at 09:11:45AM +0200, Peter Zijlstra wrote: >> On Wed, Oct 02, 2019 at 10:33:15PM -0300, Leonardo Bras wrote: .... > > And I still think all that wrong, you really shouldn't need to wait on > munmap(). > I do have a patch that does something like that. +#define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR_FULL +static inline pmd_t pmdp_huge_get_and_clear_full(struct mm_struct *mm, + unsigned long address, pmd_t *pmdp, + int full) +{ + bool serialize = true; + /* + * We don't need to serialze against a lockless page table walk if + * we are clearing the pmd due to task exit. For regular mnumap, we + * still need to serialize due the possibility of MADV_DONTNEED running + * parallel to a page fault which can convert a THP pte entry to a + * pointer to level 4 table. + * Here MADV_DONTNEED is removing the THP entry and the fault is filling + * a level 4 pte. + */ + if (full == 1) + serialize = false; + return __pmdp_huge_get_and_clear(mm, address, pmdp, serialize); } if it is a fullmm flush we can skip that serialize, But for everything else we need to serialize. MADV_DONTNEED is another case. I haven't sent this yet, because I was trying to look at what it takes to switch that MADV variant to take mmap_sem in write mode. MADV_DONTNEED has caused us multiple issues due to the fact that it can run in parallel to page fault. I am not sure whether we have a known/noticeable performance gain in allowing that with mmap_sem held in read mode. -aneesh