From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 63ADBC4321E for ; Mon, 28 Nov 2022 20:10:17 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AAEDC6B0071; Mon, 28 Nov 2022 15:10:16 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A5ED36B0072; Mon, 28 Nov 2022 15:10:16 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9273E6B0073; Mon, 28 Nov 2022 15:10:16 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 7F6246B0071 for ; Mon, 28 Nov 2022 15:10:16 -0500 (EST) Received: from smtpin30.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 473DAAB31C for ; Mon, 28 Nov 2022 20:10:16 +0000 (UTC) X-FDA: 80183942832.30.46FA3EB Received: from mail-pj1-f52.google.com (mail-pj1-f52.google.com [209.85.216.52]) by imf20.hostedemail.com (Postfix) with ESMTP id E17A01C0011 for ; Mon, 28 Nov 2022 20:10:14 +0000 (UTC) Received: by mail-pj1-f52.google.com with SMTP id e7-20020a17090a77c700b00216928a3917so15140213pjs.4 for ; Mon, 28 Nov 2022 12:10:14 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=LrWhoa/Wn/Otd/OEE6O5+GU+sWhxvmWKhnQriMwolNQ=; b=Ft3K8vNpguhkxGVviUoZ0VbAZbXap64K+Z8vTx6lvjef85eskOiDN+PNPQx1ONGzBL XqMQxY1yaif+QgDSgxQDNdhTzf4M037bMP/ZQJD4NpyGc6/1wCeMKz1s+K5kMgWRhuZS xsvPM6oN1lkGBRggysG6A59OmQmKzNKqkZNm/ns+2+sGUfY3nrAcvx0ZI5hpmhQsdHiS Rv9fS3ySEubOBKkEpCt+pqKGeahw+ADG8Sk1AbOu1DNv7GapdB0MXcRiubbImtzs7g8E GzfXasmCzGfVhX5wBQjMGw0oUzgCyaygTUEajcbLlENdGdgR4b4mRcs6ZglvP+inVymB HuJg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=LrWhoa/Wn/Otd/OEE6O5+GU+sWhxvmWKhnQriMwolNQ=; b=z1Uf9kYbuRzLazLy5Gp5A9H91k4inM35CXdMx4OMmfL0wFWoeDAixUeG6p+2UkaY09 fEAzxRA3mgfHxBPqRgWuUHigofeVY+1m3ReJaVQu1Zxra9eBSP3P0bMyTwRhse0XaPgB 9WcQo3wp2xQI11ZGHbJ0x9JQFUfqUOQC2w0wIqvIS4RmLXNae4X0BJYgf1Yg1VxMhMaB v/YvzWN54KMk4RseVBhOMMBrOLYaZo1tndCRUPEPUUa1IO2IvcuQI7KunjWL557NirQk +bdMZ7qmtGCosxPjxu1nlqjexKAGEHh3XB1LIob+JuagifZNTYnFDRP3UcEPfm5LU6K+ 69og== X-Gm-Message-State: ANoB5pmEtcz76Qh8DK6QvN/ecd6Oy1Z24Fp35DNRvbGcr9ujS/mKh2om o4nW1s5unNgpPlGlp7YuQMsr5ERlkOFBjkWgkRA= X-Google-Smtp-Source: AA0mqf4PTB+8hgR9blsjPRLxL0XGfnedeyPd8FdLPLwJS2r6KI+2cktFVM5MgcPuatdnueCBv7xFW3X8VMSQpU7dvcs= X-Received: by 2002:a17:90a:4302:b0:20a:e469:dc7d with SMTP id q2-20020a17090a430200b0020ae469dc7dmr54086687pjg.97.1669666213792; Mon, 28 Nov 2022 12:10:13 -0800 (PST) MIME-Version: 1.0 References: <20221128180252.1684965-1-jannh@google.com> <20221128180252.1684965-2-jannh@google.com> In-Reply-To: From: Yang Shi Date: Mon, 28 Nov 2022 12:10:02 -0800 Message-ID: Subject: Re: [PATCH v4 2/3] mm/khugepaged: Fix GUP-fast interaction by sending IPI To: Jann Horn Cc: security@kernel.org, Andrew Morton , David Hildenbrand , Peter Xu , John Hubbard , linux-kernel@vger.kernel.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=Ft3K8vNp; spf=pass (imf20.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.52 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1669666214; a=rsa-sha256; cv=none; b=KlOIfu94AW4LS0jIuOEFyoJSFPnJUQqlQhf/qK69WWySnus9/sZH5tLPbfFo5ztBSMoJ4R zxMNG9dZolnwTuHTLTQEbdtWPFrK9DbI9SAGfkSsiTN73wtiAnTkS3LhpPf2YoPcf8iKuP z/feEyWt+M/1QoUBN3UEpmSuFE18DwU= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1669666214; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=LrWhoa/Wn/Otd/OEE6O5+GU+sWhxvmWKhnQriMwolNQ=; b=J/EmMb7Yke+yHonc7rCQHEvxSZc96a8tM9ACX5Y5rmV06q0TOCPXLxu73LcvWAo9iEpJhf FA/GXSKYoZEAvZu+9onSVscVHCWtLLadD12FyjnPlG8kUY3q+0qDWbQVoUPoZ6speaeCGK xLo05h5s1+ZG9AqN5m+NVIG7W9ChjBE= Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=Ft3K8vNp; spf=pass (imf20.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.52 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Server: rspam01 X-Stat-Signature: i6ghrkqax8z8t4n6oq6eazgm5j91x7ye X-Rspamd-Queue-Id: E17A01C0011 X-Rspam-User: X-HE-Tag: 1669666214-296615 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 Mon, Nov 28, 2022 at 11:57 AM Jann Horn wrote: > > On Mon, Nov 28, 2022 at 8:54 PM Yang Shi wrote: > > > > On Mon, Nov 28, 2022 at 10:03 AM Jann Horn wrote: > > > > > > Since commit 70cbc3cc78a99 ("mm: gup: fix the fast GUP race against THP > > > collapse"), the lockless_pages_from_mm() fastpath rechecks the pmd_t to > > > ensure that the page table was not removed by khugepaged in between. > > > > > > However, lockless_pages_from_mm() still requires that the page table is not > > > concurrently freed or reused to store non-PTE data. Otherwise, problems > > > can occur because: > > > > > > - deposited page tables can be freed when a THP page somewhere in the > > > mm is removed > > > - some architectures store non-PTE information inside deposited page > > > tables (see radix__pgtable_trans_huge_deposit()) > > > > > > Additionally, lockless_pages_from_mm() is also somewhat brittle with > > > regards to page tables being repeatedly moved back and forth, but > > > that shouldn't be an issue in practice. > > > > > > Fix it by sending IPIs (if the architecture uses > > > semi-RCU-style page table freeing) before freeing/reusing page tables. > > > > > > As noted in mm/gup.c, on configs that define CONFIG_HAVE_FAST_GUP, > > > there are two possible cases: > > > > > > 1. CONFIG_MMU_GATHER_RCU_TABLE_FREE is set, causing > > > tlb_remove_table_sync_one() to send an IPI to synchronize with > > > lockless_pages_from_mm(). > > > 2. CONFIG_MMU_GATHER_RCU_TABLE_FREE is unset, indicating that all > > > TLB flushes are already guaranteed to send IPIs. > > > tlb_remove_table_sync_one() will do nothing, but we've already > > > run pmdp_collapse_flush(), which did a TLB flush, which must have > > > involved IPIs. > > > > I'm trying to catch up with the discussion after the holiday break. I > > understand you switched from always allocating a new page table page > > (we decided before) to sending IPIs to serialize against fast-GUP, > > this is fine to me. > > > > So the code now looks like: > > pmdp_collapse_flush() > > sending IPI > > > > But the missing part is how we reached "TLB flushes are already > > guaranteed to send IPIs" when CONFIG_MMU_GATHER_RCU_TABLE_FREE is > > unset? ARM64 doesn't do it IIRC. Or did I miss something? > > From arch/arm64/Kconfig: > > select MMU_GATHER_RCU_TABLE_FREE > > CONFIG_MMU_GATHER_RCU_TABLE_FREE is not a config option that the user > can freely toggle; it is an option selected by the architecture. Aha, I see :-) BTW, shall we revert "mm: gup: fix the fast GUP race against THP collapse"? It seems not necessary anymore if this approach is used IIUC. Reviewed-by: Yang Shi