Message-ID: <1501941942.6577.7.camel@redhat.com>
Date:   Sat, 05 Aug 2017 10:05:42 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, fweimer@...hat.com, colm@...costs.net,
        akpm@...ux-foundation.org, rppt@...ux.vnet.ibm.com,
        keescook@...omium.org, luto@...capital.net, wad@...omium.org,
        mingo@...nel.org
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

On Fri, 2017-08-04 at 16:09 -0700, Mike Kravetz wrote:
> On 08/04/2017 12:07 PM, riel@...hat.com wrote:
> > From: Rik van Riel <riel@...hat.com>
> > 
> > Introduce MADV_WIPEONFORK semantics, which result in a VMA being
> > empty in the child process after fork. This differs from
> > MADV_DONTFORK
> > in one important way.
> > 
> > If a child process accesses memory that was MADV_WIPEONFORK, it
> > will get zeroes. The address ranges are still valid, they are just
> > empty.
> > 
> This didn't seem 'quite right' to me for shared mappings and/or file
> backed mappings.  I wasn't exactly sure what it 'should' do in such
> cases.  So, I tried it with a mapping created as follows:
> 
> addr = mmap(ADDR, page_size,
>                         PROT_READ | PROT_WRITE,
>                         MAP_ANONYMOUS|MAP_SHARED, -1, 0);

Your test program is pretty much the same I used, except I
used MAP_PRIVATE instead of MAP_SHARED.

Let me see how the code paths differ for both cases...


> When setting MADV_WIPEONFORK on the vma/mapping, I got the following
> at task exit time:
> 
> [  694.558290] ------------[ cut here ]------------
> [  694.558978] kernel BUG at mm/filemap.c:212!
> [  694.559476] invalid opcode: 0000 [#1] SMP
> [  694.560023] Modules linked in: ip6t_REJECT nf_reject_ipv6
> ip6t_rpfilter xt_conntrack ebtable_broute bridge stp llc ebtable_nat
> ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6
> ip6table_raw ip6table_mangle ip6table_security iptable_nat
> nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack
> iptable_raw iptable_mangle 9p iptable_security ebtable_filter
> ebtables ip6table_filter ip6_tables snd_hda_codec_generic
> snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_seq ppdev
> snd_seq_device joydev crct10dif_pclmul crc32_pclmul crc32c_intel
> snd_pcm ghash_clmulni_intel 9pnet_virtio virtio_balloon snd_timer
> 9pnet parport_pc snd parport i2c_piix4 soundcore nfsd auth_rpcgss
> nfs_acl lockd grace sunrpc virtio_net virtio_blk virtio_console
> 8139too qxl drm_kms_helper ttm drm serio_raw 8139cp
> [  694.571554]  mii virtio_pci ata_generic virtio_ring virtio
> pata_acpi
> [  694.572608] CPU: 3 PID: 1200 Comm: test_wipe2 Not tainted 4.13.0-
> rc3+ #8
> [  694.573778] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS 1.9.1-1.fc24 04/01/2014
> [  694.574917] task: ffff880137178040 task.stack: ffffc900019d4000
> [  694.575650] RIP: 0010:__delete_from_page_cache+0x344/0x410
> [  694.576409] RSP: 0018:ffffc900019d7a88 EFLAGS: 00010082
> [  694.577238] RAX: 0000000000000021 RBX: ffffea00047d0e00 RCX:
> 0000000000000006
> [  694.578537] RDX: 0000000000000000 RSI: 0000000000000096 RDI:
> ffff88023fd0db90
> [  694.579774] RBP: ffffc900019d7ad8 R08: 00000000000882b6 R09:
> 000000000000028a
> [  694.580754] R10: ffffc900019d7da8 R11: ffffffff8211184d R12:
> ffffea00047d0e00
> [  694.582040] R13: 0000000000000000 R14: 0000000000000202 R15:
> ffff8801384439e8
> [  694.583236] FS:  0000000000000000(0000) GS:ffff88023fd00000(0000)
> knlGS:0000000000000000
> [  694.584607] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  694.585409] CR2: 00007ff77a8da618 CR3: 0000000001e09000 CR4:
> 00000000001406e0
> [  694.586547] Call Trace:
> [  694.586996]  delete_from_page_cache+0x54/0x110
> [  694.587481]  truncate_inode_page+0xab/0x120
> [  694.588110]  shmem_undo_range+0x498/0xa50
> [  694.588813]  ? save_stack_trace+0x1b/0x20
> [  694.589529]  ? set_track+0x70/0x140
> [  694.590150]  ? init_object+0x69/0xa0
> [  694.590722]  ? __inode_wait_for_writeback+0x73/0xe0
> [  694.591525]  shmem_truncate_range+0x16/0x40
> [  694.592268]  shmem_evict_inode+0xb1/0x190
> [  694.592735]  evict+0xbb/0x1c0
> [  694.593147]  iput+0x1c0/0x210
> [  694.593497]  dentry_unlink_inode+0xb4/0x150
> [  694.593982]  __dentry_kill+0xc1/0x150
> [  694.594400]  dput+0x1c8/0x1e0
> [  694.594745]  __fput+0x172/0x1e0
> [  694.595103]  ____fput+0xe/0x10
> [  694.595463]  task_work_run+0x80/0xa0
> [  694.595886]  do_exit+0x2d6/0xb50
> [  694.596323]  ? __do_page_fault+0x288/0x4a0
> [  694.596818]  do_group_exit+0x47/0xb0
> [  694.597249]  SyS_exit_group+0x14/0x20
> [  694.597682]  entry_SYSCALL_64_fastpath+0x1a/0xa5
> [  694.598198] RIP: 0033:0x7ff77a5e78c8
> [  694.598612] RSP: 002b:00007ffc5aece318 EFLAGS: 00000246 ORIG_RAX:
> 00000000000000e7
> [  694.599804] RAX: ffffffffffffffda RBX: 0000000000000000 RCX:
> 00007ff77a5e78c8
> [  694.600609] RDX: 0000000000000000 RSI: 000000000000003c RDI:
> 0000000000000000
> [  694.601424] RBP: 00007ff77a8da618 R08: 00000000000000e7 R09:
> ffffffffffffff98
> [  694.602224] R10: 0000000000000003 R11: 0000000000000246 R12:
> 0000000000000001
> [  694.603151] R13: 00007ff77a8dbc60 R14: 0000000000000000 R15:
> 0000000000000000
> [  694.603984] Code: 60 f3 c5 81 e8 2e 7e 03 00 0f 0b 48 c7 c6 60 f3
> c5 81 4c 89 e7 e8 1d 7e 03 00 0f 0b 48 c7 c6 00 f4 c5 81 4c 89 e7 e8
> 0c 7e 03 00 <0f> 0b 48 c7 c6 38 f3 c5 81 4c 89 e7 e8 fb 7d 03 00 0f
> 0b 48 c7 
> [  694.606500] RIP: __delete_from_page_cache+0x344/0x410 RSP:
> ffffc900019d7a88
> [  694.607426] ---[ end trace 55e6b04ae95d8ce3 ]---
> 
> BTW, this was on 4.13.0-rc3 + your patches.  Simple test program is
> below.
> 

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