From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f181.google.com (mail-ie0-f181.google.com [209.85.223.181]) by kanga.kvack.org (Postfix) with ESMTP id B505E6B0035 for ; Fri, 11 Jul 2014 18:27:35 -0400 (EDT) Received: by mail-ie0-f181.google.com with SMTP id rp18so1390375iec.26 for ; Fri, 11 Jul 2014 15:27:35 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id i4si5945279igj.19.2014.07.11.15.27.34 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 11 Jul 2014 15:27:34 -0700 (PDT) Date: Fri, 11 Jul 2014 15:27:32 -0700 From: Andrew Morton Subject: Re: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits Message-Id: <20140711152732.de78603744cd861497eca5dc@linux-foundation.org> In-Reply-To: <20140711081656.15654.19946.stgit@localhost.localdomain> References: <20140711081656.15654.19946.stgit@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Maxim Patlasov Cc: riel@redhat.com, linux-kernel@vger.kernel.org, mhocko@suse.cz, linux-mm@kvack.org, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, jweiner@redhat.com On Fri, 11 Jul 2014 12:18:27 +0400 Maxim Patlasov wrote: > Under memory pressure, it is possible for dirty_thresh, calculated by > global_dirty_limits() in balance_dirty_pages(), to equal zero. Under what circumstances? Really small values of vm_dirty_bytes? > Then, if > strictlimit is true, bdi_dirty_limits() tries to resolve the proportion: > > bdi_bg_thresh : bdi_thresh = background_thresh : dirty_thresh > > by dividing by zero. > > ... > > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -1306,9 +1306,9 @@ static inline void bdi_dirty_limits(struct backing_dev_info *bdi, > *bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh); > > if (bdi_bg_thresh) > - *bdi_bg_thresh = div_u64((u64)*bdi_thresh * > - background_thresh, > - dirty_thresh); > + *bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh * > + background_thresh, > + dirty_thresh) : 0; This introduces a peculiar discontinuity: if dirty_thresh==3, treat it as 3 if dirty_thresh==2, treat it as 2 if dirty_thresh==1, treat it as 1 if dirty_thresh==0, treat it as infinity Would it not make more sense to change global_dirty_limits() to convert 0 to 1? With an appropriate comment, obviously. Or maybe the fix lies elsewhere. Please do tell us how this zero comes about. -- 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