From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 01B836B004F for ; Tue, 15 Sep 2009 21:09:00 -0400 (EDT) Date: Wed, 16 Sep 2009 09:08:53 +0800 From: Wu Fengguang Subject: Re: [PATCH] hwpoison: fix uninitialized warning Message-ID: <20090916010853.GA11865@localhost> References: <20090916002329.GA8476@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Hugh Dickins Cc: Andi Kleen , Andrew Morton , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" List-ID: On Wed, Sep 16, 2009 at 08:51:06AM +0800, Hugh Dickins wrote: > On Wed, 16 Sep 2009, Wu Fengguang wrote: > > On Wed, Sep 16, 2009 at 05:19:07AM +0800, Hugh Dickins wrote: > > > Fix mmotm build warning, presumably also in linux-next: > > > mm/memory.c: In function `do_swap_page': > > > mm/memory.c:2498: warning: `pte' may be used uninitialized in this function > > > > > > Signed-off-by: Hugh Dickins > > > --- > > > I've only noticed this warning on one machine, the powerpc: certainly it > > > needs CONFIG_MIGRATION or CONFIG_MEMORY_FAILURE to see it, but I thought > > > I had one of those set on other machines - just musing in case it's being > > > masked elsewhere by some other bug... > > > The lines was introduced in this patch: > > > > entry = pte_to_swp_entry(orig_pte); > > - if (is_migration_entry(entry)) { > > - migration_entry_wait(mm, pmd, address); > > + if (unlikely(non_swap_entry(entry))) { > > + if (is_migration_entry(entry)) { > > + migration_entry_wait(mm, pmd, address); > > + } else if (is_hwpoison_entry(entry)) { > > + ret = VM_FAULT_HWPOISON; > > + } else { > > + print_bad_pte(vma, address, pte, NULL); > > + ret = VM_FAULT_OOM; > > + } > > goto out; > > } > > > > Given that currently there are only two types of non swap entries: > > migration/hwpoison, the last 'else' block is in fact dead code.. > > Ah, yes, I think it is dead code on x86 (32 and 64), where the > swp_entry_t is well packed. But not dead code on ppc64, which has > > #define __swp_type(entry) (((entry).val >> 1) & 0x3f) > > which is allowing swap types up to 63, when in fact the highest > we use is 31: that leaves space for 32 more non_swap_entry types. > > So the compiler was absolutely right to complain about the > uninitialized variable on ppc64, but not on x86. It's a little > surprising that ppc64 allows 64 swap types, but nothing wrong. Ah I know. It seems that gcc is smart enough to remove that dead code and hence the warning message in x86 :) Thanks, Fengguang -- 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