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[79.242.63.113]) by smtp.gmail.com with ESMTPSA id w9sm22106619wrs.7.2021.09.01.09.19.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 01 Sep 2021 09:19:04 -0700 (PDT) Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages To: Jason Gunthorpe Cc: Qi Zheng , 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> From: David Hildenbrand Organization: Red Hat Message-ID: Date: Wed, 1 Sep 2021 18:19:03 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210901161613.GN1721383@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01.09.21 18:16, Jason Gunthorpe wrote: > On Wed, Sep 01, 2021 at 06:13:07PM +0200, David Hildenbrand wrote: >> On 01.09.21 17:32, Jason Gunthorpe wrote: >>> On Wed, Sep 01, 2021 at 03:57:09PM +0200, David Hildenbrand wrote: >>>> On 01.09.21 15:53, Jason Gunthorpe wrote: >>>>> On Thu, Aug 19, 2021 at 11:18:55AM +0800, Qi Zheng wrote: >>>>> >>>>>> diff --git a/mm/gup.c b/mm/gup.c >>>>>> index 2630ed1bb4f4..30757f3b176c 100644 >>>>>> +++ b/mm/gup.c >>>>>> @@ -500,6 +500,9 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, >>>>>> if (unlikely(pmd_bad(*pmd))) >>>>>> return no_page_table(vma, flags); >>>>>> + if (!pte_try_get(mm, pmd)) >>>>>> + return no_page_table(vma, flags); >>>>>> + >>>>>> ptep = pte_offset_map_lock(mm, pmd, address, &ptl); >>>>> >>>>> This is not good on a performance path, the pte_try_get() is >>>>> locking/locking the same lock that pte_offset_map_lock() is getting. >>>> >>>> Yes, and we really need patch #8, anything else is just confusing reviewers. >>> >>> It is a bit better with patch 8, but it is still not optimal, we don't >>> need to do the atomic work at all if the entire ptep is accessed while >>> locked. So the above is stil not what I would expect here, even with >>> RCU. >>> >>> eg I would expect that this kind of change would work first with the >>> existing paired acessors, ie >>> >>> pte = pte_offset_map(pmd, address); >>> pte_unmap(pte); >>> >>> Should handle the refcount under the covers, and same kind of idea for >>> the _locked/_unlocked varient. >> >> See my other mail. > > Do you have a reference? Reply to the other mail you just send. > >>> Only places that don't already use that pairing should get modified. >>> >>> To do this we have to extend the API so that pte_offset_map() can >>> fail, or very cleverly return some kind of global non-present pte page >>> (I wonder if the zero page would work?) >> >> I explored both ideas (returning NULL, return a specially prepared page) and >> it didn't work in some cases where we unmap+remap etc. > > 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. -- Thanks, David / dhildenb