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[79.242.63.113]) by smtp.gmail.com with ESMTPSA id u16sm5978489wmc.41.2021.09.01.06.57.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 01 Sep 2021 06:57:10 -0700 (PDT) Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages To: Jason Gunthorpe , Qi Zheng 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> From: David Hildenbrand Organization: Red Hat Message-ID: <0c9766c9-6e8b-5445-83dc-9f2b71a76b4f@redhat.com> Date: Wed, 1 Sep 2021 15:57:09 +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: <20210901135314.GA1859446@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 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. > > This would be much better if the map_lock infra could manage the > refcount itself. > > I'm also not really keen on adding ptl level locking to all the > currently no-lock paths. If we are doing that then the no-lock paths > should rely on the ptl for alot more of their operations and avoid the > complicatred no-lock data access we have. eg 'pte_try_get()' should > also copy the pte_t under the lock. > > Also, I don't really understand how this scheme works with > get_user_pages_fast. With the RCU change it in #8 it should work just fine, because RCU synchronize has to wait either until all other CPUs have left the RCU read section, or re-enabled interrupts. -- Thanks, David / dhildenb