Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753969AbbHFERI (ORCPT ); Thu, 6 Aug 2015 00:17:08 -0400 Received: from mail-pa0-f45.google.com ([209.85.220.45]:36585 "EHLO mail-pa0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751230AbbHFERG (ORCPT ); Thu, 6 Aug 2015 00:17:06 -0400 Date: Wed, 5 Aug 2015 21:15:57 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: "Kirill A. Shutemov" cc: Andrew Morton , Andrea Arcangeli , David Rientjes , Hugh Dickins , Dave Hansen , Mel Gorman , Rik van Riel , Vlastimil Babka , Christoph Lameter , Naoya Horiguchi , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , Jerome Marchand , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: page-flags behavior on compound pages: a worry In-Reply-To: <1426784902-125149-5-git-send-email-kirill.shutemov@linux.intel.com> Message-ID: References: <1426784902-125149-1-git-send-email-kirill.shutemov@linux.intel.com> <1426784902-125149-5-git-send-email-kirill.shutemov@linux.intel.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3599 Lines: 81 Hi Kirill, I had a nasty thought this morning. Andrew had prodded me gently to re-examine my concerns with your page-flags rework in mmotm. I still dislike the bloat (my mm/built-in.o text goes up from 478513 to 490183 bytes on a non-DEBUG_VM build); but I was hoping to set that aside, to let us move forward. But looking into the bloat led me to what seems a more serious issue with it. I'd tacked a little function on to the end of mm/filemap.c: bool page_is_locked(struct page *page) { return !!PageLocked(page); } which came out as: 0000000000003a60 : 3a60: 48 8b 07 mov (%rdi),%rax 3a63: 55 push %rbp 3a64: 48 89 e5 mov %rsp,%rbp [instructions above same as without your patches; those below added by them] 3a67: f6 c4 80 test $0x80,%ah 3a6a: 74 10 je 3a7c 3a6c: 48 8b 47 30 mov 0x30(%rdi),%rax 3a70: 48 8b 17 mov (%rdi),%rdx 3a73: 80 e6 80 and $0x80,%dh 3a76: 48 0f 44 c7 cmove %rdi,%rax 3a7a: eb 03 jmp 3a7f 3a7c: 48 89 f8 mov %rdi,%rax 3a7f: 48 8b 00 mov (%rax),%rax [instructions above added by your patches; those below same as before] 3a82: 5d pop %rbp 3a83: 83 e0 01 and $0x1,%eax 3a86: c3 retq The "and $0x80,%dh" looked superfluous at first, but of course it isn't: it's from the smp_rmb() in David's 668f9abbd433 "mm: close PageTail race" (a later commit refactors compound_head() but doesn't change the story). And it's that race, or a worse race of that kind, that now worries me. Relying on smp_wmb() and smp_rmb() may be all that was needed in the case that David was fixing; and (I dare not look at them to audit!) all uses of compound_head() in our current v4.2-rc tree may well be safe, for this or that contingent reason in each place that it's used. But there is no locking within compound_head(page) to make it safe everywhere, yet your page-flags rework is changing a large number of PageWhatever()s and SetPageWhatever()s and ClearPageWhatever()s now to do a hidden compound_head(page) beneath the covers. To be more specific: if preemption, or an interrupt, or entry to SMM mode, or whatever, delays this thread somewhere in that compound_head() sequence of instructions, how can we be sure that the "head" returned by compound_head() is good? We know the page was PageTail just before looking up page->first_page, and we know it was PageTail just after, but we don't know that it was PageTail throughout, and we don't know whether page->first_page is even a good page pointer, or something else from the private/ptl/slab_cache union. Of course it would be very rare for it to go wrong; and most callsites will obviously be safe for this or that reason; though, sadly, none of them safe from holding a reference to the tail page in question, since its count is frozen at 0 and cannot be grabbed by get_page_unless_zero. But I don't see how it can be safe to rely on compound_head() inside a general purpose page-flag function, that we're all accustomed to think of as a simple bitop, that can be applied without great care. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/