Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero

[zhong jiang <zhongjiang@huawei.com>]

On 2019/10/19 0:07, Alexander Duyck wrote:
> On Fri, Oct 18, 2019 at 7:00 AM zhong jiang <zhongjiang@huawei.com> wrote:
>> On 2019/10/18 4:42, John Hubbard wrote:
>>> On 10/16/19 8:19 PM, zhong jiang wrote:
>>>> With the help of unsigned_lesser_than_zero.cocci. Unsigned 'nr_pages"'
>>>> compare with zero. And __get_user_pages_locked will return an long value.
>>>>
>>>> The patch use a new local variable to store the return value of
>>>> __get_user_pages_locked(). Then use it to compare with zero.
>>> Hi Zhong,
>>>
>>> The above are actually more like notes to yourself, and while those are
>>> good to have, but it's not yet a perfect commit description: it talks
>>> about how you got here, which is only part of the story. A higher level
>>> summary is better, and let the code itself cover the details.
>>>
>>> Like this:
>>>
>>> First line (subject):
>>>
>>>    mm/gup: allow CMA migration to propagate errors back to caller
>>>
>>> Commit description:
>>>
>>> check_and_migrate_cma_pages() was recording the result of
>>> __get_user_pages_locked() in an unsigned "nr_pages" variable. Because
>>> __get_user_pages_locked() returns a signed value that can include
>>> negative errno values, this had the effect of hiding errors.
>>>
>>> Change check_and_migrate_cma_pages() implementation so that it
>>> uses a signed variable instead, and propagates the results back
>>> to the caller just as other gup internal functions do.
>>>
>>> This was discovered with the help of unsigned_lesser_than_zero.cocci.
>>>
>>>> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
>>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>>>> ---
>>>>  mm/gup.c | 8 +++++---
>>>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>> index 8f236a3..1fe0ceb 100644
>>>> --- a/mm/gup.c
>>>> +++ b/mm/gup.c
>>>> @@ -1443,6 +1443,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>>>>      bool drain_allow = true;
>>>>      bool migrate_allow = true;
>>>>      LIST_HEAD(cma_page_list);
>>>> +    long ret;
>>> Ira pointed out that this needs initialization, see below.
>>>
>>>>  check_again:
>>>>      for (i = 0; i < nr_pages;) {
>>>> @@ -1504,17 +1505,18 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>>>>               * again migrating any new CMA pages which we failed to isolate
>>>>               * earlier.
>>>>               */
>>>> -            nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages,
>>>> +            ret = __get_user_pages_locked(tsk, mm, start, nr_pages,
>>>>                                                 pages, vmas, NULL,
>>>>                                                 gup_flags);
>>>>
>>>> -            if ((nr_pages > 0) && migrate_allow) {
>>>> +            nr_pages = ret;
>>> Although technically correct, it makes me feel odd to see the assignment
>>> done from signed to unsigned, *before* checking for >0. And Ira is hinting
>>> at the same thing, when he asks if we can return early here. See below...
>>>
>>>> +            if ((ret > 0) && migrate_allow) {
>>>>                      drain_allow = true;
>>>>                      goto check_again;
>>>>              }
>>>>      }
>>>>
>>>> -    return nr_pages;
>>>> +    return ret;
>>>>  }
>>>>  #else
>>>>  static long check_and_migrate_cma_pages(struct task_struct *tsk,
>>>>
>>> So, overall, I'd recommend this, instead:
>>>
>>> diff --git a/mm/gup.c b/mm/gup.c
>>> index 23a9f9c9d377..72bc027037fa 100644
>>> --- a/mm/gup.c
>>> +++ b/mm/gup.c
>>> @@ -1443,6 +1443,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk,
>>>         bool drain_allow = true;
>>>         bool migrate_allow = true;
>>>         LIST_HEAD(cma_page_list);
>>> +       long ret = nr_pages;
>>>
>> I think the ret should be assigned as zero. we will return ret if cma_page_list is empty.
>> I miss something?
> That wasn't the behavior before. Before if the cma_page_list was empty
> nr_pages would not be modified from what what passed. I believe that
> is the intended behavior for this since the cma_page_list seems like a
> workaround to migrate out any CMA pages if present in the longterm
> mapping. If there are no CMA pages the return result should be
> nr_pages.
Thanks for your clarification.

Sincerely,
zhong jiang
> Thanks.
>
> - Alex
>
>
> .
>