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[2003:cb:c745:2a00:d74a:a8c5:20b6:3ec3]) by smtp.gmail.com with ESMTPSA id f9-20020a0560001b0900b003143867d2ebsm12380949wrz.63.2023.11.27.07.42.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Nov 2023 07:42:26 -0800 (PST) Message-ID: <4ca4a5af-accb-4424-a5fb-e1e561900348@redhat.com> Date: Mon, 27 Nov 2023 16:42:25 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: David Hildenbrand Subject: Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios To: Ryan Roberts , Barry Song <21cnbao@gmail.com>, Steven Price Cc: akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, shy828301@gmail.com, v-songbaohua@oppo.com, wangkefeng.wang@huawei.com, willy@infradead.org, xiang@kernel.org, ying.huang@intel.com, yuzhao@google.com References: <20231114014313.67232-1-v-songbaohua@oppo.com> <864489b3-5d85-4145-b5bb-5d8a74b9b92d@redhat.com> <8c7f1a2f-57d2-4f20-abb2-394c7980008e@redhat.com> <5de66ff5-b6c8-4ffc-acd9-59aec4604ca4@redhat.com> <71c4b8b2-512a-4e50-9160-6ee77a5ec0a4@arm.com> <679a144a-db47-4d05-bbf7-b6a0514f5ed0@arm.com> <8aa8f095-1840-4a2e-ad06-3f375282ab6a@arm.com> <7065bbd3-64b3-4cd6-a2cd-146c556aac66@redhat.com> <74596ea2-e7e5-4161-9600-ad1a69b6a6fe@arm.com> In-Reply-To: <74596ea2-e7e5-4161-9600-ad1a69b6a6fe@arm.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 469DF20033 X-Stat-Signature: aoiwpo6k85fej9ofioejjq6e5t17rwgc X-HE-Tag: 1701099751-946962 X-HE-Meta: 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 4urIqh/v VZ/k30F/crstnKh9udNalKVQdkyh1isQLVNdUHMpVn1fD4B9OAkB8KkAkY+Ug7C+adyupaqyHxueiQUtG/drA28R16kKkBby4Ti+zxj5QSl2Bt256ruAKWrvz9f3aJh7FIBEzrKZBK4RMld8aokh44ZWwSbNDa7sFk51CudyQ2JuFEl0ed84GDtda5Cdk4AnOSb0KbHmQ4CqEqTZWPWSCDQRkjZgS6Sdy4DWQrNTw7KX5qMbKNTuSdTtZanc7pUuIXFmIIPIrCtnu6SeLp06LMFWIr9HeHBFiFTz2q04g/Ua5GA8IucsCMdCJF4YS4NHpFzbK2fddDsb9lZ4h7kxhBEul7vAFx7NWx0PNaPX14BKQh5fLgxIacR6cqOSWBDMj+uIc7kIpF8CK9hWZ9ARUIT11M98wMlWuklRxYNp9KhhPWek= 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: List-Subscribe: List-Unsubscribe: >>> Are you referring to Steven's suggestion of reading the tag to see if it's >>> zeros? I think that demonstrates my point that this has to be done per-page and >> >> Yes. > > OK I'm obviously being thick, because checking a page's tag to see if its zero > seems logically equivalent to checking the existing per-page flag, just more > expensive. Yes we could make that change but I don't see how it helps solve the > real problem at hand. Unless you are also deliberately trying to remove the > per-page flag at the same time, as per Matthew's master plan? I think a per-folio interface is cleaner and more future-proof, and removing the per-page flag might be a nice side product of that. Anyhow, just some thoughts from my side if it could be easily/cleanly done. At least the "easy" part does not seem to be the case, so I'm fine with deferring anything like that for now. [...] >> >> We can identify in 1) the subpages by reading the tag from HW, > > I don't think this actually works; I'm pretty sure the optimization to clear the > tag at the same time as the page clearing only happens for small pages. I don't > think this will be done when allocating a THP today. Obviously that could change. > Could be, absolutely no expert. I was primarily on the "It would be possible to reverse this scheme - we could drop the page flag and just look at the actual tag storage. If it's all zeros then obviously there's no point in storing it." comment from Steven. >> and on 2) by >> checking the datastructure. For 3), there is nothing to check. >> >> On swapout of a large folio: >> >> * For 3) we don't do anything >> * For 2) we don't do anything >> * For 1) we store all tags that are non-zero (reading all tags) and >>   transition to 2). > > Given a tag architecturally exists for every page even when unused, and we think > a folio being partially mte-tagged is the corner case, could you simplify this > further and just write out all the tags for the folio and not care if some are > not in use? Likely this could be simplified, yes. -- Cheers, David / dhildenb