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 87E5CEB64D9 for ; Tue, 27 Jun 2023 15:50:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231964AbjF0Puv (ORCPT ); Tue, 27 Jun 2023 11:50:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56826 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231814AbjF0Puj (ORCPT ); Tue, 27 Jun 2023 11:50:39 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7CC982D4B for ; Tue, 27 Jun 2023 08:49:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1687880993; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=WWk97apVhyJHDonX03+ZEK7CdXKLu5yLLv7Ry7RwGo0=; b=QI+njb1EtRKxjAOKbJbDhAle9PDzUOjg7OF9vH98T+r/0+i4cv9ummNEcUupGq7uYX+BG+ TWoR6FZxsI0dt9KhAS8NrjFM+on7sWttTaPZFXMUewS+mKOIuEVToG2owXV4DoCeBqb1ZQ kXj8k7sO8G9kEwm6LXzN9A9znm6lt1Q= Received: from mail-qv1-f70.google.com (mail-qv1-f70.google.com [209.85.219.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-551-UzjC4oLTOlGm4vslpiCfFw-1; Tue, 27 Jun 2023 11:49:52 -0400 X-MC-Unique: UzjC4oLTOlGm4vslpiCfFw-1 Received: by mail-qv1-f70.google.com with SMTP id 6a1803df08f44-62dd79f63e0so8012296d6.0 for ; Tue, 27 Jun 2023 08:49:52 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1687880992; x=1690472992; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=WWk97apVhyJHDonX03+ZEK7CdXKLu5yLLv7Ry7RwGo0=; b=b6oPUhL1keyZ+zLqm7Z1PnKf8VtUDw1FUWtFdr4lN9kQcsmm2dYvDhfPfWKR+LIvvO zXIl+L90pwRvQaGmVzvEYVX7p6fi3/xJHsXeQ9Nso3T84YMB/xK6MeDHc3Ok3cWBPgnN V6ZZC9iHRX2Jw4yuRrZr9Ex4d83MSiD1F94ypvT0Jal3NqOREg7k011ShY4JLFvmlqky SCbTmQ3eRzxXbzA2HvHfr7LsR5u+1vS2cZwyvXl7pLWFK8o30WipBxDBCpJLKHLdA30t 3gW8DQ10u7N0HWefFbYB8eZivI4e7uawbfJtu9vvolUVutuKO6VFrNyKsziR/CNch+kA irZw== X-Gm-Message-State: AC+VfDwIX9EbMVyXvo8C1Ab4oli4XNDDg63EZgbvvFfrtik++DKq33qF NuBT3Kl66/lOpvAMCiY+KsSCXhu0cMM4aJQnMLJq+WwGtpalBzBqEh9NybHZZLq+QHNefnfRyfo lrh0XQHYFlK1/dxdk6j/CtUgHGw== X-Received: by 2002:a05:6214:5186:b0:62d:eceb:f7ce with SMTP id kl6-20020a056214518600b0062decebf7cemr22553575qvb.1.1687880991691; Tue, 27 Jun 2023 08:49:51 -0700 (PDT) X-Google-Smtp-Source: ACHHUZ6CRo+5Jo3T9LBKx4Xlv3zmaVI3QEZGMWrv6tzFG6sXb500w5+qcA4m6HHVWBCmVoQxsCYlOA== X-Received: by 2002:a05:6214:5186:b0:62d:eceb:f7ce with SMTP id kl6-20020a056214518600b0062decebf7cemr22553552qvb.1.1687880991370; Tue, 27 Jun 2023 08:49:51 -0700 (PDT) Received: from x1n (cpe5c7695f3aee0-cm5c7695f3aede.cpe.net.cable.rogers.com. [99.254.144.39]) by smtp.gmail.com with ESMTPSA id g25-20020a37e219000000b0076224a4884asm4086568qki.35.2023.06.27.08.49.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Jun 2023 08:49:51 -0700 (PDT) Date: Tue, 27 Jun 2023 11:49:43 -0400 From: Peter Xu To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, willy@infradead.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, ying.huang@intel.com, david@redhat.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 v3 7/8] mm: drop VMA lock before waiting for migration Message-ID: References: <20230627042321.1763765-1-surenb@google.com> <20230627042321.1763765-8-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230627042321.1763765-8-surenb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Jun 26, 2023 at 09:23:20PM -0700, Suren Baghdasaryan wrote: > migration_entry_wait does not need VMA lock, therefore it can be > dropped before waiting. Hmm, I'm not sure.. Note that we're still dereferencing *vmf->pmd when waiting, while *pmd is on the page table and IIUC only be guaranteed if the vma is still there. If without both mmap / vma lock I don't see what makes sure the pgtable is always there. E.g. IIUC a race can happen where unmap() runs right after vma_end_read() below but before pmdp_get_lockless() (inside migration_entry_wait()), then pmdp_get_lockless() can read some random things if the pgtable is freed. > > Signed-off-by: Suren Baghdasaryan > --- > mm/memory.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 5caaa4c66ea2..bdf46fdc58d6 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3715,8 +3715,18 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > entry = pte_to_swp_entry(vmf->orig_pte); > if (unlikely(non_swap_entry(entry))) { > if (is_migration_entry(entry)) { > - migration_entry_wait(vma->vm_mm, vmf->pmd, > - vmf->address); > + /* Save mm in case VMA lock is dropped */ > + struct mm_struct *mm = vma->vm_mm; > + > + if (vmf->flags & FAULT_FLAG_VMA_LOCK) { > + /* > + * No need to hold VMA lock for migration. > + * WARNING: vma can't be used after this! > + */ > + vma_end_read(vma); > + ret |= VM_FAULT_COMPLETED; > + } > + migration_entry_wait(mm, vmf->pmd, vmf->address); > } else if (is_device_exclusive_entry(entry)) { > vmf->page = pfn_swap_entry_to_page(entry); > ret = remove_device_exclusive_entry(vmf); > -- > 2.41.0.178.g377b9f9a00-goog > -- Peter Xu