Re: [PATCH 04/16] mm/migration: reduce the rcu lock duration

["Huang, Ying" <ying.huang@intel.com>]

Miaohe Lin <linmiaohe@huawei.com> writes:

> On 2022/3/7 10:32, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@huawei.com> writes:
>> 
>>> rcu_read_lock is required by grabbing the task refcount but it's not
>>> needed for ptrace_may_access. So we could release the rcu lock after
>>> task refcount is successfully grabbed to reduce the rcu holding time.
>>>
>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>> ---
>>>  mm/migrate.c | 3 +--
>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index da5a81052468..26943bd819e8 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1907,17 +1907,16 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
>>>  		return ERR_PTR(-ESRCH);
>>>  	}
>>>  	get_task_struct(task);
>>> +	rcu_read_unlock();
>>>  
>>>  	/*
>>>  	 * Check if this process has the right to modify the specified
>>>  	 * process. Use the regular "ptrace_may_access()" checks.
>>>  	 */
>>>  	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
>>> -		rcu_read_unlock();
>>>  		mm = ERR_PTR(-EPERM);
>>>  		goto out;
>>>  	}
>>> -	rcu_read_unlock();
>>>  
>>>  	mm = ERR_PTR(security_task_movememory(task));
>>>  	if (IS_ERR(mm))
>> 
>> Digged some history via `git blame`, found that the RCU read lock is
>> extended in the following commit,
>> 
>> "
>> 3268c63eded4612a3d07b56d1e02ce7731e6608e
>> Author:     Christoph Lameter <cl@linux.com>
>> AuthorDate: Wed Mar 21 16:34:06 2012 -0700
>> Commit:     Linus Torvalds <torvalds@linux-foundation.org>
>> CommitDate: Wed Mar 21 17:54:58 2012 -0700
>> 
>> mm: fix move/migrate_pages() race on task struct
>> 
>> Migration functions perform the rcu_read_unlock too early.  As a result
>> the task pointed to may change from under us.  This can result in an oops,
>> as reported by Dave Hansen in https://lkml.org/lkml/2012/2/23/302.
>> 
>> The following patch extend the period of the rcu_read_lock until after the
>> permissions checks are done.  We also take a refcount so that the task
>> reference is stable when calling security check functions and performing
>> cpuset node validation (which takes a mutex).
>> 
>> The refcount is dropped before actual page migration occurs so there is no
>> change to the refcounts held during page migration.
>> 
>> Also move the determination of the mm of the task struct to immediately
>> before the do_migrate*() calls so that it is clear that we switch from
>> handling the task during permission checks to the mm for the actual
>> migration.  Since the determination is only done once and we then no
>> longer use the task_struct we can be sure that we operate on a specific
>> address space that will not change from under us.
>> "
>> 
>> After that, the permission checking has been changed from __task_cred()
>> to ptrace_may_access().  So the situation may change somewhat.  Cced
>
> In ptrace_may_access, __task_cred is access while holding the rcu read lock.
> It seems this is ensured by the ptrace_may_access itself.

Please read the patch above.  Before extending rcu_read_lock protected
region, __task_cred() is protected by rcu_read_lock already.  The patch
above combines 2 regions into 1.

Best Regards,
Huang, Ying

>> some names found in git history to verify.
>
> Thanks for your carefulness.
>
>> 
>> Best Regards,
>> Huang, Ying
>> .
>>