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[24.6.216.183]) by smtp.gmail.com with ESMTPSA id 79-20020a621452000000b0056b9df2a15esm1590688pfu.62.2022.10.29.14.03.47 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Sat, 29 Oct 2022 14:03:48 -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: Sat, 29 Oct 2022 14:03:46 -0700 Cc: Peter Zijlstra , Jann Horn , X86 ML , Matthew Wilcox , Andrew Morton , kernel list , Linux-MM , Andrea Arcangeli , "Kirill A . Shutemov" , jroedel@suse.de, ubizjak@gmail.com, Alistair Popple Content-Transfer-Encoding: quoted-printable Message-Id: <9880C4F0-FF13-46BA-9044-30949D155158@gmail.com> References: <20221022111403.531902164@infradead.org> <2c800ed1-d17a-def4-39e1-09281ee78d05@nvidia.com> <6C548A9A-3AF3-4EC1-B1E5-47A7FFBEB761@gmail.com> <47678198-C502-47E1-B7C8-8A12352CDA95@gmail.com> <3a57cfc5-5db4-bdc9-1ddf-5305a37ffa62@nvidia.com> To: Linus Torvalds , John Hubbard X-Mailer: Apple Mail (2.3696.120.41.1.1) ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1667077430; 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=y0XCKXWN9AOEQpnItiZWYYfOLH2B1OdimWu6qHPiTXU=; b=tPINmU1MFtnKqjlz5GHIzD32CHUe3DuKj2wCLVML17XI8B0+HKKbTIsdPdkeh3ZpeO3MPX w26xs01X4wY+5JTHLuH8AZCY7NKlBsOQZVUWdEZHl1YlqSJwZaiwmUcMoqoST/Q2v2gZAL x7hY5cyt8ppmH0/TZAPKwI1s9mIVCIo= ARC-Authentication-Results: i=1; imf03.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=MykEozAV; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf03.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.214.181 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1667077430; a=rsa-sha256; cv=none; b=waQXzNsE2lrasZEfn6JFeAj4j4ujHz9tB78kdQWtYxoHycf3ZNcxo5fvaNtTlI6hZ851Vt NIfwGD7t+xPsJly3hG+mUkK3oSQc9yDdrPBtNGr48T2QzITLwfHyNUDu0DxS2XVnIhk0x5 KUlhoDH6ZHI5wQX2jT8btz+sFcPzLhk= X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 4D3CD2002B X-Rspam-User: Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=MykEozAV; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf03.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.214.181 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com X-Stat-Signature: am7uoes554jk6nkexxwe7wrt8bq6b83j X-HE-Tag: 1667077430-881087 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 29, 2022, at 1:56 PM, Nadav Amit wrote: > On Oct 29, 2022, at 1:15 PM, Linus Torvalds = wrote: >=20 >> (b) we could move the "page_remove_rmap()" into the "flush-and-free" = path too >>=20 >> And (b) would be fairly easy - same model as that dirty bit patch, >> just a 'do page_remove_rmap too' - except page_remove_rmap() wants = the >> vma as well (and we delay the TLB flush over multiple vma's, so it's >> not just a "save vma in mmu_gather=E2=80=9D). >=20 > (b) sounds reasonable and may potentially allow future performance > improvements (batching, doing stuff without locks). >=20 > It does appear to break a potential hidden assumption that rmap is = removed > while the ptl is acquired (at least in the several instances I = samples). > Yet, anyhow page_vma_mapped_walk() checks the PTE before calling the > function, so it should be fine. >=20 > I=E2=80=99ll give it a try. I have just seen John=E2=80=99s and your emails. It seems (b) fell off. = (a) is out of my =E2=80=9Czone=E2=80=9D, and anyhow assuming it would not be solved = soon, deferring page_remove_rmap() might cause regressions. (c) might be more intrusive and potentially induce overheads. If we need = a small backportable solution, I think the approach that I proposed = (marking the page dirty after the invalidation, before the PTL is released) is = the simplest one. Please advise how to proceed.