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[84.110.50.27]) by smtp.gmail.com with ESMTPSA id o1-20020a05600c338100b003a3170a7af9sm2601374wmp.4.2022.10.26.12.43.22 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Wed, 26 Oct 2022 12:43:23 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\)) Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment From: Nadav Amit In-Reply-To: Date: Wed, 26 Oct 2022 22:43:21 +0300 Cc: Jann Horn , John Hubbard , X86 ML , Matthew Wilcox , Linus Torvalds , Andrew Morton , kernel list , Linux-MM , Andrea Arcangeli , "Kirill A . Shutemov" , jroedel@suse.de, ubizjak@gmail.com Content-Transfer-Encoding: quoted-printable Message-Id: References: <20221022111403.531902164@infradead.org> <20221022114424.515572025@infradead.org> <2c800ed1-d17a-def4-39e1-09281ee78d05@nvidia.com> To: Peter Zijlstra X-Mailer: Apple Mail (2.3696.120.41.1.1) ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=PRu7uBQe; spf=pass (imf05.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.221.52 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666813406; a=rsa-sha256; cv=none; b=1b2EFYPG6K/fFlJj2zn45dvTCaBZ9KmTER9efIS8W8MNmOFPy1EqCZJylez4rVhb87wQ+m bHsBaFboCo2ER7UFKKjlWsFngQlvYqhQLDmoKTJjQyBbBY7bsBOOLAxtqPSnZeN515bd9O PFKqn2/CGk84Vif7jW8dlalo6DDktyw= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666813406; 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=SaQg4HnVfQQh2mYdE59PhhkkYlSdwKzsH9fh8OFcl80=; b=na8cl3eg5x4KAExr7ynJkxxgLi2woBzvzHMxIMvfUbbw5+KPco/iEnA4hkpP4IhbmCKqeU lJiI2dv4vsek6EXvpk7PYUvmNk7mwAOjh89d1R4HhQ0qYyep5gwF4gdaQwkYB/JNgDm17O Ucil9/YzYm/rVbFQJF+yKrbPqxmxDsE= X-Rspamd-Queue-Id: 79B7F100003 Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=PRu7uBQe; spf=pass (imf05.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.221.52 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Server: rspam02 X-Rspam-User: X-Stat-Signature: fhikwdorh1bkioj9yipa8mb8nx9dgk56 X-HE-Tag: 1666813406-585973 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 Oct 25, 2022, at 6:06 PM, Peter Zijlstra = wrote: > if (!force_flush && !tlb->fullmm && details && > + details->zap_flags & ZAP_FLAG_FORCE_FLUSH) > + force_flush =3D 1; Isn=E2=80=99t it too big of a hammer? At the same time, the whole reasoning about TLB flushes is not getting = any simpler. We had cases in which MADV_DONTNEED and another concurrent operation that effectively zapped PTEs (e.g., another MADV_DONTNEED) = caused the zap_pte_range() to skip entries since pte_none() was true. To = resolve these cases we relied on tlb_finish_mmu() to flush the range when needed (i.e., flush the whole range when mm_tlb_flush_nested()). Now, I do not have a specific broken scenario in mind following this = change, but it is all sounds to me a bit dangerous and at same time can = potentially introduce new overheads. One alternative may be using mm_tlb_flush_pending() when setting a new = PTE to check for pending flushes and flushing the TLB if that is the case. = This is somewhat similar to what ptep_clear_flush() does. Anyhow, I guess = this might induce some overheads. As noted before, it is possible to track pending TLB flushes in VMA/page-table granularity, with different = tradeoffs of overheads.