Re: [PATCH] Fix do_move_pages_to_node() error handling

[Piotr Jaroszynski <pjaroszynski@nvidia.com>]

On 11/14/18 1:22 PM, Michal Hocko wrote:
> On Wed 14-11-18 10:04:45, Piotr Jaroszynski wrote:
>> On 11/14/18 3:29 AM, Michal Hocko wrote:
>>> On Wed 14-11-18 08:34:15, Michal Hocko wrote:
>>>> On Tue 13-11-18 16:40:59, p.jaroszynski@gmail.com wrote:
>>>>> From: Piotr Jaroszynski <pjaroszynski@nvidia.com>
>>>>>
>>>>> migrate_pages() can return the number of pages that failed to migrate
>>>>> instead of 0 or an error code. If that happens, the positive return is
>>>>> treated as an error all the way up through the stack leading to the
>>>>> move_pages() syscall returning a positive number. I believe this
>>>>> regressed with commit a49bd4d71637 ("mm, numa: rework do_pages_move")
>>>>> that refactored a lot of this code.
>>>>
>>>> Yes this is correct.
>>>>
>>>>> Fix this by treating positive returns as success in
>>>>> do_move_pages_to_node() as that seems to most closely follow the
>>>>> previous code. This still leaves the question whether silently
>>>>> considering this case a success is the right thing to do as even the
>>>>> status of the pages will be set as if they were successfully migrated,
>>>>> but that seems to have been the case before as well.
>>>>
>>>> Yes, I believe the previous semantic was just wrong and we want to fix
>>>> it. Jan has already brought this up [1]. I believe we want to update the
>>>> documentation rather than restore the previous hazy semantic.
>>
>> That's probably fair although at least some code we have will have to be
>> updated as it just checks for non-zero returns from move_pages() and
>> assumes errno is set when that happens.
> 
> Can you tell me more about your usecase plase? I definitely do not want
> to break any existing userspace. Making the syscall return code more
> reasonable is still attractive. So if this new semantic can work better
> for you it would be one argument more to keep it this way.
>  

One of our APIs exposes a way to move a VA range to a GPU NUMA node or one of
the CPU NUMA nodes. The code keeps retrying move_pages() and relies on
the reported page status to decide whether each page is done, needs a
retry (EAGAIN or EBUSY) or possibly needs a fallback (EMEM).

With the previous behaviour we would get a success, but the page status
would be reported incorrectly. That's bad as we skip the migration
without knowing about it.

With the current code we get what we interpret as success as errno is 0,
but the page status is gargabe/untouched. That's also bad.

The proposed solution adds a new case to handle, but it will just tell
us the page status is unusable and all we can do is just retry blindly.
If it was possible to plumb through the migration status for each page
accurately that would allow us to save redoing the call for pages that
actually worked. Perhaps we would need a special status for pages
skipped due to errors.

But maybe this is all a tiny corner case short of the bug I hit (see
more below) and it's not worth thinking too much about.

>>>> Just wondering, how have you found out? Is there any real application
>>>> failing because of the change or this is a result of some test?
>>
>> I have a test that creates a tmp file, mmaps it as shared, memsets the
>> memory and then attempts to move it to a different node. It used to
>> work, but now fails. I suspect the filesystem's migratepage() callback
>> regressed and will look into it next. So far I have only tested this on
>> powerpc with the xfs filesystem.
> 
> I would be surprise if the rewor changed the migration behavior.

It didn't, I tracked it down to the new fs/iomap.c code used by xfs not
being compatible with migrate_page_move_mapping() and prepared a perhaps
naive fix in [1].

> 
> [...]
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index f7e4bfdc13b7..aa53ebc523eb 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1615,8 +1615,16 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>>>  			goto out_flush;
>>>  
>>>  		err = do_move_pages_to_node(mm, &pagelist, current_node);
>>> -		if (err)
>>> +		if (err) {
>>> +			/*
>>> +			 * Possitive err means the number of failed pages to
>>> +			 * migrate. Make sure to report the rest of the
>>> +			 * nr_pages is not migrated as well.
>>> +			 */
>>> +			if (err > 0)
>>> +				err += nr_pages - i - 1;
>>>  			goto out;
>>
>> Ok, so we give up after the first failure to migrate everything. That
>> probably makes sense although I don't have a good idea about how
>> frequent it is for the migration to give up in such a manner (short of
>> the issue I'm seeing that I suspect is a separate bug). In this case,
>> should the status of each page be updated to something instead of being
>> left undefined? Or should it be specified that page status is only valid
>> for the first N - not migrated pages?
> 
> I believe this is consistent with the previous behavior. I do not
> remember in detail but I believe we haven't set the status for the
> remaining pages. With this patch it seems straightforward to skip over
> exact number of pages that failed.
> 

Yeah, I was just wondering whether the semantics could be improved
further, not arguing that the previous behaviour was any better.

[1] - https://lore.kernel.org/lkml/20181115003000.1358007-1-pjaroszynski@nvidia.com/

Thanks,
Piotr