From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f70.google.com (mail-pl0-f70.google.com [209.85.160.70]) by kanga.kvack.org (Postfix) with ESMTP id DCC636B000C for ; Fri, 13 Apr 2018 08:13:02 -0400 (EDT) Received: by mail-pl0-f70.google.com with SMTP id b11-v6so5812488pla.19 for ; Fri, 13 Apr 2018 05:13:02 -0700 (PDT) Received: from smtp.codeaurora.org (smtp.codeaurora.org. [198.145.29.96]) by mx.google.com with ESMTPS id u23-v6si4889721plk.516.2018.04.13.05.13.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 13 Apr 2018 05:13:01 -0700 (PDT) Subject: Re: [PATCH] mm: vmalloc: Remove double execution of vunmap_page_range References: <1523611019-17679-1-git-send-email-cpandya@codeaurora.org> <8da9f826-2a3d-e618-e512-4fc8d45c16f2@codeaurora.org> <20180413110949.GA17670@dhcp22.suse.cz> <696fedc5-6bcd-f0a0-62f5-4f9e7b7c602a@codeaurora.org> <20180413114133.GJ17484@dhcp22.suse.cz> From: Chintan Pandya Message-ID: <7674bfda-6186-8b32-0144-62c666e05e3c@codeaurora.org> Date: Fri, 13 Apr 2018 17:42:53 +0530 MIME-Version: 1.0 In-Reply-To: <20180413114133.GJ17484@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Anshuman Khandual , vbabka@suse.cz, labbott@redhat.com, catalin.marinas@arm.com, hannes@cmpxchg.org, f.fainelli@gmail.com, xieyisheng1@huawei.com, ard.biesheuvel@linaro.org, richard.weiyang@gmail.com, byungchul.park@lge.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 4/13/2018 5:11 PM, Michal Hocko wrote: > On Fri 13-04-18 16:57:06, Chintan Pandya wrote: >> >> >> On 4/13/2018 4:39 PM, Michal Hocko wrote: >>> On Fri 13-04-18 16:15:26, Chintan Pandya wrote: >>>> >>>> >>>> On 4/13/2018 4:10 PM, Anshuman Khandual wrote: >>>>> On 04/13/2018 03:47 PM, Chintan Pandya wrote: >>>>>> >>>>>> >>>>>> On 4/13/2018 3:29 PM, Anshuman Khandual wrote: >>>>>>> On 04/13/2018 02:46 PM, Chintan Pandya wrote: >>>>>>>> Unmap legs do call vunmap_page_range() irrespective of >>>>>>>> debug_pagealloc_enabled() is enabled or not. So, remove >>>>>>>> redundant check and optional vunmap_page_range() routines. >>>>>>> >>>>>>> vunmap_page_range() tears down the page table entries and does >>>>>>> not really flush related TLB entries normally unless page alloc >>>>>>> debug is enabled where it wants to make sure no stale mapping is >>>>>>> still around for debug purpose. Deferring TLB flush improves >>>>>>> performance. This patch will force TLB flush during each page >>>>>>> table tear down and hence not desirable. >>>>>>> >>>>>> Deferred TLB invalidation will surely improve performance. But force >>>>>> flush can help in detecting invalid access right then and there. I >>>>> >>>>> Deferred TLB invalidation was a choice made some time ago with the >>>>> commit db64fe02258f1507e ("mm: rewrite vmap layer") as these vmalloc >>>>> mappings wont be used other than inside the kernel and TLB gets >>>>> flushed when they are reused. This way it can still avail the benefit >>>>> of deferred TLB flushing without exposing itself to invalid accesses. >>>>> >>>>>> chose later. May be I should have clean up the vmap tear down code >>>>>> as well where it actually does the TLB invalidation. >>>>>> >>>>>> Or make TLB invalidation in free_unmap_vmap_area() be dependent upon >>>>>> debug_pagealloc_enabled(). >>>>> >>>>> Immediate TLB invalidation needs to be dependent on debug_pagealloc_ >>>>> enabled() and should be done only for debug purpose. Contrary to that >>>>> is not desirable. >>>>> >>>> Okay. I will raise v2 for that. >>> >>> More importantly. Your changelog absolutely lacks the _why_ part. It >>> just states what the code does which is not all that hard to read from >>> the diff. It is usually much more important to present _why_ the patch >>> is an improvement and worth merging. >>> >> >> It is improving performance in debug scenario. > > Do not forget to add some numbers presenting the benefits when > resubmitting. Okay. > >> More than that, I see it >> as a clean up. Sure, I will try to address *why* in next change log. > > As Anshuman pointed out the current code layout is deliberate. If you > believe that reasons mentioned previously are not valid then dispute > them and provide your arguments in the changelog. > Here, the trade off is, performance vs catching use-after-free. Original code is preferring performance gains. At first, it seemed to me that stability is more important than performance. But giving more thoughts on this (and reading commit db64fe02258f1507e ("mm: rewrite vmap layer")), I feel that use-after-free is client side wrong-doing. vmap layer need not loose its best case settings for potential client side mistakes. For that, vmap layer can provide debug settings. So, I plan to do TLB flush conditional on debug settings. Chintan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project