On 2019/10/21 20:57, Vlastimil Babka wrote:
> On 10/17/19 10:42 PM, 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.
> Agreed.
>
>>> Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Signed-off-by: zhong jiang <zhongjiang@xxxxxxxxxx>
>>> ---
>>> 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:
> Also agreed. Zhong, can you resend it like that?
Ok, I will resend it right now. Thanks
Sincerely,
zhong jiang
>> 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;
>>
>> 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) {
>> + if ((ret > 0) && migrate_allow) {
>> + nr_pages = ret;
>> drain_allow = true;
>> goto check_again;
>> }
>> }
>>
>> - return nr_pages;
>> + return ret;
>> }
>> #else
>> static long check_and_migrate_cma_pages(struct task_struct *tsk,
>>
>>
>> thanks,
>>
>> John Hubbard
>> NVIDIA
>>
>
> .
>
