From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f72.google.com (mail-it0-f72.google.com [209.85.214.72]) by kanga.kvack.org (Postfix) with ESMTP id 826DC6B0279 for ; Thu, 1 Jun 2017 10:38:48 -0400 (EDT) Received: by mail-it0-f72.google.com with SMTP id r63so43276071itc.2 for ; Thu, 01 Jun 2017 07:38:48 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id y127si20939781itc.54.2017.06.01.07.38.47 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Jun 2017 07:38:47 -0700 (PDT) Received: from mail-ua0-f182.google.com (mail-ua0-f182.google.com [209.85.217.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E11CB23A0C for ; Thu, 1 Jun 2017 14:38:46 +0000 (UTC) Received: by mail-ua0-f182.google.com with SMTP id y4so28599640uay.2 for ; Thu, 01 Jun 2017 07:38:46 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20170601143344.GA3961@redhat.com> References: <20170531150349.4816-1-jglisse@redhat.com> <20170601143344.GA3961@redhat.com> From: Andy Lutomirski Date: Thu, 1 Jun 2017 07:38:25 -0700 Message-ID: Subject: Re: [PATCH] x86/mm: do not BUG_ON() on stall pgd entries Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Jerome Glisse Cc: Andy Lutomirski , "linux-mm@kvack.org" , Ingo Molnar , "Kirill A . Shutemov" On Thu, Jun 1, 2017 at 7:33 AM, Jerome Glisse wrote: > On Thu, Jun 01, 2017 at 06:59:04AM -0700, Andy Lutomirski wrote: >> On Wed, May 31, 2017 at 8:03 AM, J=C3=A9r=C3=B4me Glisse wrote: >> > Since af2cf278ef4f ("Don't remove PGD entries in remove_pagetable()") >> > we no longer cleanup stall pgd entries and thus the BUG_ON() inside >> > sync_global_pgds() is wrong. >> > >> > This patch remove the BUG_ON() and unconditionaly update stall pgd >> > entries. >> > >> > Signed-off-by: J=C3=A9r=C3=B4me Glisse >> > Cc: Ingo Molnar >> > Cc: Kirill A. Shutemov >> > --- >> > arch/x86/mm/init_64.c | 7 +------ >> > 1 file changed, 1 insertion(+), 6 deletions(-) >> > >> > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c >> > index ff95fe8..36b9020 100644 >> > --- a/arch/x86/mm/init_64.c >> > +++ b/arch/x86/mm/init_64.c >> > @@ -123,12 +123,7 @@ void sync_global_pgds(unsigned long start, unsign= ed long end) >> > pgt_lock =3D &pgd_page_get_mm(page)->page_tabl= e_lock; >> > spin_lock(pgt_lock); >> > >> > - if (!p4d_none(*p4d_ref) && !p4d_none(*p4d)) >> > - BUG_ON(p4d_page_vaddr(*p4d) >> > - !=3D p4d_page_vaddr(*p4d_ref)); >> > - >> > - if (p4d_none(*p4d)) >> > - set_p4d(p4d, *p4d_ref); >> > + set_p4d(p4d, *p4d_ref); >> >> If we have a mismatch in the vmalloc range, vmalloc_fault is going to >> screw up and we'll end up using incorrect page tables. >> >> What's causing the mismatch? If you're hitting this BUG in practice, >> I suspect we have a bug elsewhere. > > No bug elsewhere, simply hotplug memory then hotremove same memory you > just hotplugged then hotplug it again and you will trigger this as on > the first hotplug we allocate p4d/pud for the struct pages area, then on > hot remove we free that memory and clear the p4d/pud in the mm_init pgd > but not in any of the other pgds. That sounds like a bug to me. Either we should remove the stale entries and fix all the attendant races, or we should unconditionally allocate second-highest-level kernel page tables in unremovable memory and never free them. I prefer the latter even though it's slightly slower. > So at that point the next hotplug > will trigger the BUG because of stall entries from the first hotplug. By the time we have a pgd with an entry pointing off into the woods, we've already lost. Removing the BUG just hides the problem. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org