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From: Muchun Song In-Reply-To: <7be98c64-88a1-3bee-7f92-67bb1f4f495b@huawei.com> Date: Thu, 18 Aug 2022 15:59:43 +0800 Cc: "Yin, Fengwei" , Andrew Morton , Mike Kravetz , Muchun Song , Linux MM , linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <3B1463C2-9DC4-43D0-93EC-2D2334A20502@linux.dev> References: <20220816130553.31406-1-linmiaohe@huawei.com> <20220816130553.31406-5-linmiaohe@huawei.com> <0EAF1279-6A1C-41FA-9A32-414C36B3792A@linux.dev> <019c1272-9d01-9d51-91a0-2d656b25c318@intel.com> <18adbf89-473e-7ba6-9a2b-522e1592bdc6@huawei.com> <9c791de0-b702-1bbe-38a4-30e87d9d1b95@intel.com> <931536E2-3948-40AB-88A7-E36F67954AAA@linux.dev> <7be98c64-88a1-3bee-7f92-67bb1f4f495b@huawei.com> To: Miaohe Lin X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660816338; a=rsa-sha256; cv=none; b=u6c42kkicTFspaOQT8+nV9D8YFF4HrQfj8NYBHFGZGqhz1WBw90vtQfqcTRp1EJs8EK7O0 Niu56AhWeJ7zsFnwgWQWbAZ5ccf74qq2XYcGBrN3sE3gVElrUq+vDZO1TcXNrZ7x69Iosl ERyao3FYz3UY7gika1m8RQZt1JPLp74= ARC-Authentication-Results: i=1; imf03.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="CgBP/vjk"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf03.hostedemail.com: domain of muchun.song@linux.dev designates 91.121.223.63 as permitted sender) smtp.mailfrom=muchun.song@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1660816338; 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:dkim-signature; bh=rx2a0SfNDi1opj3mz7ReHR+gfYYq0rf6lV0w0JKT45s=; b=3plPKAqJcUCVVkJjwc6+PExDRPE8VeUOlcXuxWRQl/kw0Rhh6ne+pdVFGK79K8rNv0qTzD rjwPJzxI2b/AF3CF5mIt4A590Zh7dn8CGwPNCu4Mi+8ELP4H40bNNXuKSMc4aiGHgqlGsV yw8HbamsVOKQADYdERBSDYYk/S8ibEs= X-Stat-Signature: 3uf6jugmk9byg8nz9bwgqcnox8gzrwh9 X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 8DFF5208E4 X-Rspam-User: Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="CgBP/vjk"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf03.hostedemail.com: domain of muchun.song@linux.dev designates 91.121.223.63 as permitted sender) smtp.mailfrom=muchun.song@linux.dev X-HE-Tag: 1660816339-439944 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: > On Aug 18, 2022, at 15:52, Miaohe Lin wrote: >=20 > On 2022/8/18 10:47, Muchun Song wrote: >>=20 >>=20 >>> On Aug 18, 2022, at 10:00, Yin, Fengwei = wrote: >>>=20 >>>=20 >>>=20 >>> On 8/18/2022 9:55 AM, Miaohe Lin wrote: >>>>>>> /* >>>>>>> * The memory barrier inside __SetPageUptodate makes = sure that >>>>>>> * preceding stores to the page contents become visible = before >>>>>>> * the set_pte_at() write. >>>>>>> */ >>>>>>> __SetPageUptodate(page); >>>>>> IIUC, the case here we should make sure others (CPUs) can see new = page=E2=80=99s >>>>>> contents after they have saw PG_uptodate is set. I think commit = 0ed361dec369 >>>>>> can tell us more details. >>>>>>=20 >>>>>> I also looked at commit 52f37629fd3c to see why we need a barrier = before >>>>>> set_pte_at(), but I didn=E2=80=99t find any info to explain why. = I guess we want >>>>>> to make sure the order between the page=E2=80=99s contents and = subsequent memory >>>>>> accesses using the corresponding virtual address, do you agree = with this? >>>>> This is my understanding also. Thanks. >>>> That's also my understanding. Thanks both. >>> I have an unclear thing (not related with this patch directly): Who = is response >>> for the read barrier in the read side in this case? >>>=20 >>> For SetPageUptodate, there are paring write/read memory barrier. >>>=20 >>=20 >> I have the same question. So I think the example proposed by Miaohe = is a little >> difference from the case (hugetlb_vmemmap) here. >=20 > Per my understanding, memory barrier in PageUptodate() is needed = because user might access the > page contents using page_address() (corresponding pagetable entry = already exists) soon. But for > the above proposed case, if user wants to access the page contents, = the corresponding pagetable > should be visible first or the page contents can't be accessed. So = there should be a data dependency > acting as memory barrier between pagetable entry is loaded and page = contents is accessed. > Or am I miss something? Yep, it is a data dependency. The difference between hugetlb_vmemmap and = PageUptodate() is that the page table (a pointer to the mapped page frame) is loaded by MMU = while PageUptodate() is loaded by CPU. Seems like the data dependency should be inserted between = the MMU access and the CPU access. Maybe it is hardware=E2=80=99s guarantee? >=20 > Thanks, > Miaohe Lin