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    SubjectRe: [patch] mm, hugetlb: add thread name and pid to SHM_HUGETLB mlock rlimit warning
    On Thu, 8 Mar 2012 13:37:57 -0800 (PST)
    David Rientjes <rientjes@google.com> wrote:

    > On Thu, 8 Mar 2012, Andrew Morton wrote:
    >
    > > > --- a/fs/hugetlbfs/inode.c
    > > > +++ b/fs/hugetlbfs/inode.c
    > > > @@ -946,7 +946,11 @@ struct file *hugetlb_file_setup(const char *name, size_t size,
    > > > if (creat_flags == HUGETLB_SHMFS_INODE && !can_do_hugetlb_shm()) {
    > > > *user = current_user();
    > > > if (user_shm_lock(size, *user)) {
    > > > - printk_once(KERN_WARNING "Using mlock ulimits for SHM_HUGETLB is deprecated\n");
    > > > + task_lock(current);
    > > > + printk_once(KERN_WARNING
    > > > + "%s (%d): Using mlock ulimits for SHM_HUGETLB is deprecated\n",
    > > > + current->comm, current->pid);
    > > > + task_unlock(current);
    > >
    > > I assume the task_lock() is there to protect current->comm.
    >
    > Yup.
    >
    > > If so, it
    > > is unneeded - we're protecting against prctl(PR_SET_NAME), and
    > > PR_SET_NAME only operates on current, and we know this task isn't
    > > currently running PR_SET_NAME.
    > >
    > > If there's a way for another task to alter this task's ->comm then we
    > > _do_ need locking. But there isn't a way, I hope.
    > >
    >
    > I wish there wasn't as well, it would prevent a lot of the currently buggy
    > reads to current->comm and allow us to avoid so many otherwise pointless
    > task_lock()s.
    >
    > This protects against /proc/pid/comm, which is writable by threads in the
    > same thread group.

    Oh crap.

    > We have a get_task_comm() that does the task_lock()
    > internally but requires a TASK_COMM_LEN buffer in the calling code. It's
    > just easier for the calling code to the task_lock() itself for a tiny
    > little printk().

    Well for a tiny little printk we could just omit the locking? The
    printk() won't oops and once in a million years one person will see a
    garbled comm[] string?



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