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Thu, 02 Sep 2021 00:05:02 -0700 (PDT) Received: from [10.254.207.253] ([139.177.225.243]) by smtp.gmail.com with ESMTPSA id ev12sm984854pjb.57.2021.09.02.00.04.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 02 Sep 2021 00:05:02 -0700 (PDT) Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages To: Jason Gunthorpe , David Hildenbrand Cc: akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com References: <20210819031858.98043-1-zhengqi.arch@bytedance.com> <20210819031858.98043-7-zhengqi.arch@bytedance.com> <20210901135314.GA1859446@nvidia.com> <0c9766c9-6e8b-5445-83dc-9f2b71a76b4f@redhat.com> <20210901153247.GJ1721383@nvidia.com> <7789261d-6a64-c47b-be6c-c9be680e5d33@redhat.com> <20210901161613.GN1721383@nvidia.com> <20210901171039.GO1721383@nvidia.com> <20210901175547.GP1721383@nvidia.com> From: Qi Zheng Message-ID: Date: Thu, 2 Sep 2021 15:04:53 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <20210901175547.GP1721383@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=bytedance-com.20150623.gappssmtp.com header.s=20150623 header.b=gpq0+vBy; spf=pass (imf25.hostedemail.com: domain of zhengqi.arch@bytedance.com designates 209.85.214.174 as permitted sender) smtp.mailfrom=zhengqi.arch@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2774CB000181 X-Stat-Signature: fs7ca3pgtneky8zjdyqjanihz7jt9mwk X-HE-Tag: 1630566305-594880 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2021/9/2 AM1:55, Jason Gunthorpe wrote: > On Wed, Sep 01, 2021 at 07:49:23PM +0200, David Hildenbrand wrote: >> On 01.09.21 19:10, Jason Gunthorpe wrote: >>> On Wed, Sep 01, 2021 at 06:19:03PM +0200, David Hildenbrand wrote: >>> >>>>> I wouldn't think it works everywhere, bit it works in a lot of places, >>>>> and it is a heck of a lot better than what is proposed here. I'd >>>>> rather see the places that can use it be moved, and the few places >>>>> that can't be opencoded. >>>> >>>> Well, I used ptep_get_map_lock() and friends. But hacking directly into >>>> ptep_map_lock() and friends wasn't possible due to all the corner cases. >>> >>> Sure, I'm not surprised you can't get every single case, but that just >>> suggest we need two API families, today's to support the special cases >>> and a different one for the other regular simple cases. >>> >>> A new function family pte_try_map/_locked() and paired unmap that can >>> internally do the recounting and THP trickery and convert the easy >>> callsites. >>> >>> Very rough counting suggest at least half of the pte_offset_map_lock() >>> call sites can trivially use the simpler API. >>> >>> The other cases can stay as is and get open coded refcounts, or maybe >>> someone will have a better idea once they are more clearly identified. >>> >>> But I don't think we should take a performance hit of additional >>> atomics in cases like GUP where this is trivially delt with by using a >>> better API. >> >> Right, but as I said in the cover letter, we can happily optimize once we >> have the basic infrastructure in place and properly reviewed. Getting rid of >> some unnecessary atomics by introducing additional fancy helpers falls under >> that category. > > I'm not sure I agree given how big and wide this patch series is. It > would be easier to review if it was touching less places. The helpers > are not fancy, it is a logical re-arrangement of existing code that > shrinks the LOC of this series and makes it more reviewable. > > Or stated another way, a niche feature like this try much harder not > to add more complexity everywhere. Totally agree, I will rework this patch series based on you and David's suggestions. Thank you very much, Qi > > Jason >