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[PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

Started by	riel@redhat.com
First post	2017-08-04 21:10 +0200
Last post	2017-08-05 16:10 +0200
Articles	3 — 3 participants

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  [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK riel@redhat.com - 2017-08-04 21:10 +0200
    Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK Mike Kravetz <mike.kravetz@oracle.com> - 2017-08-05 01:20 +0200
      Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK Rik van Riel <riel@redhat.com> - 2017-08-05 16:10 +0200

#1704172 — [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

From	riel@redhat.com
Date	2017-08-04 21:10 +0200
Subject	[PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK
Message-ID	<uaQ8W-2CK-21@gated-at.bofh.it>

From: Rik van Riel <riel@redhat.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.

If a child process accesses memory that was MADV_DONTFORK, it will
get a segmentation fault, since those address ranges are no longer
valid in the child after fork.

Since MADV_DONTFORK also seems to be used to allow very large
programs to fork in systems with strict memory overcommit restrictions,
changing the semantics of MADV_DONTFORK might break existing programs.

The use case is libraries that store or cache information, and
want to know that they need to regenerate it in the child process
after fork.

Examples of this would be:
- systemd/pulseaudio API checks (fail after fork)
  (replacing a getpid check, which is too slow without a PID cache)
- PKCS#11 API reinitialization check (mandated by specification)
- glibc's upcoming PRNG (reseed after fork)
- OpenSSL PRNG (reseed after fork)

The security benefits of a forking server having a re-inialized
PRNG in every child process are pretty obvious. However, due to
libraries having all kinds of internal state, and programs getting
compiled with many different versions of each library, it is
unreasonable to expect calling programs to re-initialize everything
manually after fork.

A further complication is the proliferation of clone flags,
programs bypassing glibc's functions to call clone directly,
and programs calling unshare, causing the glibc pthread_atfork
hook to not get called.

It would be better to have the kernel take care of this automatically.

This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO:

    https://man.openbsd.org/minherit.2

Reported-by: Florian Weimer <fweimer@redhat.com>
Reported-by: Colm MacCártaigh <colm@allcosts.net>
Signed-off-by: Rik van Riel <riel@redhat.com>
---
 arch/alpha/include/uapi/asm/mman.h     |  3 +++
 arch/mips/include/uapi/asm/mman.h      |  3 +++
 arch/parisc/include/uapi/asm/mman.h    |  3 +++
 arch/xtensa/include/uapi/asm/mman.h    |  3 +++
 fs/proc/task_mmu.c                     |  1 +
 include/linux/mm.h                     |  2 +-
 include/uapi/asm-generic/mman-common.h |  3 +++
 kernel/fork.c                          |  8 ++++++--
 mm/madvise.c                           |  8 ++++++++
 mm/memory.c                            | 10 ++++++++++
 10 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
index 02760f6e6ca4..2a708a792882 100644
--- a/arch/alpha/include/uapi/asm/mman.h
+++ b/arch/alpha/include/uapi/asm/mman.h
@@ -64,6 +64,9 @@
 					   overrides the coredump filter bits */
 #define MADV_DODUMP	17		/* Clear the MADV_NODUMP flag */
 
+#define MADV_WIPEONFORK 18		/* Zero memory on fork, child only */
+#define MADV_KEEPONFORK 19		/* Undo MADV_WIPEONFORK */
+
 /* compatibility flags */
 #define MAP_FILE	0
 
diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
index 655e2fb5395b..d59c57d60d7d 100644
--- a/arch/mips/include/uapi/asm/mman.h
+++ b/arch/mips/include/uapi/asm/mman.h
@@ -91,6 +91,9 @@
 					   overrides the coredump filter bits */
 #define MADV_DODUMP	17		/* Clear the MADV_NODUMP flag */
 
+#define MADV_WIPEONFORK 18		/* Zero memory on fork, child only */
+#define MADV_KEEPONFORK 19		/* Undo MADV_WIPEONFORK */
+
 /* compatibility flags */
 #define MAP_FILE	0
 
diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
index 5979745815a5..e205e0179642 100644
--- a/arch/parisc/include/uapi/asm/mman.h
+++ b/arch/parisc/include/uapi/asm/mman.h
@@ -60,6 +60,9 @@
 					   overrides the coredump filter bits */
 #define MADV_DODUMP	70		/* Clear the MADV_NODUMP flag */
 
+#define MADV_WIPEONFORK 71		/* Zero memory on fork, child only */
+#define MADV_KEEPONFORK 72		/* Undo MADV_WIPEONFORK */
+
 /* compatibility flags */
 #define MAP_FILE	0
 #define MAP_VARIABLE	0
diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
index 24365b30aae9..ed23e0a1b30d 100644
--- a/arch/xtensa/include/uapi/asm/mman.h
+++ b/arch/xtensa/include/uapi/asm/mman.h
@@ -103,6 +103,9 @@
 					   overrides the coredump filter bits */
 #define MADV_DODUMP	17		/* Clear the MADV_NODUMP flag */
 
+#define MADV_WIPEONFORK 18		/* Zero memory on fork, child only */
+#define MADV_KEEPONFORK 19		/* Undo MADV_WIPEONFORK */
+
 /* compatibility flags */
 #define MAP_FILE	0
 
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index b836fd61ed87..2591e70216ff 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -651,6 +651,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_NORESERVE)]	= "nr",
 		[ilog2(VM_HUGETLB)]	= "ht",
 		[ilog2(VM_ARCH_1)]	= "ar",
+		[ilog2(VM_WIPEONFORK)]	= "wf",
 		[ilog2(VM_DONTDUMP)]	= "dd",
 #ifdef CONFIG_MEM_SOFT_DIRTY
 		[ilog2(VM_SOFTDIRTY)]	= "sd",
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7550eeb06ccf..58788c1b9e9d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -189,7 +189,7 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_NORESERVE	0x00200000	/* should the VM suppress accounting */
 #define VM_HUGETLB	0x00400000	/* Huge TLB Page VM */
 #define VM_ARCH_1	0x01000000	/* Architecture-specific flag */
-#define VM_ARCH_2	0x02000000
+#define VM_WIPEONFORK	0x02000000	/* Wipe VMA contents in child. */
 #define VM_DONTDUMP	0x04000000	/* Do not include in the core dump */
 
 #ifdef CONFIG_MEM_SOFT_DIRTY
diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
index 8c27db0c5c08..49e2b1d78093 100644
--- a/include/uapi/asm-generic/mman-common.h
+++ b/include/uapi/asm-generic/mman-common.h
@@ -58,6 +58,9 @@
 					   overrides the coredump filter bits */
 #define MADV_DODUMP	17		/* Clear the MADV_DONTDUMP flag */
 
+#define MADV_WIPEONFORK 18		/* Zero memory on fork, child only */
+#define MADV_KEEPONFORK 19		/* Undo MADV_WIPEONFORK */
+
 /* compatibility flags */
 #define MAP_FILE	0
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 17921b0390b4..2dd0d0cae3bb 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -628,7 +628,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 
 	prev = NULL;
 	for (mpnt = oldmm->mmap; mpnt; mpnt = mpnt->vm_next) {
-		struct file *file;
+		struct file *file = NULL;
 
 		if (mpnt->vm_flags & VM_DONTCOPY) {
 			vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
@@ -658,7 +658,11 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 			goto fail_nomem_anon_vma_fork;
 		tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
 		tmp->vm_next = tmp->vm_prev = NULL;
-		file = tmp->vm_file;
+
+		/* With VM_WIPEONFORK, the child gets an empty VMA. */
+		if (!(tmp->vm_flags & VM_WIPEONFORK))
+			file = tmp->vm_file;
+
 		if (file) {
 			struct inode *inode = file_inode(file);
 			struct address_space *mapping = file->f_mapping;
diff --git a/mm/madvise.c b/mm/madvise.c
index 9976852f1e1c..9e644c0ed4dc 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -80,6 +80,12 @@ static long madvise_behavior(struct vm_area_struct *vma,
 		}
 		new_flags &= ~VM_DONTCOPY;
 		break;
+	case MADV_WIPEONFORK:
+		new_flags |= VM_WIPEONFORK;
+		break;
+	case MADV_KEEPONFORK:
+		new_flags &= ~VM_WIPEONFORK;
+		break;
 	case MADV_DONTDUMP:
 		new_flags |= VM_DONTDUMP;
 		break;
@@ -689,6 +695,8 @@ madvise_behavior_valid(int behavior)
 #endif
 	case MADV_DONTDUMP:
 	case MADV_DODUMP:
+	case MADV_WIPEONFORK:
+	case MADV_KEEPONFORK:
 #ifdef CONFIG_MEMORY_FAILURE
 	case MADV_SOFT_OFFLINE:
 	case MADV_HWPOISON:
diff --git a/mm/memory.c b/mm/memory.c
index 0e517be91a89..f9b0ad7feb57 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1134,6 +1134,16 @@ int copy_page_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 			!vma->anon_vma)
 		return 0;
 
+	/*
+	 * With VM_WIPEONFORK, the child inherits the VMA from the
+	 * parent, but not its contents.
+	 *
+	 * A child accessing VM_WIPEONFORK memory will see all zeroes;
+	 * a child accessing VM_DONTCOPY memory receives a segfault.
+	 */
+	if (vma->vm_flags & VM_WIPEONFORK)
+		return 0;
+
 	if (is_vm_hugetlb_page(vma))
 		return copy_hugetlb_page_range(dst_mm, src_mm, vma);
 
-- 
2.9.4

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#1704278

From	Mike Kravetz <mike.kravetz@oracle.com>
Date	2017-08-05 01:20 +0200
Message-ID	<uaU2R-560-15@gated-at.bofh.it>
In reply to	#1704172

On 08/04/2017 12:07 PM, riel@redhat.com wrote:
> From: Rik van Riel <riel@redhat.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.
> 
> If a child process accesses memory that was MADV_DONTFORK, it will
> get a segmentation fault, since those address ranges are no longer
> valid in the child after fork.
> 
> Since MADV_DONTFORK also seems to be used to allow very large
> programs to fork in systems with strict memory overcommit restrictions,
> changing the semantics of MADV_DONTFORK might break existing programs.
> 
> The use case is libraries that store or cache information, and
> want to know that they need to regenerate it in the child process
> after fork.
> 
> Examples of this would be:
> - systemd/pulseaudio API checks (fail after fork)
>   (replacing a getpid check, which is too slow without a PID cache)
> - PKCS#11 API reinitialization check (mandated by specification)
> - glibc's upcoming PRNG (reseed after fork)
> - OpenSSL PRNG (reseed after fork)
> 
> The security benefits of a forking server having a re-inialized
> PRNG in every child process are pretty obvious. However, due to
> libraries having all kinds of internal state, and programs getting
> compiled with many different versions of each library, it is
> unreasonable to expect calling programs to re-initialize everything
> manually after fork.
> 
> A further complication is the proliferation of clone flags,
> programs bypassing glibc's functions to call clone directly,
> and programs calling unshare, causing the glibc pthread_atfork
> hook to not get called.
> 
> It would be better to have the kernel take care of this automatically.
> 
> This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO:
> 
>     https://man.openbsd.org/minherit.2
> 
> Reported-by: Florian Weimer <fweimer@redhat.com>
> Reported-by: Colm MacCártaigh <colm@allcosts.net>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
>  arch/alpha/include/uapi/asm/mman.h     |  3 +++
>  arch/mips/include/uapi/asm/mman.h      |  3 +++
>  arch/parisc/include/uapi/asm/mman.h    |  3 +++
>  arch/xtensa/include/uapi/asm/mman.h    |  3 +++
>  fs/proc/task_mmu.c                     |  1 +
>  include/linux/mm.h                     |  2 +-
>  include/uapi/asm-generic/mman-common.h |  3 +++
>  kernel/fork.c                          |  8 ++++++--
>  mm/madvise.c                           |  8 ++++++++
>  mm/memory.c                            | 10 ++++++++++
>  10 files changed, 41 insertions(+), 3 deletions(-)
> 

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);

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.

-- 
Mike Kravetz


#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/mman.h>
#include <errno.h>

#define MADV_WIPEONFORK 18
#define ADDR (void *)(0x0UL)

int main(int argc, char ** argv)
{
	unsigned long page_size;
	int ret;
	void *addr;
	char foo;

	page_size = sysconf(_SC_PAGE_SIZE);

	addr = mmap(ADDR, page_size,
			PROT_READ | PROT_WRITE,
			MAP_ANONYMOUS|MAP_SHARED, -1, 0);
	if (addr == MAP_FAILED) {
		perror("mmap");
		exit (1);
	}

	printf("Parent writing 'a' to page\n");
	*((char *)addr) = 'a'; 

	ret = madvise(addr, page_size, MADV_WIPEONFORK);
	if (ret) {
		perror("madvise");
		exit (1);
	}

	if (fork()) {
		/* In parent */
		sleep(1);
	} else {
		/* In child */
		foo = *((char *)addr);
		printf("child read '%c' from page\n", foo);
	}

	return ret;
}

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#1704653

From	Rik van Riel <riel@redhat.com>
Date	2017-08-05 16:10 +0200
Message-ID	<ub7Wa-61f-13@gated-at.bofh.it>
In reply to	#1704278

On Fri, 2017-08-04 at 16:09 -0700, Mike Kravetz wrote:
> On 08/04/2017 12:07 PM, riel@redhat.com wrote:
> > From: Rik van Riel <riel@redhat.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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[PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

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