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[2003:cb:c733:f100:75e7:a0a4:9ac2:1abb]) by smtp.gmail.com with ESMTPSA id g11-20020a05600c4ecb00b00412e293bee9sm4890462wmq.38.2024.03.04.13.15.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Mar 2024 13:15:03 -0800 (PST) Message-ID: <804524c8-772c-42d0-93a5-90d77f13f304@redhat.com> Date: Mon, 4 Mar 2024 22:15:02 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio To: Barry Song <21cnbao@gmail.com> Cc: Ryan Roberts , akpm@linux-foundation.org, linux-mm@kvack.org, chrisl@kernel.org, yuzhao@google.com, hanchuanhua@oppo.com, linux-kernel@vger.kernel.org, willy@infradead.org, ying.huang@intel.com, xiang@kernel.org, mhocko@suse.com, shy828301@gmail.com, wangkefeng.wang@huawei.com, Barry Song , Hugh Dickins References: <20240304103757.235352-1-21cnbao@gmail.com> <706b7129-85f6-4470-9fd9-f955a8e6bd7c@arm.com> <37f1e6da-412b-4bb4-88b7-4c49f21f5fe9@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: A84421C0006 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: akukoe63ghct5pquuenpaxjzi5i39uqt X-HE-Tag: 1709586909-586718 X-HE-Meta: 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 HSqpAqEH uYzS4XA1lO0wRg6I2/5570Hv2BuDKVUArjAgxVNbal443Cy7PUkchS8uyOzNhwjqjpstxKP5WtjOpgHKgejwrPyh/t1fUwr/Uop8DQh0LcUIuKfFx9NImkNxn7N2HXCO8Thv33LHD9QK3+6CLewdkW6ji/p75rtVCjVMT 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: >>> Do we need a Fixes tag? > > I am not quite sure which commit should be here for a fixes tag. > I think it's more of an optimization. Good, that helps! > >>> >> >> What would be the description of the problem we are fixing? >> >> 1) failing to unmap? >> >> That can happen with small folios as well IIUC. >> >> 2) Putting the large folio on the deferred split queue? >> >> That sounds more reasonable. > > I don't feel it is reasonable. Avoiding this kind of accident splitting > from the kernel's improper code is a more reasonable approach > as there is always a price to pay for splitting and unfolding PTEs > etc. > > While we can't avoid splitting coming from userspace's > MADV_DONTNEED, munmap, mprotect, we have a way > to ensure the kernel itself doesn't accidently break up a > large folio. Note that on the next vmscan we would retry, find the remaining present entries and swapout that thing completely :) > > In OPPO's phones, we ran into some weird bugs due to skipped PTEs > in try_to_unmap_one. hardly could we fix it from the root cause. with > various races, figuring out their timings was really a big pain :-) > I can imagine. I assume, though, that it might be related to the way the cont-pte bit was handled. Ryan's implementation should be able to cope with that. > But we did "resolve" those bugs by entirely untouching all PTEs if we > found some PTEs were skipped in try_to_unmap_one [1]. > > While we find we only get the PTL from 2nd, 3rd but not > 1st PTE, we entirely give up on try_to_unmap_one, and leave > all PTEs untouched. > > /* we are not starting from head */ > if (!IS_ALIGNED((unsigned long)pvmw.pte, CONT_PTES * sizeof(*pvmw.pte))) { > ret = false; > atomic64_inc(&perf_stat.mapped_walk_start_from_non_head); > set_pte_at(mm, address, pvmw.pte, pteval); > page_vma_mapped_walk_done(&pvmw); > break; > } > This will ensure all PTEs still have a unified state such as CONT-PTE > after try_to_unmap fails. > I feel this could have some false postive because when racing > with unmap, 1st PTE might really become pte_none. So explicitly > holding PTL from 1st PTE seems a better way. Can we estimate the "cost" of holding the PTL? -- Cheers, David / dhildenb