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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1709629697; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8Epj7If0zvVbvccdyifxUIrmrXlDu3t1G5CCtAyXP6Q=; b=8jvkWwfdVA5opKh91L51cRA7ODDy9N7oblVN2RwySytU5s2ZWo41TBMja/px92svVlAH7u aAdftuY25V7gvO0TLRBapMvEnELpw9kJuL+y8a+SIy7fQ5ySJZAD/lgRP3RLk+HoXOLMW3 sdvE2kYO9S8DGck21rWX4/NgDoRS2mY= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2A7271FB; Tue, 5 Mar 2024 01:08:53 -0800 (PST) Received: from [10.57.68.162] (unknown [10.57.68.162]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3A6D43F762; Tue, 5 Mar 2024 01:08:14 -0800 (PST) Message-ID: <6c16c7c5-8cf4-4c30-b3a5-a9ab55b21114@arm.com> Date: Tue, 5 Mar 2024 09:08:12 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio Content-Language: en-GB To: Barry Song <21cnbao@gmail.com>, "Huang, Ying" Cc: akpm@linux-foundation.org, linux-mm@kvack.org, david@redhat.com, chrisl@kernel.org, yuzhao@google.com, hanchuanhua@oppo.com, linux-kernel@vger.kernel.org, willy@infradead.org, xiang@kernel.org, mhocko@suse.com, shy828301@gmail.com, wangkefeng.wang@huawei.com, Barry Song , Hugh Dickins References: <20240304103757.235352-1-21cnbao@gmail.com> <878r2x9ly3.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: A145D4000E X-Stat-Signature: cdzewwofkrdjytobgedo7cd5pwwy8i18 X-Rspam-User: X-HE-Tag: 1709629697-424712 X-HE-Meta: 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 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: On 05/03/2024 08:56, Barry Song wrote: > are writing pte to zero(break) before writing a new value(make). while As an aside, "break-before-make" as defined in the Arm architecture would also require a TLBI, which usually isn't done for these write-0-modify-prots-write-back operations. Arm doesn't require "break-before-make" in these situations so its legal (as long as only certain bits are changed). To my understanding purpose of doing this is to avoid races with HW access/dirty flag updates; if the MMU wants to set either flag and finds the PTE is 0 (invalid) it will cause an exception which will be queued waiting for the PTL. So I don't think you really mean break-before-make here. > this behavior is within PTL in another thread, page_vma_mapped_walk() > of try_to_unmap_one thread won't take PTL till it meets a present PTE. > for example, if another threads are modifying nr_pages PTEs under PTL, > but we don't hold PTL, we might skip one or two PTEs at the beginning of > a large folio. > For a large folio, after try_to_unmap_one(), we may result in PTE0 and PTE1 > untouched but PTE2~nr_pages-1 are set to swap entries. > > by holding PTL from PTE0 for large folios, we won't get these intermediate > values. At the moment we get PTL, other threads have done.