Re: [PATCH 2/4] mm: gup: add get_user_pages_locked and get_user_pages_unlocked

[Andres Lagar-Cavilla <andreslc@google.com>]

On Wed, Oct 1, 2014 at 8:51 AM, Peter Feiner <pfeiner@google.com> wrote:
> On Wed, Oct 01, 2014 at 10:56:35AM +0200, Andrea Arcangeli wrote:
>> +static inline long __get_user_pages_locked(struct task_struct *tsk,
>> +                                        struct mm_struct *mm,
>> +                                        unsigned long start,
>> +                                        unsigned long nr_pages,
>> +                                        int write, int force,
>> +                                        struct page **pages,
>> +                                        struct vm_area_struct **vmas,
>> +                                        int *locked,
>> +                                        bool notify_drop)
>> +{
>> +     int flags = FOLL_TOUCH;
>> +     long ret, pages_done;
>> +     bool lock_dropped;
>> +
>> +     if (locked) {
>> +             /* if VM_FAULT_RETRY can be returned, vmas become invalid */
>> +             BUG_ON(vmas);
>> +             /* check caller initialized locked */
>> +             BUG_ON(*locked != 1);
>> +     }
>> +
>> +     if (pages)
>> +             flags |= FOLL_GET;
>> +     if (write)
>> +             flags |= FOLL_WRITE;
>> +     if (force)
>> +             flags |= FOLL_FORCE;
>> +
>> +     pages_done = 0;
>> +     lock_dropped = false;
>> +     for (;;) {
>> +             ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
>> +                                    vmas, locked);
>> +             if (!locked)
>> +                     /* VM_FAULT_RETRY couldn't trigger, bypass */
>> +                     return ret;
>> +
>> +             /* VM_FAULT_RETRY cannot return errors */
>> +             if (!*locked) {
>> +                     BUG_ON(ret < 0);
>> +                     BUG_ON(nr_pages == 1 && ret);
>
> If I understand correctly, this second BUG_ON is asserting that when
> __get_user_pages is asked for a single page and it is successfully gets the
> page, then it shouldn't have dropped the mmap_sem. If that's the case, then
> you could generalize this assertion to
>
>                         BUG_ON(nr_pages == ret);

Even more strict:
     BUG_ON(ret >= nr_pages);

Reviewed-by: Andres Lagar-Cavilla <andreslc@google.com>

>
> Otherwise, looks good!
>
> Peter



-- 
Andres Lagar-Cavilla | Google Kernel Team | andreslc@google.com

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