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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C7279C32793 for ; Wed, 18 Jan 2023 13:15:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229518AbjARNPf (ORCPT ); Wed, 18 Jan 2023 08:15:35 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46972 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231391AbjARNPI (ORCPT ); Wed, 18 Jan 2023 08:15:08 -0500 Received: from mail-io1-xd36.google.com (mail-io1-xd36.google.com [IPv6:2607:f8b0:4864:20::d36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3B16C4740A for ; Wed, 18 Jan 2023 04:38:41 -0800 (PST) Received: by mail-io1-xd36.google.com with SMTP id z194so4531938iof.10 for ; Wed, 18 Jan 2023 04:38:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.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=ne1ZZ8g0CKjtUM0flXLQwaKYgCBQx784VijV19d+6xc=; b=rheJ1RUUcAkj+1owhrmAToGN1JKqVwS5uNfucsbLgdY6jiwLnv20ZvMr/zG5UmC3MR r6w4F2M4P+L39RFGiPrp7Pc4YsCSMo2eypVoYy7uAZH0skF0cb9KGqSqKmQtTROWTdZw Y4onbS8Bh9xurv/ecxH2l9+6u4n2k+Du7eEvZ+3bilOaoZOJlQlDDMN5a0ev1/1AkMIv 6hGk1h8D1872yYIqM9YnRK4SbQiqQUWcQ1xmAUvd1tSMlevfjfCKbIpNjzx92FXNNtTK eamrPPtqzoYNgbP3fcy329YWOqfU15+ST4GR/7gu/oa+wU+FfArYkk5BOsi4UvDxXYDv dylg== 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=ne1ZZ8g0CKjtUM0flXLQwaKYgCBQx784VijV19d+6xc=; b=daJJlPddh3G3ld7NBmk8eVVlt5wu/DINT/X3SCBhlWO1wPTazzGT/wrfxYNhKexBKn pxPk6MK9c41uRd4Aj8SsgMy8rbrn39onUgMUPURyuhc8bzkEUGkFiDY7cuW0xICqf9Dx bNNg5mttgWVf8hSrVnDI6svP3UUegAs/8wNjBy2vvNagYbRYchqY9KL6WAiX5N5poqsZ 2k3Lz3dx9d1B4ZVoL7aCvw47+gIc4vtHs+zjgGTr/VofNn2QJ9aUR4sQzLx6AHL8ZeYY ZwWcEWqWE915dF2EZji0EWTWYrwAzs0mL6oJc7XOXHJPpWriK9YWZeoROiX3ia8ZMow3 nskQ== X-Gm-Message-State: AFqh2kr3f/WU+SUhX5pKCfF8Y9wjPf6tb/dTxV1kfT/e/LS9TBzzRGQH Drgzb1CMak/5g4dBoIg7CmpA2fvr/lobjdQZRhsg2A== X-Google-Smtp-Source: AMrXdXtdPpTPNrN7OiYOv16BmNfqKkmp2Nm/DssJpSsitfPp+gX8QsWWZQOd27RnCdytJ0z4l4VIPkrlODp7fq/3+OE= X-Received: by 2002:a02:cb45:0:b0:39e:6dd8:6c96 with SMTP id k5-20020a02cb45000000b0039e6dd86c96mr543739jap.246.1674045520432; Wed, 18 Jan 2023 04:38:40 -0800 (PST) MIME-Version: 1.0 References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-19-surenb@google.com> In-Reply-To: From: Jann Horn Date: Wed, 18 Jan 2023 13:38:03 +0100 Message-ID: Subject: Re: [PATCH 18/41] mm/khugepaged: write-lock VMA while collapsing a huge page To: Michal Hocko Cc: Suren Baghdasaryan , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 18, 2023 at 10:40 AM Michal Hocko wrote: > On Tue 17-01-23 21:28:06, Jann Horn wrote: > > On Tue, Jan 17, 2023 at 4:25 PM Michal Hocko wrote: > > > On Mon 09-01-23 12:53:13, Suren Baghdasaryan wrote: > > > > Protect VMA from concurrent page fault handler while collapsing a huge > > > > page. Page fault handler needs a stable PMD to use PTL and relies on > > > > per-VMA lock to prevent concurrent PMD changes. pmdp_collapse_flush(), > > > > set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will > > > > not be detected by a page fault handler without proper locking. > > > > > > I am struggling with this changelog. Maybe because my recollection of > > > the THP collapsing subtleties is weak. But aren't you just trying to say > > > that the current #PF handling and THP collapsing need to be mutually > > > exclusive currently so in order to keep that assumption you have mark > > > the vma write locked? > > > > > > Also it is not really clear to me how that handles other vmas which can > > > share the same thp? > > > > It's not about the hugepage itself, it's about how the THP collapse > > operation frees page tables. > > > > Before this series, page tables can be walked under any one of the > > mmap lock, the mapping lock, and the anon_vma lock; so when khugepaged > > unlinks and frees page tables, it must ensure that all of those either > > are locked or don't exist. This series adds a fourth lock under which > > page tables can be traversed, and so khugepaged must also lock out that one. > > > > There is a codepath in khugepaged that iterates through all mappings > > of a file to zap page tables (retract_page_tables()), which locks each > > visited mm with mmap_write_trylock() and now also does > > vma_write_lock(). > > OK, I see. This would be a great addendum to the changelog. > > > I think one aspect of this patch that might cause trouble later on, if > > support for non-anonymous VMAs is added, is that retract_page_tables() > > now does vma_write_lock() while holding the mapping lock; the page > > fault handling path would probably take the locks the other way > > around, leading to a deadlock? So the vma_write_lock() in > > retract_page_tables() might have to become a trylock later on. > > This, right? > #PF retract_page_tables > vma_read_lock > i_mmap_lock_write > i_mmap_lock_read > vma_write_lock > > > I might be missing something but I have only found huge_pmd_share to be > called from the #PF path. That one should be safe as it cannot be a > target for THP. Not that it would matter much because such a dependency > chain would be really subtle. Oops, yeah. Now that I'm looking closer I also don't see a path from the #PF path to i_mmap_lock_read. Sorry for sending you on a wild goose chase.