From: Khalid Aziz <khalid.aziz@oracle.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
peterz@infradead.org, akpm@linux-foundation.org,
andi.kleen@intel.com, rob@landley.net, viro@zeniv.linux.org.uk,
venki@google.com, linux-kernel@vger.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace
Date: Tue, 04 Mar 2014 10:44:54 -0700 [thread overview]
Message-ID: <53161116.9050109@oracle.com> (raw)
In-Reply-To: <20140304135624.GA6846@redhat.com>
Thanks for the review. Please see my comments inline below.
On 03/04/2014 06:56 AM, Oleg Nesterov wrote:
> On 03/03, Khalid Aziz wrote:
>> kernel/sched/preempt_delay.c | 39 ++++++
>
> Why? This can go into proc/ as well.
>
Sure. No strong reason to keep these functions in separate file. These
functions can go into proc/fs/base.c.
>> +static void
>> +close_preempt_delay_vmops(struct vm_area_struct *vma)
>> +{
>> + struct preemp_delay_mmap_state *state;
>> +
>> + state = (struct preemp_delay_mmap_state *) vma->vm_private_data;
>> + BUG_ON(!state || !state->task);
>> +
>> + state->page->mapping = NULL;
>> + /* point delay request flag pointer back to old flag in task_struct */
>> + state->task->sched_preempt_delay.delay_req =
>> + &state->task->sched_preempt_delay.delay_flag;
>> + state->task->sched_preempt_delay.mmap_state = NULL;
>> + vfree(state->kaddr);
>> + kfree(state);
>> + vma->vm_private_data = NULL;
>> +}
>
> Suppose that state->task != current. Then this can race with do_exit()
> which cleanups ->mmap_state too. OTOH do_exit() unmaps this region, it
> is not clear why it can't rely in vm_ops->close().
>
> Hmm. In fact I think do_exit() should crash after munmap? ->mmap_state
> should be NULL ?? Perhaps I misread this patch completely...
do_exit() unmaps mmap_state->uaddr, and frees up mmap_state->kaddr and
mmap_state. mmap_state should not be NULL after unmap. vfree() and
kfree() are tolerant of pointers that have already been freed. On the
other hand mmap_state can be NULL in do_exit() if do_exit() and
close_preempt_delay_vmops() were to race since
close_preempt_delay_vmops() sets mmap_state to NULL just before it frees
it up. Could they indeed race, because the thread happens to be killed
just as it had called munmap()? I can protect against that with a
refcount in mmap_state. Do you feel this is necessary/helpful to do?
>
>> +static int
>> +tid_preempt_delay_mmap(struct file *file, struct vm_area_struct *vma)
>> +{
>> + int retval = 0;
>> + void *kaddr = NULL;
>> + struct preemp_delay_mmap_state *state = NULL;
>> + struct inode *inode = file_inode(file);
>> + struct task_struct *task;
>> + struct page *page;
>> +
>> + /*
>> + * Validate args:
>> + * - Only offset 0 support for now
>> + * - size should be PAGE_SIZE
>> + */
>> + if (vma->vm_pgoff != 0 || (vma->vm_end - vma->vm_start) != PAGE_SIZE) {
>> + retval = -EINVAL;
>> + goto error;
>> + }
>> +
>> + /*
>> + * Only one mmap allowed at a time
>> + */
>> + if (current->sched_preempt_delay.mmap_state != NULL) {
>> + retval = -EEXIST;
>> + goto error;
>
> This assumes that we are going to setup current->sched_preempt_delay.mmap_state,
> but what if the task opens /proc/random_tid/sched_preempt_delay ?
Good point. A thread should not be allowed to request preemption delay
for another thread. I would recommend leaving this code alone and adding
following code before this:
if (get_proc_task(inode) != current) {
retval = -EPERM;
goto error;
}
Sounds reasonable?
>
>> + state = kzalloc(sizeof(struct preemp_delay_mmap_state), GFP_KERNEL);
>> + kaddr = vmalloc_user(PAGE_SIZE);
>
> Why vmalloc() ? We only need a single page?
>
Makes sense. I will switch to get_zeroed_page().
>> + task = get_proc_task(inode);
>
> And it seems that nobody does put_task_struct(state->task);
Good catch. I had caught the other two instances of get_proc_task() but
missed this one.
>
>> + state->page = page;
>> + state->kaddr = kaddr;
>> + state->uaddr = (void *)vma->vm_start;
>
> This is used by do_exit(). But ->vm_start can be changed by mremap() ?
>
>
> Hmm. And mremap() can do vm_ops->close() too. But the new vma will
> have the same vm_ops/vm_private_data, so exit_mmap() will try to do
> this again... Perhaps I missed something, but I bet this all can't be
> right.
Would you say sys_munmap() of mmap_state->uaddr is not even needed since
exit_mm() will do this any way further down in do_exit()? If I were to
remove this sys_munmap(), that could simplify the race issues as well.
>
>> + state->task = task;
>> +
>> + /* Clear the current delay request flag */
>> + task->sched_preempt_delay.delay_flag = 0;
>> +
>> + /* Point delay request flag pointer to the newly allocated memory */
>> + task->sched_preempt_delay.delay_req = (unsigned char *)kaddr;
>> +
>> + task->sched_preempt_delay.mmap_state = state;
>> + vma->vm_private_data = state;
>> + vma->vm_ops = &preempt_delay_vmops;
>> + vma->vm_flags |= VM_DONTCOPY | VM_DONTEXPAND | VM_SHARED | VM_WRITE;
>
> This probably also needs VM_IO, to protect from madvise(MADV_DOFORK).
Yes, you are right. I will add that.
> VM_SHARED/VM_WRITE doesn't look right.
VM_SHARED is wrong but VM_WRITE is needed I think since the thread will
write to the mmap'd page to signal to request preemption delay.
>
> Oleg.
>
I appreciate your taking the time to review this code. Thank you very much.
--
Khalid
next prev parent reply other threads:[~2014-03-04 17:45 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-03 18:07 [RFC] [PATCH] Pre-emption control for userspace Khalid Aziz
2014-03-03 21:51 ` Davidlohr Bueso
2014-03-03 23:29 ` Khalid Aziz
2014-03-04 13:56 ` Oleg Nesterov
2014-03-04 17:44 ` Khalid Aziz [this message]
2014-03-04 18:38 ` Al Viro
2014-03-04 19:01 ` Khalid Aziz
2014-03-04 19:03 ` Oleg Nesterov
2014-03-04 20:14 ` Khalid Aziz
2014-03-05 14:38 ` Oleg Nesterov
2014-03-05 16:12 ` Oleg Nesterov
2014-03-05 17:10 ` Khalid Aziz
2014-03-04 21:12 ` H. Peter Anvin
2014-03-04 21:39 ` Khalid Aziz
2014-03-04 22:23 ` One Thousand Gnomes
2014-03-04 22:44 ` Khalid Aziz
2014-03-05 0:39 ` Thomas Gleixner
2014-03-05 0:51 ` Andi Kleen
2014-03-05 11:10 ` Peter Zijlstra
2014-03-05 17:29 ` Khalid Aziz
2014-03-05 19:58 ` Khalid Aziz
2014-03-06 9:57 ` Peter Zijlstra
2014-03-06 16:08 ` Khalid Aziz
2014-03-06 11:14 ` Thomas Gleixner
2014-03-06 16:32 ` Khalid Aziz
2014-03-05 14:54 ` Oleg Nesterov
2014-03-05 15:56 ` Andi Kleen
2014-03-05 16:36 ` Oleg Nesterov
2014-03-05 17:22 ` Khalid Aziz
2014-03-05 23:13 ` David Lang
2014-03-05 23:48 ` Khalid Aziz
2014-03-05 23:56 ` H. Peter Anvin
2014-03-06 0:02 ` Khalid Aziz
2014-03-06 0:13 ` H. Peter Anvin
2014-03-05 23:59 ` David Lang
2014-03-06 0:17 ` Khalid Aziz
2014-03-06 0:36 ` David Lang
2014-03-06 1:22 ` Khalid Aziz
2014-03-06 14:23 ` David Lang
2014-03-06 12:13 ` Kevin Easton
2014-03-06 13:59 ` Peter Zijlstra
2014-03-06 22:41 ` Andi Kleen
2014-03-06 14:25 ` David Lang
2014-03-06 16:12 ` Khalid Aziz
2014-03-06 13:24 ` Rasmus Villemoes
2014-03-06 13:34 ` Peter Zijlstra
2014-03-06 13:45 ` Rasmus Villemoes
2014-03-06 14:02 ` Peter Zijlstra
2014-03-06 14:33 ` Thomas Gleixner
2014-03-06 14:34 ` H. Peter Anvin
2014-03-06 14:04 ` Thomas Gleixner
2014-03-25 17:17 ` [PATCH v2] " Khalid Aziz
2014-03-25 17:44 ` Andrew Morton
2014-03-25 17:56 ` Khalid Aziz
2014-03-25 18:14 ` Andrew Morton
2014-03-25 17:46 ` Oleg Nesterov
2014-03-25 17:59 ` Khalid Aziz
2014-03-25 18:20 ` Andi Kleen
2014-03-25 18:47 ` Khalid Aziz
2014-03-25 19:47 ` Andi Kleen
2014-03-25 18:59 ` Eric W. Biederman
2014-03-25 19:15 ` Khalid Aziz
2014-03-25 20:31 ` Eric W. Biederman
2014-03-25 21:37 ` Khalid Aziz
2014-03-26 6:03 ` Mike Galbraith
2014-03-25 23:01 ` [RFC] [PATCH] " Davidlohr Bueso
2014-03-25 23:29 ` Khalid Aziz
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