[PATCH 0/1] mm/gup: finish consolidating error handling

[PATCH 0/1] mm/gup: finish consolidating error handling

[john.hubbard]

From: John Hubbard <jhubbard@nvidia.com>

Hi,

Keith Busch and Dan Williams noticed that this patch
(which was part of my RFC[1] for the get_user_pages + DMA
fix) also fixes a bug. Accordingly, I'm adjusting
the changelog and posting this as it's own patch.

[1] https://lkml.kernel.org/r/20181110085041.10071-1-jhubbard@nvidia.com

John Hubbard (1):
  mm/gup: finish consolidating error handling

 mm/gup.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

-- 
2.19.1

[PATCH] mm/gup: finish consolidating error handling

[john.hubbard]

From: John Hubbard <jhubbard@nvidia.com>

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.

Fixes: df06b37ffe5a4 ("mm/gup: cache dev_pagemap while pinning pages")

Reviewed-by: Keith Busch <keith.busch@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index aa43620a3270..8cb68a50dbdf 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -702,12 +702,11 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		if (!vma || start >= vma->vm_end) {
 			vma = find_extend_vma(mm, start);
 			if (!vma && in_gate_area(mm, start)) {
-				int ret;
 				ret = get_gate_page(mm, start & PAGE_MASK,
 						gup_flags, &vma,
 						pages ? &pages[i] : NULL);
 				if (ret)
-					return i ? : ret;
+					goto out;
 				ctx.page_mask = 0;
 				goto next_page;
 			}
-- 
2.19.1

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

[Andrew Morton]

On Wed, 21 Nov 2018 00:14:02 -0800 john.hubbard@gmail.com 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?

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@gmail.com 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