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[91.12.96.79]) by smtp.gmail.com with ESMTPSA id n8sm363484wrp.95.2021.11.10.08.53.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Nov 2021 08:53:58 -0800 (PST) Message-ID: Date: Wed, 10 Nov 2021 17:53:57 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v3 00/15] Free user PTE page table pages Content-Language: en-US To: Matthew Wilcox Cc: Jason Gunthorpe , Qi Zheng , akpm@linux-foundation.org, tglx@linutronix.de, 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, zhouchengming@bytedance.com References: <20211110105428.32458-1-zhengqi.arch@bytedance.com> <20211110125601.GQ1740502@nvidia.com> <8d0bc258-58ba-52c5-2e0d-a588489f2572@redhat.com> <20211110143859.GS1740502@nvidia.com> <6ac9cc0d-7dea-0e19-51b3-625ec6561ac7@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10.11.21 17:49, Matthew Wilcox wrote: > On Wed, Nov 10, 2021 at 04:37:14PM +0100, David Hildenbrand wrote: >>> I'd suggest to make this new lock a special rwsem which allows either >>> concurrent read access OR concurrent PTL access, but not both. This >> >> I looked into such a lock recently in similar context and something like >> that does not exist yet (and fairness will be challenging). You either >> have a single reader or multiple writer. I'd be interested if someone >> knows of something like that. > > We've talked about having such a lock before for filesystems which want > to permit either many direct-IO accesses or many buffered-IO accesses, but > want to exclude a mixture of direct-IO and buffered-IO. The name we came > up with for such a lock was the red-blue lock. Either Team Red has the > lock, or Team Blue has the lock (or it's free). Indicate free with velue > zero, Team Red with positive numbers and Team Blue with negative numbers. > If we need to indicate an opposing team is waiting for the semaphore, > we can use a high bit (1 << 30) to indicate no new team members can > acquire the lock. Not sure whether anybody ever coded it up. Interesting, thanks for sharing! My excessive google search didn't reveal anything back then (~3 months ago) -- only questions on popular coding websites asking essentially for the same thing without any helpful replies. But maybe I used the wrong keywords (e.g., "multiple reader, multiple writer", I certainly didn't search for "red-blue lock", but I do like the name :) ). Fairness might still be the biggest issue, but I am certainly no locking expert. -- Thanks, David / dhildenb