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 8B08DC001B0 for ; Thu, 10 Aug 2023 22:16:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230419AbjHJWQs (ORCPT ); Thu, 10 Aug 2023 18:16:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56322 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229539AbjHJWQr (ORCPT ); Thu, 10 Aug 2023 18:16:47 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 27D9A2717; Thu, 10 Aug 2023 15:16:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=kZPovxlvK/roVyxwyrIHXjZdgR6OfruTG8lx5laTMjE=; b=Wu9JD/vLovqSrk7O9Ir9t5qFsz B7QVo0bG4GprbraPb6elzFPJ3NE/0hGrj9rgwFwVX0NDoTwdn5+NFUTNBSqdhXrTeSAegMaRIfYS3 sBlOmjAdS7vP+kUu7bSDI8NMwxfhWi0JJ6PeN1v0CCn1P9iRRp1bNjkboCR6gCltj0pWY2RPXoqir tzEbcfCPZm/LCQ50fGWMeFvYFV7mNpl1EQZOQC1XHuGqWbUk4gkZ7IYoX3rgb94KtusmIUVxgv1kp 30OyN/78yXUBl3imywAb+maKp42gUxHzQxhb0MVHJ0rrbZWzqjQOSRMDTeHddGq8L6aUWWwHyjyhy RMI8I8aQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qUDwr-00F0qE-VO; Thu, 10 Aug 2023 22:16:06 +0000 Date: Thu, 10 Aug 2023 23:16:05 +0100 From: Matthew Wilcox To: Suren Baghdasaryan Cc: David Hildenbrand , akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz, minchan@google.com, dave@stgolabs.net, punit.agrawal@bytedance.com, lstoakes@gmail.com, hdanton@sina.com, apopple@nvidia.com, peterx@redhat.com, ying.huang@intel.com, yuzhao@google.com, dhowells@redhat.com, hughd@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, pasha.tatashin@soleen.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults Message-ID: References: <20230630211957.1341547-1-surenb@google.com> <0ab6524a-6917-efe2-de69-f07fb5cdd9d2@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Aug 10, 2023 at 06:24:15AM +0000, Suren Baghdasaryan wrote: > Ok, I think I found the issue. wp_page_shared() -> > fault_dirty_shared_page() can drop mmap_lock (see the comment saying > "Drop the mmap_lock before waiting on IO, if we can...", therefore we > have to ensure we are not doing this under per-VMA lock. ... or we could change maybe_unlock_mmap_for_io() the same way that we changed folio_lock_or_retry(): +++ b/mm/internal.h @@ -706,7 +706,7 @@ static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf, if (fault_flag_allow_retry_first(flags) && !(flags & FAULT_FLAG_RETRY_NOWAIT)) { fpin = get_file(vmf->vma->vm_file); - mmap_read_unlock(vmf->vma->vm_mm); + release_fault_lock(vmf); } return fpin; } What do you think? > I think what happens is that this path is racing with another page > fault which took mmap_lock for read. fault_dirty_shared_page() > releases this lock which was taken by another page faulting thread and > that thread generates an assertion when it finds out the lock it just > took got released from under it. I'm confused that our debugging didn't catch this earlier. lockdep should always catch this.