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 0ECFCC38A2D for ; Mon, 24 Oct 2022 21:00:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233345AbiJXVAd (ORCPT ); Mon, 24 Oct 2022 17:00:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57324 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233073AbiJXU7h (ORCPT ); Mon, 24 Oct 2022 16:59:37 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 19AEE244C59 for ; Mon, 24 Oct 2022 12:06:07 -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=t9FHfeIcloVdmFbUJWpclGjePHUA/HlGyPuL3wK+kr0=; b=tCmUJuvA77NpSI0qj14vZyfAqC XkRqamylOY2ncbRiW/TvVx3cmRphmaPB9fzoLsV5xqRkIWJxnzVjLY8fKwe4rKDCtIhJ4IxpQyf54 fpZZ31JcJYxZNTcWN2NXjbVnCdhRXQOBSedBlYztVQmLQe+nHU8/wTHTTy5CchnforEZSGLVanqGe pXFOj/SQXIqSUIaGIOnIKQDv1P4/nYA8F5iEtaDSgET1K7oaCjk/inmHoeKok+qrfQSOrKI04BxNq YNGRdOdmafgr89NVU+KhpKmKKV78Dne39j/ZO1sbfdJxEq6RrWZ3vYE+6VXPv1Fw+77QHIq6ZxcQn v3PYj9HQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1on22A-00Fdca-8R; Mon, 24 Oct 2022 18:18:46 +0000 Date: Mon, 24 Oct 2022 19:18:46 +0100 From: Matthew Wilcox To: Ira Weiny Cc: Andrew Morton , Randy Dunlap , Peter Xu , Andrea Arcangeli , kernel test robot , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH for rc] mm/shmem: Ensure proper fallback if page faults Message-ID: References: <20221024043305.1491403-1-ira.weiny@intel.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-kernel@vger.kernel.org On Mon, Oct 24, 2022 at 09:54:30AM -0700, Ira Weiny wrote: > On Sun, Oct 23, 2022 at 09:33:05PM -0700, Ira wrote: > > From: Ira Weiny > > > > The kernel test robot flagged a recursive lock as a result of a > > conversion from kmap_atomic() to kmap_local_folio()[Link] > > > > The cause was due to the code depending on the kmap_atomic() side effect > > of disabling page faults. In that case the code expects the fault to > > fail and take the fallback case. > > > > git archaeology implied that the recursion may not be an actual bug.[1] > > However, the mmap_lock needed in the fault may be the one held.[2] > > > > Add an explicit pagefault_disable() and a big comment to explain this > > for future souls looking at this code. > > > > [1] https://lore.kernel.org/all/Y1MymJ%2FINb45AdaY@iweiny-desk3/ > > [2] https://lore.kernel.org/all/Y1M2p9OtBGnKwGUE@x1n/ > > > > Fixes: 7a7256d5f512 ("shmem: convert shmem_mfill_atomic_pte() to use a folio") > > Cc: Andrew Morton > > Cc: Randy Dunlap > > Cc: Peter Xu > > Cc: Andrea Arcangeli > > Reported-by: Matthew Wilcox (Oracle) > > Reported-by: kernel test robot > > Link: https://lore.kernel.org/r/202210211215.9dc6efb5-yujie.liu@intel.com > > Signed-off-by: Ira Weiny > > > > --- > > Thanks to Matt and Andrew for initial diagnosis. > > Thanks to Randy for pointing out C code needs ';' :-D > > Thanks to Andrew for suggesting an elaborate comment > > Thanks to Peter for pointing out that the mm's may be the same. > > --- > > mm/shmem.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/mm/shmem.c b/mm/shmem.c > > index 8280a5cb48df..c1bca31cd485 100644 > > --- a/mm/shmem.c > > +++ b/mm/shmem.c > > @@ -2424,9 +2424,16 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm, > > > > if (!zeropage) { /* COPY */ > > page_kaddr = kmap_local_folio(folio, 0); > > + /* > > + * The mmap_lock is held here. Disable page faults to > > + * prevent deadlock should copy_from_user() fault. The > > + * copy will be retried outside the mmap_lock. > > + */ > > Offline Dave Hansen and I were discussing this and he was concerned that this > comment implies that a deadlock would always occur rather than might occur. > > I was not clear on this as I was thinking the read mmap_lock was non-recursive. > > So I think we have 3 cases only 1 of which will actually deadlock and is, as > Dave puts it, currently theoretical. > > 1) Different mm's are in play (no issue) > 2) Readlock implementation is recursive and same mm is in play (no issue) > 3) Readlock implementation is _not_ recursive (issue) > > In both 1 and 2 lockdep is incorrectly flagging the issue but 3 is a problem > and I think this is what Andrea was thinking. The readlock implementation is only recursive if nobody else has taken a write lock. AIUI, no other process can take a write lock on the mmap_lock (other processes can take read locks by examining /proc/$pid/maps, for example), although maybe ptrace can take the mmap_lock for write? But if you have a multithreaded process, one of the other threads can call mmap() and that will prevent recursion (due to fairness). Even if it's a different process that you're trying to acquire the mmap read lock on, you can still get into a deadly embrace. eg: process A thread 1 takes read lock on own mmap_lock process A thread 2 calls mmap, blocks taking write lock process B thread 1 takes page fault, read lock on own mmap lock process B thread 2 calls mmap, blocks taking write lock process A thread 1 blocks taking read lock on process B process B thread 1 blocks taking read lock on process A Now all four threads are blocked waiting for each other.