[v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi]

mmap_sem is on the hot path of kernel, and it very contended, but it is
abused too. It is used to protect arg_start|end and evn_start|end when
reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
sense since those proc files just expect to read 4 values atomically and
not related to VM, they could be set to arbitrary values by C/R.

And, the mmap_sem contention may cause unexpected issue like below:

INFO: task ps:14018 blocked for more than 120 seconds.
       Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
message.
 ps              D    0 14018      1 0x00000004
  ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
  ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
  00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
 Call Trace:
  [<ffffffff817154d0>] ? __schedule+0x250/0x730
  [<ffffffff817159e6>] schedule+0x36/0x80
  [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
  [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
  [<ffffffff81717db0>] down_read+0x20/0x40
  [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
  [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
  [<ffffffff81241d87>] __vfs_read+0x37/0x150
  [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
  [<ffffffff81242266>] vfs_read+0x96/0x130
  [<ffffffff812437b5>] SyS_read+0x55/0xc0
  [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5

Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
for them to mitigate the abuse of mmap_sem.

So, introduce a new spinlock in mm_struct to protect the concurrent
access to arg_start|end, env_start|end and others, as well as replace
write map_sem to read to protect the race condition between prctl and
sys_brk which might break check_data_rlimit(), and makes prctl more
friendly to other VM operations.

This patch just eliminates the abuse of mmap_sem, but it can't resolve the
above hung task warning completely since the later access_remote_vm() call
needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
future.

Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mateusz Guzik <mguzik@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
---
v3 --> v4:
* Protected values update with down_read + spin_lock to prevent from race
  condition between prctl and sys_brk and made prctl more friendly to VM
  operations per Michal's suggestion

v2 --> v3:
* Restored down_write in prctl syscall
* Elaborate the limitation of this patch suggested by Michal
* Protect those fields by the new lock except brk and start_brk per Michal's
  suggestion
* Based off Cyrill's non PR_SET_MM_MAP oprations deprecation patch
  (https://lkml.org/lkml/2018/4/5/541)

v1 --> v2:
* Use spinlock instead of rwlock per Mattew's suggestion
* Replace down_write to down_read in prctl_set_mm (see commit log for details)
 fs/proc/base.c           | 8 ++++----
 include/linux/mm_types.h | 2 ++
 kernel/fork.c            | 1 +
 kernel/sys.c             | 6 ++++--
 mm/init-mm.c             | 1 +
 5 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index eafa39a..3551757 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -239,12 +239,12 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 		goto out_mmput;
 	}
 
-	down_read(&mm->mmap_sem);
+	spin_lock(&mm->arg_lock);
 	arg_start = mm->arg_start;
 	arg_end = mm->arg_end;
 	env_start = mm->env_start;
 	env_end = mm->env_end;
-	up_read(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
 
 	BUG_ON(arg_start > arg_end);
 	BUG_ON(env_start > env_end);
@@ -929,10 +929,10 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 	if (!mmget_not_zero(mm))
 		goto free;
 
-	down_read(&mm->mmap_sem);
+	spin_lock(&mm->arg_lock);
 	env_start = mm->env_start;
 	env_end = mm->env_end;
-	up_read(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
 
 	while (count > 0) {
 		size_t this_len, max_len;
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2161234..49dd59e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -413,6 +413,8 @@ struct mm_struct {
 	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE & ~VM_STACK */
 	unsigned long stack_vm;		/* VM_STACK */
 	unsigned long def_flags;
+
+	spinlock_t arg_lock; /* protect the below fields */
 	unsigned long start_code, end_code, start_data, end_data;
 	unsigned long start_brk, brk, start_stack;
 	unsigned long arg_start, arg_end, env_start, env_end;
diff --git a/kernel/fork.c b/kernel/fork.c
index 242c8c9..295f903 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -900,6 +900,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	mm->pinned_vm = 0;
 	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
 	spin_lock_init(&mm->page_table_lock);
+	spin_lock_init(&mm->arg_lock);
 	mm_init_cpumask(mm);
 	mm_init_aio(mm);
 	mm_init_owner(mm, p);
diff --git a/kernel/sys.c b/kernel/sys.c
index f16725e..0cc5a1c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2011,7 +2011,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 			return error;
 	}
 
-	down_write(&mm->mmap_sem);
+	down_read(&mm->mmap_sem);
 
 	/*
 	 * We don't validate if these members are pointing to
@@ -2025,6 +2025,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 	 *    to any problem in kernel itself
 	 */
 
+	spin_lock(&mm->arg_lock);
 	mm->start_code	= prctl_map.start_code;
 	mm->end_code	= prctl_map.end_code;
 	mm->start_data	= prctl_map.start_data;
@@ -2036,6 +2037,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 	mm->arg_end	= prctl_map.arg_end;
 	mm->env_start	= prctl_map.env_start;
 	mm->env_end	= prctl_map.env_end;
+	spin_unlock(&mm->arg_lock);
 
 	/*
 	 * Note this update of @saved_auxv is lockless thus
@@ -2048,7 +2050,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 	if (prctl_map.auxv_size)
 		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
 
-	up_write(&mm->mmap_sem);
+	up_read(&mm->mmap_sem);
 	return 0;
 }
 #endif /* CONFIG_CHECKPOINT_RESTORE */
diff --git a/mm/init-mm.c b/mm/init-mm.c
index f94d5d1..f0179c9 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -22,6 +22,7 @@ struct mm_struct init_mm = {
 	.mm_count	= ATOMIC_INIT(1),
 	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
 	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
+	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
 	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
 	.user_ns	= &init_user_ns,
 	INIT_MM_CONTEXT(init_mm)
-- 
1.8.3.1

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Cyrill Gorcunov]

On Sun, Apr 15, 2018 at 02:24:51AM +0800, Yang Shi wrote:
> mmap_sem is on the hot path of kernel, and it very contended, but it is
> abused too. It is used to protect arg_start|end and evn_start|end when
> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> sense since those proc files just expect to read 4 values atomically and
> not related to VM, they could be set to arbitrary values by C/R.
> 
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
>  ps              D    0 14018      1 0x00000004
>   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>  Call Trace:
>   [<ffffffff817154d0>] ? __schedule+0x250/0x730
>   [<ffffffff817159e6>] schedule+0x36/0x80
>   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffff81717db0>] down_read+0x20/0x40
>   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>   [<ffffffff81241d87>] __vfs_read+0x37/0x150
>   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>   [<ffffffff81242266>] vfs_read+0x96/0x130
>   [<ffffffff812437b5>] SyS_read+0x55/0xc0
>   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new spinlock in mm_struct to protect the concurrent
> access to arg_start|end, env_start|end and others, as well as replace
> write map_sem to read to protect the race condition between prctl and
> sys_brk which might break check_data_rlimit(), and makes prctl more
> friendly to other VM operations.
> 
> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
> above hung task warning completely since the later access_remote_vm() call
> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
> future.
> 
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mateusz Guzik <mguzik@redhat.com>
> Cc: Cyrill Gorcunov <gorcunov@gmail.com>
> ---

Looks ok to me, thanks!
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Andrew Morton]

On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:

> mmap_sem is on the hot path of kernel, and it very contended, but it is
> abused too. It is used to protect arg_start|end and evn_start|end when
> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> sense since those proc files just expect to read 4 values atomically and
> not related to VM, they could be set to arbitrary values by C/R.
> 
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
>  ps              D    0 14018      1 0x00000004
>   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>  Call Trace:
>   [<ffffffff817154d0>] ? __schedule+0x250/0x730
>   [<ffffffff817159e6>] schedule+0x36/0x80
>   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffff81717db0>] down_read+0x20/0x40
>   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>   [<ffffffff81241d87>] __vfs_read+0x37/0x150
>   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>   [<ffffffff81242266>] vfs_read+0x96/0x130
>   [<ffffffff812437b5>] SyS_read+0x55/0xc0
>   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new spinlock in mm_struct to protect the concurrent
> access to arg_start|end, env_start|end and others, as well as replace
> write map_sem to read to protect the race condition between prctl and
> sys_brk which might break check_data_rlimit(), and makes prctl more
> friendly to other VM operations.

(We should move check_data_rlimit() out of the .h file)

It seems inconsistent to be using mmap_sem to protect ->start_brk and
friends in sys_brk().  We've already declared that these are protected
by arg_lock so that's what we should be using?  And getting this
consistent should permit us to stop using mmap_sem in prctl()
altogether?

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Cyrill Gorcunov]

On Tue, Apr 17, 2018 at 11:29:57AM -0700, Andrew Morton wrote:
> On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
> 
> > mmap_sem is on the hot path of kernel, and it very contended, but it is
> > abused too. It is used to protect arg_start|end and evn_start|end when
> > reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> > sense since those proc files just expect to read 4 values atomically and
> > not related to VM, they could be set to arbitrary values by C/R.
> > 
> > And, the mmap_sem contention may cause unexpected issue like below:
> > 
> > INFO: task ps:14018 blocked for more than 120 seconds.
> >        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
> >  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > message.
> >  ps              D    0 14018      1 0x00000004
> >   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
> >   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
> >   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
> >  Call Trace:
> >   [<ffffffff817154d0>] ? __schedule+0x250/0x730
> >   [<ffffffff817159e6>] schedule+0x36/0x80
> >   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
> >   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
> >   [<ffffffff81717db0>] down_read+0x20/0x40
> >   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
> >   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
> >   [<ffffffff81241d87>] __vfs_read+0x37/0x150
> >   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
> >   [<ffffffff81242266>] vfs_read+0x96/0x130
> >   [<ffffffff812437b5>] SyS_read+0x55/0xc0
> >   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> > 
> > Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> > for them to mitigate the abuse of mmap_sem.
> > 
> > So, introduce a new spinlock in mm_struct to protect the concurrent
> > access to arg_start|end, env_start|end and others, as well as replace
> > write map_sem to read to protect the race condition between prctl and
> > sys_brk which might break check_data_rlimit(), and makes prctl more
> > friendly to other VM operations.
> 
> (We should move check_data_rlimit() out of the .h file)
> 
> It seems inconsistent to be using mmap_sem to protect ->start_brk and
> friends in sys_brk().  We've already declared that these are protected
> by arg_lock so that's what we should be using?  And getting this
> consistent should permit us to stop using mmap_sem in prctl()
> altogether?

Nope, we still can't. Look, the down_read part order the call with
sys_brk. while arg_lock orders prctl call itself. That said if
someone is calling sys_brk while we're in a middle of prctl it
should wait until prctl finished. But two simultaneous prcl
may proceed without taking a write lock using arg_lock as
a barrier.

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi]

On 4/17/18 11:29 AM, Andrew Morton wrote:
> On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>> mmap_sem is on the hot path of kernel, and it very contended, but it is
>> abused too. It is used to protect arg_start|end and evn_start|end when
>> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
>> sense since those proc files just expect to read 4 values atomically and
>> not related to VM, they could be set to arbitrary values by C/R.
>>
>> And, the mmap_sem contention may cause unexpected issue like below:
>>
>> INFO: task ps:14018 blocked for more than 120 seconds.
>>         Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
>> message.
>>   ps              D    0 14018      1 0x00000004
>>    ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>>    ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>>    00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>>   Call Trace:
>>    [<ffffffff817154d0>] ? __schedule+0x250/0x730
>>    [<ffffffff817159e6>] schedule+0x36/0x80
>>    [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>>    [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>>    [<ffffffff81717db0>] down_read+0x20/0x40
>>    [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>>    [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>>    [<ffffffff81241d87>] __vfs_read+0x37/0x150
>>    [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>>    [<ffffffff81242266>] vfs_read+0x96/0x130
>>    [<ffffffff812437b5>] SyS_read+0x55/0xc0
>>    [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
>>
>> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
>> for them to mitigate the abuse of mmap_sem.
>>
>> So, introduce a new spinlock in mm_struct to protect the concurrent
>> access to arg_start|end, env_start|end and others, as well as replace
>> write map_sem to read to protect the race condition between prctl and
>> sys_brk which might break check_data_rlimit(), and makes prctl more
>> friendly to other VM operations.
> (We should move check_data_rlimit() out of the .h file)

I don't get the point, check_data_rlimit() is still used by both prctl 
and sys_brk.

>
> It seems inconsistent to be using mmap_sem to protect ->start_brk and
> friends in sys_brk().  We've already declared that these are protected
> by arg_lock so that's what we should be using?  And getting this
> consistent should permit us to stop using mmap_sem in prctl()
> altogether?

Cyrill already helped to elaborate the reason. arg_lock just can protect 
the concurrent access of brk between prctl calls, but not sys_brk.

Thanks,
Yang

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Michal Hocko]

On Tue 17-04-18 23:39:19, Cyrill Gorcunov wrote:
> On Tue, Apr 17, 2018 at 11:29:57AM -0700, Andrew Morton wrote:
> > On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
> > 
> > > mmap_sem is on the hot path of kernel, and it very contended, but it is
> > > abused too. It is used to protect arg_start|end and evn_start|end when
> > > reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> > > sense since those proc files just expect to read 4 values atomically and
> > > not related to VM, they could be set to arbitrary values by C/R.
> > > 
> > > And, the mmap_sem contention may cause unexpected issue like below:
> > > 
> > > INFO: task ps:14018 blocked for more than 120 seconds.
> > >        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
> > >  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > > message.
> > >  ps              D    0 14018      1 0x00000004
> > >   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
> > >   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
> > >   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
> > >  Call Trace:
> > >   [<ffffffff817154d0>] ? __schedule+0x250/0x730
> > >   [<ffffffff817159e6>] schedule+0x36/0x80
> > >   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
> > >   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
> > >   [<ffffffff81717db0>] down_read+0x20/0x40
> > >   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
> > >   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
> > >   [<ffffffff81241d87>] __vfs_read+0x37/0x150
> > >   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
> > >   [<ffffffff81242266>] vfs_read+0x96/0x130
> > >   [<ffffffff812437b5>] SyS_read+0x55/0xc0
> > >   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> > > 
> > > Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> > > for them to mitigate the abuse of mmap_sem.
> > > 
> > > So, introduce a new spinlock in mm_struct to protect the concurrent
> > > access to arg_start|end, env_start|end and others, as well as replace
> > > write map_sem to read to protect the race condition between prctl and
> > > sys_brk which might break check_data_rlimit(), and makes prctl more
> > > friendly to other VM operations.
> > 
> > (We should move check_data_rlimit() out of the .h file)
> > 
> > It seems inconsistent to be using mmap_sem to protect ->start_brk and
> > friends in sys_brk().  We've already declared that these are protected
> > by arg_lock so that's what we should be using?  And getting this
> > consistent should permit us to stop using mmap_sem in prctl()
> > altogether?
> 
> Nope, we still can't. Look, the down_read part order the call with
> sys_brk. while arg_lock orders prctl call itself. That said if
> someone is calling sys_brk while we're in a middle of prctl it
> should wait until prctl finished. But two simultaneous prcl
> may proceed without taking a write lock using arg_lock as
> a barrier.

A small comment would be due. The changelog mentions this but it would
be nicer to comment why we care about mmap_sem for read in prctl.

-- 
Michal Hocko
SUSE Labs

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Michal Hocko]

On Sun 15-04-18 02:24:51, Yang Shi wrote:
> mmap_sem is on the hot path of kernel, and it very contended, but it is
> abused too. It is used to protect arg_start|end and evn_start|end when
> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> sense since those proc files just expect to read 4 values atomically and
> not related to VM, they could be set to arbitrary values by C/R.
> 
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
>  ps              D    0 14018      1 0x00000004
>   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>  Call Trace:
>   [<ffffffff817154d0>] ? __schedule+0x250/0x730
>   [<ffffffff817159e6>] schedule+0x36/0x80
>   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffff81717db0>] down_read+0x20/0x40
>   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>   [<ffffffff81241d87>] __vfs_read+0x37/0x150
>   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>   [<ffffffff81242266>] vfs_read+0x96/0x130
>   [<ffffffff812437b5>] SyS_read+0x55/0xc0
>   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new spinlock in mm_struct to protect the concurrent
> access to arg_start|end, env_start|end and others, as well as replace
> write map_sem to read to protect the race condition between prctl and
> sys_brk which might break check_data_rlimit(), and makes prctl more
> friendly to other VM operations.
> 
> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
> above hung task warning completely since the later access_remote_vm() call
> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
> future.
> 
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mateusz Guzik <mguzik@redhat.com>
> Cc: Cyrill Gorcunov <gorcunov@gmail.com>

Yes, looks good to me. As mentioned in other emails prctl_set_mm_map
really deserves a comment explaining why we are doing the down_read

What about something like the following?
"
arg_lock protects concurent updates but we still need mmap_sem for read
to exclude races with do_brk.
"
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> v3 --> v4:
> * Protected values update with down_read + spin_lock to prevent from race
>   condition between prctl and sys_brk and made prctl more friendly to VM
>   operations per Michal's suggestion
> 
> v2 --> v3:
> * Restored down_write in prctl syscall
> * Elaborate the limitation of this patch suggested by Michal
> * Protect those fields by the new lock except brk and start_brk per Michal's
>   suggestion
> * Based off Cyrill's non PR_SET_MM_MAP oprations deprecation patch
>   (https://lkml.org/lkml/2018/4/5/541)
> 
> v1 --> v2:
> * Use spinlock instead of rwlock per Mattew's suggestion
> * Replace down_write to down_read in prctl_set_mm (see commit log for details)
>  fs/proc/base.c           | 8 ++++----
>  include/linux/mm_types.h | 2 ++
>  kernel/fork.c            | 1 +
>  kernel/sys.c             | 6 ++++--
>  mm/init-mm.c             | 1 +
>  5 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index eafa39a..3551757 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -239,12 +239,12 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
>  		goto out_mmput;
>  	}
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	arg_start = mm->arg_start;
>  	arg_end = mm->arg_end;
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	BUG_ON(arg_start > arg_end);
>  	BUG_ON(env_start > env_end);
> @@ -929,10 +929,10 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>  	if (!mmget_not_zero(mm))
>  		goto free;
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	while (count > 0) {
>  		size_t this_len, max_len;
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2161234..49dd59e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -413,6 +413,8 @@ struct mm_struct {
>  	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE & ~VM_STACK */
>  	unsigned long stack_vm;		/* VM_STACK */
>  	unsigned long def_flags;
> +
> +	spinlock_t arg_lock; /* protect the below fields */
>  	unsigned long start_code, end_code, start_data, end_data;
>  	unsigned long start_brk, brk, start_stack;
>  	unsigned long arg_start, arg_end, env_start, env_end;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 242c8c9..295f903 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -900,6 +900,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mm->pinned_vm = 0;
>  	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
>  	spin_lock_init(&mm->page_table_lock);
> +	spin_lock_init(&mm->arg_lock);
>  	mm_init_cpumask(mm);
>  	mm_init_aio(mm);
>  	mm_init_owner(mm, p);
> diff --git a/kernel/sys.c b/kernel/sys.c
> index f16725e..0cc5a1c 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2011,7 +2011,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  			return error;
>  	}
>  
> -	down_write(&mm->mmap_sem);
> +	down_read(&mm->mmap_sem);
>  
>  	/*
>  	 * We don't validate if these members are pointing to
> @@ -2025,6 +2025,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	 *    to any problem in kernel itself
>  	 */
>  
> +	spin_lock(&mm->arg_lock);
>  	mm->start_code	= prctl_map.start_code;
>  	mm->end_code	= prctl_map.end_code;
>  	mm->start_data	= prctl_map.start_data;
> @@ -2036,6 +2037,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	mm->arg_end	= prctl_map.arg_end;
>  	mm->env_start	= prctl_map.env_start;
>  	mm->env_end	= prctl_map.env_end;
> +	spin_unlock(&mm->arg_lock);
>  
>  	/*
>  	 * Note this update of @saved_auxv is lockless thus
> @@ -2048,7 +2050,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	if (prctl_map.auxv_size)
>  		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
>  
> -	up_write(&mm->mmap_sem);
> +	up_read(&mm->mmap_sem);
>  	return 0;
>  }
>  #endif /* CONFIG_CHECKPOINT_RESTORE */
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index f94d5d1..f0179c9 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -22,6 +22,7 @@ struct mm_struct init_mm = {
>  	.mm_count	= ATOMIC_INIT(1),
>  	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
>  	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
> +	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
>  	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
>  	.user_ns	= &init_user_ns,
>  	INIT_MM_CONTEXT(init_mm)
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Cyrill Gorcunov]

On Wed, Apr 18, 2018 at 10:05:55AM +0200, Michal Hocko wrote:
> 
> Yes, looks good to me. As mentioned in other emails prctl_set_mm_map
> really deserves a comment explaining why we are doing the down_read
> 
> What about something like the following?
> "
> arg_lock protects concurent updates but we still need mmap_sem for read
> to exclude races with do_brk.
> "
> Acked-by: Michal Hocko <mhocko@suse.com>

Yes, thanks! Andrew, could you slightly update the changelog please?

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Michal Hocko]

On Wed 18-04-18 12:02:17, Cyrill Gorcunov wrote:
> On Wed, Apr 18, 2018 at 10:05:55AM +0200, Michal Hocko wrote:
> > 
> > Yes, looks good to me. As mentioned in other emails prctl_set_mm_map
> > really deserves a comment explaining why we are doing the down_read
> > 
> > What about something like the following?
> > "
> > arg_lock protects concurent updates but we still need mmap_sem for read
> > to exclude races with do_brk.
> > "
> > Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Yes, thanks! Andrew, could you slightly update the changelog please?

No, I meant it to be a comment in the _code_.

-- 
Michal Hocko
SUSE Labs

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Cyrill Gorcunov]

On Wed, Apr 18, 2018 at 11:03:14AM +0200, Michal Hocko wrote:
> > > 
> > > What about something like the following?
> > > "
> > > arg_lock protects concurent updates but we still need mmap_sem for read
> > > to exclude races with do_brk.
> > > "
> > > Acked-by: Michal Hocko <mhocko@suse.com>
> > 
> > Yes, thanks! Andrew, could you slightly update the changelog please?
> 
> No, I meant it to be a comment in the _code_.

Ah, I see. Then small patch on top should do the trick.

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Kirill Tkhai]

On 14.04.2018 21:24, Yang Shi wrote:
> mmap_sem is on the hot path of kernel, and it very contended, but it is
> abused too. It is used to protect arg_start|end and evn_start|end when
> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> sense since those proc files just expect to read 4 values atomically and
> not related to VM, they could be set to arbitrary values by C/R.
> 
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
>  ps              D    0 14018      1 0x00000004
>   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>  Call Trace:
>   [<ffffffff817154d0>] ? __schedule+0x250/0x730
>   [<ffffffff817159e6>] schedule+0x36/0x80
>   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffff81717db0>] down_read+0x20/0x40
>   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>   [<ffffffff81241d87>] __vfs_read+0x37/0x150
>   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>   [<ffffffff81242266>] vfs_read+0x96/0x130
>   [<ffffffff812437b5>] SyS_read+0x55/0xc0
>   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new spinlock in mm_struct to protect the concurrent
> access to arg_start|end, env_start|end and others, as well as replace
> write map_sem to read to protect the race condition between prctl and
> sys_brk which might break check_data_rlimit(), and makes prctl more
> friendly to other VM operations.
> 
> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
> above hung task warning completely since the later access_remote_vm() call
> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
> future.
> 
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mateusz Guzik <mguzik@redhat.com>
> Cc: Cyrill Gorcunov <gorcunov@gmail.com>
> ---
> v3 --> v4:
> * Protected values update with down_read + spin_lock to prevent from race
>   condition between prctl and sys_brk and made prctl more friendly to VM
>   operations per Michal's suggestion
> 
> v2 --> v3:
> * Restored down_write in prctl syscall
> * Elaborate the limitation of this patch suggested by Michal
> * Protect those fields by the new lock except brk and start_brk per Michal's
>   suggestion
> * Based off Cyrill's non PR_SET_MM_MAP oprations deprecation patch
>   (https://lkml.org/lkml/2018/4/5/541)
> 
> v1 --> v2:
> * Use spinlock instead of rwlock per Mattew's suggestion
> * Replace down_write to down_read in prctl_set_mm (see commit log for details)
>  fs/proc/base.c           | 8 ++++----
>  include/linux/mm_types.h | 2 ++
>  kernel/fork.c            | 1 +
>  kernel/sys.c             | 6 ++++--
>  mm/init-mm.c             | 1 +
>  5 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index eafa39a..3551757 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -239,12 +239,12 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
>  		goto out_mmput;
>  	}
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	arg_start = mm->arg_start;
>  	arg_end = mm->arg_end;
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	BUG_ON(arg_start > arg_end);
>  	BUG_ON(env_start > env_end);
> @@ -929,10 +929,10 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>  	if (!mmget_not_zero(mm))
>  		goto free;
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	while (count > 0) {
>  		size_t this_len, max_len;
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2161234..49dd59e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -413,6 +413,8 @@ struct mm_struct {
>  	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE & ~VM_STACK */
>  	unsigned long stack_vm;		/* VM_STACK */
>  	unsigned long def_flags;
> +
> +	spinlock_t arg_lock; /* protect the below fields */

What the reason is spinlock is used to protect this fields?
There may be several readers, say, doing "ps axf" and it's
OK for them to access these fields in parallel. Why should
we delay them by each other?

rw_lock seems be more suitable for here.

>  	unsigned long start_code, end_code, start_data, end_data;
>  	unsigned long start_brk, brk, start_stack;
>  	unsigned long arg_start, arg_end, env_start, env_end;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 242c8c9..295f903 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -900,6 +900,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mm->pinned_vm = 0;
>  	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
>  	spin_lock_init(&mm->page_table_lock);
> +	spin_lock_init(&mm->arg_lock);
>  	mm_init_cpumask(mm);
>  	mm_init_aio(mm);
>  	mm_init_owner(mm, p);
> diff --git a/kernel/sys.c b/kernel/sys.c
> index f16725e..0cc5a1c 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2011,7 +2011,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  			return error;
>  	}
>  
> -	down_write(&mm->mmap_sem);
> +	down_read(&mm->mmap_sem);

This down_read() looks confusing for a reader. Comment to spinlock says it protects the fields:

	+	spinlock_t arg_lock; /* protect the below fields */

but in real life there is not obvious dependence with sys_brk(), which is nowhere described.
This hunk protects us from "execution of sys_brk()" and this looks very confusing.

The generic locking theory says, we should protect data and not a function execution.
Protection of function execution prevents their modification in the future, makes it
more difficult.

Can we take write_lock() (after we introduce it instead of spinlock) in sys_brk()
and make the locking scheme more visible and intuitive?

>  
>  	/*
>  	 * We don't validate if these members are pointing to
> @@ -2025,6 +2025,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	 *    to any problem in kernel itself
>  	 */
>  
> +	spin_lock(&mm->arg_lock);
>  	mm->start_code	= prctl_map.start_code;
>  	mm->end_code	= prctl_map.end_code;
>  	mm->start_data	= prctl_map.start_data;
> @@ -2036,6 +2037,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	mm->arg_end	= prctl_map.arg_end;
>  	mm->env_start	= prctl_map.env_start;
>  	mm->env_end	= prctl_map.env_end;
> +	spin_unlock(&mm->arg_lock);
>  
>  	/*
>  	 * Note this update of @saved_auxv is lockless thus
> @@ -2048,7 +2050,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>  	if (prctl_map.auxv_size)
>  		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
>  
> -	up_write(&mm->mmap_sem);
> +	up_read(&mm->mmap_sem);
>  	return 0;
>  }
>  #endif /* CONFIG_CHECKPOINT_RESTORE */
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index f94d5d1..f0179c9 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -22,6 +22,7 @@ struct mm_struct init_mm = {
>  	.mm_count	= ATOMIC_INIT(1),
>  	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
>  	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
> +	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
>  	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
>  	.user_ns	= &init_user_ns,
>  	INIT_MM_CONTEXT(init_mm)

Kirill

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi]

On 4/18/18 2:40 AM, Cyrill Gorcunov wrote:
> On Wed, Apr 18, 2018 at 11:03:14AM +0200, Michal Hocko wrote:
>>>> What about something like the following?
>>>> "
>>>> arg_lock protects concurent updates but we still need mmap_sem for read
>>>> to exclude races with do_brk.
>>>> "
>>>> Acked-by: Michal Hocko <mhocko@suse.com>
>>> Yes, thanks! Andrew, could you slightly update the changelog please?
>> No, I meant it to be a comment in the _code_.
> Ah, I see. Then small patch on top should do the trick.

Will send out an incremental patch soon.

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi]

On 4/18/18 3:11 AM, Kirill Tkhai wrote:
> On 14.04.2018 21:24, Yang Shi wrote:
>> mmap_sem is on the hot path of kernel, and it very contended, but it is
>> abused too. It is used to protect arg_start|end and evn_start|end when
>> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
>> sense since those proc files just expect to read 4 values atomically and
>> not related to VM, they could be set to arbitrary values by C/R.
>>
>> And, the mmap_sem contention may cause unexpected issue like below:
>>
>> INFO: task ps:14018 blocked for more than 120 seconds.
>>         Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
>> message.
>>   ps              D    0 14018      1 0x00000004
>>    ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>>    ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>>    00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>>   Call Trace:
>>    [<ffffffff817154d0>] ? __schedule+0x250/0x730
>>    [<ffffffff817159e6>] schedule+0x36/0x80
>>    [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>>    [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>>    [<ffffffff81717db0>] down_read+0x20/0x40
>>    [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>>    [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>>    [<ffffffff81241d87>] __vfs_read+0x37/0x150
>>    [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>>    [<ffffffff81242266>] vfs_read+0x96/0x130
>>    [<ffffffff812437b5>] SyS_read+0x55/0xc0
>>    [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
>>
>> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
>> for them to mitigate the abuse of mmap_sem.
>>
>> So, introduce a new spinlock in mm_struct to protect the concurrent
>> access to arg_start|end, env_start|end and others, as well as replace
>> write map_sem to read to protect the race condition between prctl and
>> sys_brk which might break check_data_rlimit(), and makes prctl more
>> friendly to other VM operations.
>>
>> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
>> above hung task warning completely since the later access_remote_vm() call
>> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
>> future.
>>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> Cc: Alexey Dobriyan <adobriyan@gmail.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Mateusz Guzik <mguzik@redhat.com>
>> Cc: Cyrill Gorcunov <gorcunov@gmail.com>
>> ---
>> v3 --> v4:
>> * Protected values update with down_read + spin_lock to prevent from race
>>    condition between prctl and sys_brk and made prctl more friendly to VM
>>    operations per Michal's suggestion
>>
>> v2 --> v3:
>> * Restored down_write in prctl syscall
>> * Elaborate the limitation of this patch suggested by Michal
>> * Protect those fields by the new lock except brk and start_brk per Michal's
>>    suggestion
>> * Based off Cyrill's non PR_SET_MM_MAP oprations deprecation patch
>>    (https://lkml.org/lkml/2018/4/5/541)
>>
>> v1 --> v2:
>> * Use spinlock instead of rwlock per Mattew's suggestion
>> * Replace down_write to down_read in prctl_set_mm (see commit log for details)
>>   fs/proc/base.c           | 8 ++++----
>>   include/linux/mm_types.h | 2 ++
>>   kernel/fork.c            | 1 +
>>   kernel/sys.c             | 6 ++++--
>>   mm/init-mm.c             | 1 +
>>   5 files changed, 12 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index eafa39a..3551757 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -239,12 +239,12 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
>>   		goto out_mmput;
>>   	}
>>   
>> -	down_read(&mm->mmap_sem);
>> +	spin_lock(&mm->arg_lock);
>>   	arg_start = mm->arg_start;
>>   	arg_end = mm->arg_end;
>>   	env_start = mm->env_start;
>>   	env_end = mm->env_end;
>> -	up_read(&mm->mmap_sem);
>> +	spin_unlock(&mm->arg_lock);
>>   
>>   	BUG_ON(arg_start > arg_end);
>>   	BUG_ON(env_start > env_end);
>> @@ -929,10 +929,10 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>>   	if (!mmget_not_zero(mm))
>>   		goto free;
>>   
>> -	down_read(&mm->mmap_sem);
>> +	spin_lock(&mm->arg_lock);
>>   	env_start = mm->env_start;
>>   	env_end = mm->env_end;
>> -	up_read(&mm->mmap_sem);
>> +	spin_unlock(&mm->arg_lock);
>>   
>>   	while (count > 0) {
>>   		size_t this_len, max_len;
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 2161234..49dd59e 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -413,6 +413,8 @@ struct mm_struct {
>>   	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE & ~VM_STACK */
>>   	unsigned long stack_vm;		/* VM_STACK */
>>   	unsigned long def_flags;
>> +
>> +	spinlock_t arg_lock; /* protect the below fields */
> What the reason is spinlock is used to protect this fields?
> There may be several readers, say, doing "ps axf" and it's
> OK for them to access these fields in parallel. Why should
> we delay them by each other?
>
> rw_lock seems be more suitable for here.

Thanks a lot for the suggestion. We did think about using rwlock, but it 
sounds the benefit might be not worth the overhead. If it turns out 
rwlock is worth, we definitely can change it to rwlock later.

>
>>   	unsigned long start_code, end_code, start_data, end_data;
>>   	unsigned long start_brk, brk, start_stack;
>>   	unsigned long arg_start, arg_end, env_start, env_end;
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 242c8c9..295f903 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -900,6 +900,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>>   	mm->pinned_vm = 0;
>>   	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
>>   	spin_lock_init(&mm->page_table_lock);
>> +	spin_lock_init(&mm->arg_lock);
>>   	mm_init_cpumask(mm);
>>   	mm_init_aio(mm);
>>   	mm_init_owner(mm, p);
>> diff --git a/kernel/sys.c b/kernel/sys.c
>> index f16725e..0cc5a1c 100644
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -2011,7 +2011,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>   			return error;
>>   	}
>>   
>> -	down_write(&mm->mmap_sem);
>> +	down_read(&mm->mmap_sem);
> This down_read() looks confusing for a reader. Comment to spinlock says it protects the fields:
>
> 	+	spinlock_t arg_lock; /* protect the below fields */
>
> but in real life there is not obvious dependence with sys_brk(), which is nowhere described.
> This hunk protects us from "execution of sys_brk()" and this looks very confusing.
>
> The generic locking theory says, we should protect data and not a function execution.
> Protection of function execution prevents their modification in the future, makes it
> more difficult.

Yes, it is used to protect data. As I mentioned in the commit log, 
check_data_rlimit() use brk, start_brk, start_data and end_data to check 
if the address space is beyond the limit. But, prctl may modify them at 
the same time.

>
> Can we take write_lock() (after we introduce it instead of spinlock) in sys_brk()
> and make the locking scheme more visible and intuitive?

Take a lock for what? Do you mean replace mmap_sem to the lock?

Thanks,
Yang

>
>>   
>>   	/*
>>   	 * We don't validate if these members are pointing to
>> @@ -2025,6 +2025,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>   	 *    to any problem in kernel itself
>>   	 */
>>   
>> +	spin_lock(&mm->arg_lock);
>>   	mm->start_code	= prctl_map.start_code;
>>   	mm->end_code	= prctl_map.end_code;
>>   	mm->start_data	= prctl_map.start_data;
>> @@ -2036,6 +2037,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>   	mm->arg_end	= prctl_map.arg_end;
>>   	mm->env_start	= prctl_map.env_start;
>>   	mm->env_end	= prctl_map.env_end;
>> +	spin_unlock(&mm->arg_lock);
>>   
>>   	/*
>>   	 * Note this update of @saved_auxv is lockless thus
>> @@ -2048,7 +2050,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>   	if (prctl_map.auxv_size)
>>   		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
>>   
>> -	up_write(&mm->mmap_sem);
>> +	up_read(&mm->mmap_sem);
>>   	return 0;
>>   }
>>   #endif /* CONFIG_CHECKPOINT_RESTORE */
>> diff --git a/mm/init-mm.c b/mm/init-mm.c
>> index f94d5d1..f0179c9 100644
>> --- a/mm/init-mm.c
>> +++ b/mm/init-mm.c
>> @@ -22,6 +22,7 @@ struct mm_struct init_mm = {
>>   	.mm_count	= ATOMIC_INIT(1),
>>   	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
>>   	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
>> +	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
>>   	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
>>   	.user_ns	= &init_user_ns,
>>   	INIT_MM_CONTEXT(init_mm)
> Kirill

Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Kirill Tkhai]

On 18.04.2018 21:48, Yang Shi wrote:
> 
> 
> On 4/18/18 3:11 AM, Kirill Tkhai wrote:
>> On 14.04.2018 21:24, Yang Shi wrote:
>>> mmap_sem is on the hot path of kernel, and it very contended, but it is
>>> abused too. It is used to protect arg_start|end and evn_start|end when
>>> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
>>> sense since those proc files just expect to read 4 values atomically and
>>> not related to VM, they could be set to arbitrary values by C/R.
>>>
>>> And, the mmap_sem contention may cause unexpected issue like below:
>>>
>>> INFO: task ps:14018 blocked for more than 120 seconds.
>>>         Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>>>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
>>> message.
>>>   ps              D    0 14018      1 0x00000004
>>>    ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>>>    ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>>>    00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>>>   Call Trace:
>>>    [<ffffffff817154d0>] ? __schedule+0x250/0x730
>>>    [<ffffffff817159e6>] schedule+0x36/0x80
>>>    [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>>>    [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>>>    [<ffffffff81717db0>] down_read+0x20/0x40
>>>    [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
>>>    [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>>>    [<ffffffff81241d87>] __vfs_read+0x37/0x150
>>>    [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>>>    [<ffffffff81242266>] vfs_read+0x96/0x130
>>>    [<ffffffff812437b5>] SyS_read+0x55/0xc0
>>>    [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
>>>
>>> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
>>> for them to mitigate the abuse of mmap_sem.
>>>
>>> So, introduce a new spinlock in mm_struct to protect the concurrent
>>> access to arg_start|end, env_start|end and others, as well as replace
>>> write map_sem to read to protect the race condition between prctl and
>>> sys_brk which might break check_data_rlimit(), and makes prctl more
>>> friendly to other VM operations.
>>>
>>> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
>>> above hung task warning completely since the later access_remote_vm() call
>>> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
>>> future.
>>>
>>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>>> Cc: Alexey Dobriyan <adobriyan@gmail.com>
>>> Cc: Michal Hocko <mhocko@kernel.org>
>>> Cc: Matthew Wilcox <willy@infradead.org>
>>> Cc: Mateusz Guzik <mguzik@redhat.com>
>>> Cc: Cyrill Gorcunov <gorcunov@gmail.com>
>>> ---
>>> v3 --> v4:
>>> * Protected values update with down_read + spin_lock to prevent from race
>>>    condition between prctl and sys_brk and made prctl more friendly to VM
>>>    operations per Michal's suggestion
>>>
>>> v2 --> v3:
>>> * Restored down_write in prctl syscall
>>> * Elaborate the limitation of this patch suggested by Michal
>>> * Protect those fields by the new lock except brk and start_brk per Michal's
>>>    suggestion
>>> * Based off Cyrill's non PR_SET_MM_MAP oprations deprecation patch
>>>    (https://lkml.org/lkml/2018/4/5/541)
>>>
>>> v1 --> v2:
>>> * Use spinlock instead of rwlock per Mattew's suggestion
>>> * Replace down_write to down_read in prctl_set_mm (see commit log for details)
>>>   fs/proc/base.c           | 8 ++++----
>>>   include/linux/mm_types.h | 2 ++
>>>   kernel/fork.c            | 1 +
>>>   kernel/sys.c             | 6 ++++--
>>>   mm/init-mm.c             | 1 +
>>>   5 files changed, 12 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>> index eafa39a..3551757 100644
>>> --- a/fs/proc/base.c
>>> +++ b/fs/proc/base.c
>>> @@ -239,12 +239,12 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
>>>           goto out_mmput;
>>>       }
>>>   -    down_read(&mm->mmap_sem);
>>> +    spin_lock(&mm->arg_lock);
>>>       arg_start = mm->arg_start;
>>>       arg_end = mm->arg_end;
>>>       env_start = mm->env_start;
>>>       env_end = mm->env_end;
>>> -    up_read(&mm->mmap_sem);
>>> +    spin_unlock(&mm->arg_lock);
>>>         BUG_ON(arg_start > arg_end);
>>>       BUG_ON(env_start > env_end);
>>> @@ -929,10 +929,10 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>>>       if (!mmget_not_zero(mm))
>>>           goto free;
>>>   -    down_read(&mm->mmap_sem);
>>> +    spin_lock(&mm->arg_lock);
>>>       env_start = mm->env_start;
>>>       env_end = mm->env_end;
>>> -    up_read(&mm->mmap_sem);
>>> +    spin_unlock(&mm->arg_lock);
>>>         while (count > 0) {
>>>           size_t this_len, max_len;
>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>>> index 2161234..49dd59e 100644
>>> --- a/include/linux/mm_types.h
>>> +++ b/include/linux/mm_types.h
>>> @@ -413,6 +413,8 @@ struct mm_struct {
>>>       unsigned long exec_vm;        /* VM_EXEC & ~VM_WRITE & ~VM_STACK */
>>>       unsigned long stack_vm;        /* VM_STACK */
>>>       unsigned long def_flags;
>>> +
>>> +    spinlock_t arg_lock; /* protect the below fields */
>> What the reason is spinlock is used to protect this fields?
>> There may be several readers, say, doing "ps axf" and it's
>> OK for them to access these fields in parallel. Why should
>> we delay them by each other?
>>
>> rw_lock seems be more suitable for here.
> 
> Thanks a lot for the suggestion. We did think about using rwlock, but it sounds the benefit might be not worth the overhead. If it turns out rwlock is worth, we definitely can change it to rwlock later.

I missed your discussion, and failed to find the overhead details.
Could you please to point that or shortly describe, what is the overhead
of rwlock in comparison to plain spinlock?

>>
>>>       unsigned long start_code, end_code, start_data, end_data;
>>>       unsigned long start_brk, brk, start_stack;
>>>       unsigned long arg_start, arg_end, env_start, env_end;
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 242c8c9..295f903 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -900,6 +900,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>>>       mm->pinned_vm = 0;
>>>       memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
>>>       spin_lock_init(&mm->page_table_lock);
>>> +    spin_lock_init(&mm->arg_lock);
>>>       mm_init_cpumask(mm);
>>>       mm_init_aio(mm);
>>>       mm_init_owner(mm, p);
>>> diff --git a/kernel/sys.c b/kernel/sys.c
>>> index f16725e..0cc5a1c 100644
>>> --- a/kernel/sys.c
>>> +++ b/kernel/sys.c
>>> @@ -2011,7 +2011,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>>               return error;
>>>       }
>>>   -    down_write(&mm->mmap_sem);
>>> +    down_read(&mm->mmap_sem);
>> This down_read() looks confusing for a reader. Comment to spinlock says it protects the fields:
>>
>>     +    spinlock_t arg_lock; /* protect the below fields */
>>
>> but in real life there is not obvious dependence with sys_brk(), which is nowhere described.
>> This hunk protects us from "execution of sys_brk()" and this looks very confusing.
>>
>> The generic locking theory says, we should protect data and not a function execution.
>> Protection of function execution prevents their modification in the future, makes it
>> more difficult.
> 
> Yes, it is used to protect data. As I mentioned in the commit log, check_data_rlimit() use brk, start_brk, start_data and end_data to check if the address space is beyond the limit. But, prctl may modify them at the same time.

But sys_brk() does not take the new lock. Prctl protects against sys_brk() via
down_read(), so it's impossible to say a single lock, which protects brk/etc.
This will be a problem if someone decides to implement something on top of this.
Which lock should he/she use to get consistent brk? Both of them.

> 
>>
>> Can we take write_lock() (after we introduce it instead of spinlock) in sys_brk()
>> and make the locking scheme more visible and intuitive?
> 
> Take a lock for what? Do you mean replace mmap_sem to the lock?

No, we should take both of the locks there. mmap_sem will protect
more variables, of course, while the new lock will protect only
brk/etc fields the comment in structure says. Then we'll have the
clear synchronization scheme, and all the locks will protect data,
but not one function against the function execution.

What problems we have to insert the new lock into sys_brk()? Is this
because it's sleepable?

> 
> Thanks,
> Yang
> 
>>
>>>         /*
>>>        * We don't validate if these members are pointing to
>>> @@ -2025,6 +2025,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>>        *    to any problem in kernel itself
>>>        */
>>>   +    spin_lock(&mm->arg_lock);
>>>       mm->start_code    = prctl_map.start_code;
>>>       mm->end_code    = prctl_map.end_code;
>>>       mm->start_data    = prctl_map.start_data;
>>> @@ -2036,6 +2037,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>>       mm->arg_end    = prctl_map.arg_end;
>>>       mm->env_start    = prctl_map.env_start;
>>>       mm->env_end    = prctl_map.env_end;
>>> +    spin_unlock(&mm->arg_lock);
>>>         /*
>>>        * Note this update of @saved_auxv is lockless thus
>>> @@ -2048,7 +2050,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>>       if (prctl_map.auxv_size)
>>>           memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
>>>   -    up_write(&mm->mmap_sem);
>>> +    up_read(&mm->mmap_sem);
>>>       return 0;
>>>   }
>>>   #endif /* CONFIG_CHECKPOINT_RESTORE */
>>> diff --git a/mm/init-mm.c b/mm/init-mm.c
>>> index f94d5d1..f0179c9 100644
>>> --- a/mm/init-mm.c
>>> +++ b/mm/init-mm.c
>>> @@ -22,6 +22,7 @@ struct mm_struct init_mm = {
>>>       .mm_count    = ATOMIC_INIT(1),
>>>       .mmap_sem    = __RWSEM_INITIALIZER(init_mm.mmap_sem),
>>>       .page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
>>> +    .arg_lock    =  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
>>>       .mmlist        = LIST_HEAD_INIT(init_mm.mmlist),
>>>       .user_ns    = &init_user_ns,
>>>       INIT_MM_CONTEXT(init_mm)
>> Kirill

Kirill