From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753836AbbJ0JbB (ORCPT ); Tue, 27 Oct 2015 05:31:01 -0400 Received: from mail-lb0-f182.google.com ([209.85.217.182]:34715 "EHLO mail-lb0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752101AbbJ0Ja6 (ORCPT ); Tue, 27 Oct 2015 05:30:58 -0400 Date: Tue, 27 Oct 2015 11:30:55 +0200 From: "Kirill A. Shutemov" To: Naoya Horiguchi Cc: "Kirill A. Shutemov" , Andrew Morton , Andrea Arcangeli , Hugh Dickins , Dave Hansen , Mel Gorman , Rik van Riel , Vlastimil Babka , Christoph Lameter , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , Jerome Marchand , Sasha Levin , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: Re: [PATCHv12 26/37] mm: rework mapcount accounting to enable 4k mapping of THPs Message-ID: <20151027093055.GA27031@node.shutemov.name> References: <1444145044-72349-1-git-send-email-kirill.shutemov@linux.intel.com> <1444145044-72349-27-git-send-email-kirill.shutemov@linux.intel.com> <20151027061800.GA336@hori1.linux.bs1.fc.nec.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151027061800.GA336@hori1.linux.bs1.fc.nec.co.jp> 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 On Tue, Oct 27, 2015 at 06:18:01AM +0000, Naoya Horiguchi wrote: > On Tue, Oct 06, 2015 at 06:23:53PM +0300, Kirill A. Shutemov wrote: > > We're going to allow mapping of individual 4k pages of THP compound. > > It means we need to track mapcount on per small page basis. > > > > Straight-forward approach is to use ->_mapcount in all subpages to track > > how many time this subpage is mapped with PMDs or PTEs combined. But > > this is rather expensive: mapping or unmapping of a THP page with PMD > > would require HPAGE_PMD_NR atomic operations instead of single we have > > now. > > > > The idea is to store separately how many times the page was mapped as > > whole -- compound_mapcount. This frees up ->_mapcount in subpages to > > track PTE mapcount. > > > > We use the same approach as with compound page destructor and compound > > order to store compound_mapcount: use space in first tail page, > > ->mapping this time. > > > > Any time we map/unmap whole compound page (THP or hugetlb) -- we > > increment/decrement compound_mapcount. When we map part of compound page > > with PTE we operate on ->_mapcount of the subpage. > > > > page_mapcount() counts both: PTE and PMD mappings of the page. > > > > Basically, we have mapcount for a subpage spread over two counters. > > It makes tricky to detect when last mapcount for a page goes away. > > > > We introduced PageDoubleMap() for this. When we split THP PMD for the > > first time and there's other PMD mapping left we offset up ->_mapcount > > in all subpages by one and set PG_double_map on the compound page. > > These additional references go away with last compound_mapcount. > > > > This approach provides a way to detect when last mapcount goes away on > > per small page basis without introducing new overhead for most common > > cases. > > > > Signed-off-by: Kirill A. Shutemov > > Tested-by: Aneesh Kumar K.V > > Acked-by: Jerome Marchand > > I found that recent mmotm hit the following BUG_ON() by reading > /proc/kpageflags over pfn backed by a thp. > > [ 268.024519] page:ffffea00033e0000 count:0 mapcount:0 mapping: (null) index:0x700000200 > [ 268.026076] flags: 0x4000000000000000() > [ 268.026778] page dumped because: VM_BUG_ON_PAGE(!PageHead(page)) > [ 268.027816] page->mem_cgroup:ffff88021588cc00 > [ 268.028638] ------------[ cut here ]------------ > [ 268.029932] kernel BUG at /src/linux-dev/include/linux/page-flags.h:552! > [ 268.031092] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC > [ 268.032125] Modules linked in: cfg80211 rfkill crc32c_intel virtio_balloon serio_raw i2c_piix4 virtio_blk virtio_net ata_generic pata_acpi > [ 268.032598] CPU: 0 PID: 1183 Comm: page-types Not tainted 4.2.0-mmotm-2015-10-21-14-41-151027-1418-00014-41+ #179 > [ 268.032598] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 > [ 268.032598] task: ffff880214a08bc0 ti: ffff880213e2c000 task.ti: ffff880213e2c000 > [ 268.032598] RIP: 0010:[] [] stable_page_flags+0x336/0x340 > [ 268.032598] RSP: 0018:ffff880213e2fda8 EFLAGS: 00010292 > [ 268.032598] RAX: 0000000000000021 RBX: ffff8802150a39c0 RCX: 0000000000000000 > [ 268.032598] RDX: ffff88021ec0ff38 RSI: ffff88021ec0d658 RDI: ffff88021ec0d658 > [ 268.032598] RBP: ffff880213e2fdc8 R08: 000000000000000a R09: 000000000000132f > [ 268.032598] R10: 0000000000000000 R11: 000000000000132f R12: 4000000000000000 > [ 268.032598] R13: ffffea00033e6340 R14: 00007fff8449e430 R15: ffffea00033e6340 > [ 268.032598] FS: 00007ff7f9525700(0000) GS:ffff88021ec00000(0000) knlGS:0000000000000000 > [ 268.032598] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 268.032598] CR2: 000000000063b800 CR3: 00000000d9e71000 CR4: 00000000000006f0 > [ 268.032598] Stack: > [ 268.032598] ffff8800db82df80 ffff8802150a39c0 0000000000000008 00000000000cf98d > [ 268.032598] ffff880213e2fe18 ffffffff81243588 00007fff8449e430 ffff880213e2ff20 > [ 268.032598] 000000000063b800 ffff8802150a39c0 fffffffffffffffb ffff880213e2ff20 > [ 268.032598] Call Trace: > [ 268.032598] [] kpageflags_read+0xc8/0x130 > [ 268.032598] [] proc_reg_read+0x48/0x70 > [ 268.032598] [] __vfs_read+0x28/0xd0 > [ 268.032598] [] ? security_file_permission+0xae/0xc0 > [ 268.032598] [] ? rw_verify_area+0x53/0xf0 > [ 268.032598] [] vfs_read+0x8a/0x130 > [ 268.032598] [] SyS_pread64+0x77/0x90 > [ 268.032598] [] entry_SYSCALL_64_fastpath+0x12/0x6a > [ 268.032598] Code: ca 00 00 40 01 48 39 c1 48 0f 44 da e9 a2 fd ff ff 48 c7 c6 50 a6 a1 8 1 e8 58 ab f4 ff 0f 0b 48 c7 c6 90 a2 a1 81 e8 4a ab f4 ff <0f> 0b 0f 1f 84 00 00 00 00 00 66 66 66 66 90 55 48 89 e5 41 57 > [ 268.032598] RIP [] stable_page_flags+0x336/0x340 > [ 268.032598] RSP > [ 268.070504] ---[ end trace e5d18553088c026a ]--- > > page_mapcount() could be called for a tail page, so VM_BUG_ON_PAGE(!PageHead()) > in PageDoubleMap() introduced by this patch seems too strong restriction. Hm. page_mapcount() calls PageDoubleMap() only for head pages. > Could you handle this? Do you have a reproducer? -- Kirill A. Shutemov