Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753301AbcCALRG (ORCPT ); Tue, 1 Mar 2016 06:17:06 -0500 Received: from mail-wm0-f54.google.com ([74.125.82.54]:35113 "EHLO mail-wm0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753086AbcCALQ7 (ORCPT ); Tue, 1 Mar 2016 06:16:59 -0500 Date: Tue, 1 Mar 2016 14:16:56 +0300 From: "Kirill A. Shutemov" To: Hugh Dickins Cc: Andrew Morton , "Kirill A. Shutemov" , Joonsoo Kim , Sasha Levin , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: __delete_from_page_cache show Bad page if mapped Message-ID: <20160301111656.GA19095@node.shutemov.name> References: <20160229095216.GA9616@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1406 Lines: 28 On Mon, Feb 29, 2016 at 10:45:59PM -0800, Hugh Dickins wrote: > Commit e1534ae95004 ("mm: differentiate page_mapped() from page_mapcount() > for compound pages") changed the famous BUG_ON(page_mapped(page)) in > __delete_from_page_cache() to VM_BUG_ON_PAGE(page_mapped(page)): which > gives us more info when CONFIG_DEBUG_VM=y, but nothing at all when not. > > Although it has not usually been very helpul, being hit long after the > error in question, we do need to know if it actually happens on users' > systems; but reinstating a crash there is likely to be opposed :) > > In the non-debug case, pr_alert("BUG: Bad page cache") plus dump_page(), > dump_stack(), add_taint() - I don't really believe LOCKDEP_NOW_UNRELIABLE, > but that seems to be the standard procedure now. Move that, or the > VM_BUG_ON_PAGE(), up before the deletion from tree: so that the > unNULLified page->mapping gives a little more information. > > If the inode is being evicted (rather than truncated), it won't have > any vmas left, so it's safe(ish) to assume that the raised mapcount is > erroneous, and we can discount it from page_count to avoid leaking the > page (I'm less worried by leaking the occasional 4kB, than losing a > potential 2MB page with each 4kB page leaked). > > Signed-off-by: Hugh Dickins Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov