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[24.6.216.183]) by smtp.gmail.com with ESMTPSA id q2-20020a170902f78200b0016d1f474653sm7437465pln.52.2022.08.15.13.52.51 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Mon, 15 Aug 2022 13:52:51 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\)) Subject: Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations From: Nadav Amit In-Reply-To: Date: Mon, 15 Aug 2022 13:52:49 -0700 Cc: "Huang, Ying" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Minchan Kim , David Hildenbrand , Andrew Morton , Hugh Dickins , Vlastimil Babka , Andrea Arcangeli , Andi Kleen , "Kirill A . Shutemov" Content-Transfer-Encoding: quoted-printable Message-Id: <5B21352C-2BE6-4070-BB6B-C1B7A5D4D225@gmail.com> References: <20220809220100.20033-1-peterx@redhat.com> <20220809220100.20033-6-peterx@redhat.com> <87pmh6dwdr.fsf@yhuang6-desk2.ccr.corp.intel.com> To: Peter Xu X-Mailer: Apple Mail (2.3696.120.41.1.1) ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660596773; a=rsa-sha256; cv=none; b=f/1AXEHteKclGdoEJCk2Ee+mC6FQKt020vy+djIyii//rcuD294IFZANSJbJE2v17uEZHR 2ppEJeCjLuHlIcpDjaaDFLWAUnAYAcLV555OMH9qRKd1DKVUg1d2PcfFb8LArtM0aZh/wC E6Qvc0ZJO9fqYZIR4H65qo8LKqEUuuw= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=TS0+ZzP9; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf01.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.215.177 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1660596773; 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=c+KhdgocO4zsjTWrfEV5k5L4Uxolaa6Gk/jRei7Y/Oc=; b=cnPL0P+kXyObcUWgZxZdPznRGDis5/YalCCnkpagRNe+ZuMd8UkmSsVb186Sgly5kc0iQK VtG8w1i0+Suqn02uYxL5WRVsgFcaTKkArVVtkdLXThRLEKkQDjpl/vMLC6rOm8dsZ7n/jZ ik0ubJmjJkhRqdcDJkQEZvHqdl+btJ8= X-Stat-Signature: sf7wcetr6bsjncz5xk1tidg1c5iw6f5s X-Rspamd-Queue-Id: CAB7940050 Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=TS0+ZzP9; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf01.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.215.177 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1660596773-594574 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 15, 2022, at 12:18 PM, Peter Xu wrote: > On Fri, Aug 12, 2022 at 10:32:48AM +0800, Huang, Ying wrote: >> Peter Xu writes: >>=20 >>> On Tue, Aug 09, 2022 at 06:00:58PM -0400, Peter Xu wrote: >>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>> index 27fb37d65476..699f821b8443 100644 >>>> --- a/mm/migrate_device.c >>>> +++ b/mm/migrate_device.c >>>> @@ -221,6 +221,10 @@ static int migrate_vma_collect_pmd(pmd_t = *pmdp, >>>> else >>>> entry =3D make_readable_migration_entry( >>>> = page_to_pfn(page)); >>>> + if (pte_young(pte)) >>>> + entry =3D = make_migration_entry_young(entry); >>>> + if (pte_dirty(pte)) >>>> + entry =3D = make_migration_entry_dirty(entry); >>>> swp_pte =3D swp_entry_to_pte(entry); >>>> if (pte_present(pte)) { >>>> if (pte_soft_dirty(pte)) >>>=20 >>> This change needs to be wrapped with pte_present() at least.. >>>=20 >>> I also just noticed that this change probably won't help anyway = because: >>>=20 >>> (1) When ram->device, the pte will finally be replaced with a = device >>> private entry, and device private entry does not yet support = A/D, it >>> means A/D will be dropped again, >>>=20 >>> (2) When device->ram, we are missing information on either A/D = bits, or >>> even if device private entries start to suport A/D, it's still = not >>> clear whether we should take device read/write into = considerations >>> too on the page A/D bits to be accurate. >>>=20 >>> I think I'll probably keep the code there for completeness, but I = think it >>> won't really help much until more things are done. >>=20 >> It appears that there are more issues. Between "pte =3D *ptep" and = pte >> clear, CPU may set A/D bit in PTE, so we may need to update pte when >> clearing PTE. >=20 > Agreed, I didn't see it a huge problem with current code, but it = should be > better in that way. >=20 >> And I don't find the TLB is flushed in some cases after PTE is = cleared. >=20 > I think it's okay to not flush tlb if pte not present. But maybe = you're > talking about something else? I think Huang refers to situation in which the PTE is cleared, still not flushed, and then A/D is being set by the hardware. At least on x86, the hardware is not supposed to do so. The only case I remember (and sometimes misremembers) is with KNL erratum, which perhaps needs to be considered: https://lore.kernel.org/all/20160708001911.9A3FD2B6@viggo.jf.intel.com/