Re: [PATCH] mm/gup: finish consolidating error handling

[John Hubbard]

On 11/21/18 2:44 PM, Andrew Morton wrote:
> On Wed, 21 Nov 2018 00:14:02 -0800 john.hubbard@xxxxxxxxx wrote:
> 
>> Commit df06b37ffe5a4 ("mm/gup: cache dev_pagemap while pinning pages")
>> attempted to operate on each page that get_user_pages had retrieved. In
>> order to do that, it created a common exit point from the routine.
>> However, one case was missed, which this patch fixes up.
>>
>> Also, there was still an unnecessary shadow declaration (with a
>> different type) of the "ret" variable, which this patch removes.
>>
> 
> What is the bug which this supposedly fixes and what is that bug's
> user-visible impact?
> 

Keith's description of the situation is:

  This also fixes a potentially leaked dev_pagemap reference count if a
  failure occurs when an iteration crosses a vma boundary. I don't think
  it's normal to have different vma's on a users mapped zone device memory,
  but good to fix anyway.

I actually thought that this code:

    /* first iteration or cross vma bound */
    if (!vma || start >= vma->vm_end) {
        vma = find_extend_vma(mm, start);
        if (!vma && in_gate_area(mm, start)) {
            ret = get_gate_page(mm, start & PAGE_MASK,
                    gup_flags, &vma,
                    pages ? &pages[i] : NULL);
            if (ret)
                goto out;

...dealt with the "you're trying to pin the gate page, as part of this call",
rather than the generic case of crossing a vma boundary. (I think there's a fine
point that I must be overlooking.) But it's still a valid case, either way.

-- 
thanks,
John Hubbard
NVIDIA