Re: [PATCH v4 2/2] mm/gup.c: Refactor check_and_migrate_movable_pages()

[Alistair Popple <apopple@nvidia.com>]

Jason Gunthorpe <jgg@nvidia.com> writes:

> On Fri, Aug 12, 2022 at 12:13:09PM +1000, Alistair Popple wrote:
>> When pinning pages with FOLL_LONGTERM check_and_migrate_movable_pages()
>> is called to migrate pages out of zones which should not contain any
>> longterm pinned pages.
>>
>> When migration succeeds all pages will have been unpinned so pinning
>> needs to be retried. Migration can also fail, in which case the pages
>> will also have been unpinned but the operation should not be retried. If
>> all pages are in the correct zone nothing will be unpinned and no retry
>> is required.
>>
>> The logic in check_and_migrate_movable_pages() tracks unnecessary state
>> and the return codes for each case are difficult to follow. Refactor the
>> code to clean this up. No behaviour change is intended.
>>
>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>>
>> ---
>
> This seems like the cleanest version yet!

Thanks, the feedback from John and yourself has been very useful!

>> +static long check_and_migrate_movable_pages(unsigned long nr_pages,
>> +					    struct page **pages)
>> +{
>> +	int ret;
>> +	unsigned long collected;
>> +	LIST_HEAD(movable_page_list);
>> +
>> +	collected = collect_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);
>> +	if (!collected)
>> +		return 0;
>> +
>> +	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);
>> +	if (!ret)
>> +		return -EAGAIN;
>> +	else
>> +		return ret;
>
> I would drop the else path and just return zero

Unless I've misunderstood you I don't think that's correct though.
check_and_migrate_movable_pages() needs to return one of 3 conditions:

 - 0: All pages are in the correct zone and are still pinned (ie. "success").
 - -EAGAIN: Some pages were migrated, all pages need re-pinning.
 - errno: Migration failed, pins were dropped and PUP should fail.

John's suggested comment update spells this out more clearly.

> Arguably migrate_longterm_unpinnable_pages() should do the same?

migrate_longterm_unpinnable_pages() returns 0 for success, errno for
some kind of "permanent" failure that needs propagating.

>> @@ -2051,10 +2079,10 @@ static long __gup_longterm_locked(struct mm_struct *mm,
>>  			break;
>>
>>  		rc = check_and_migrate_movable_pages(rc, pages);
>> -	} while (!rc);
>> +	} while (rc == -EAGAIN);
>
> Since the only reader only cares about errno or not errno..

See above. Errors other than -EAGAIN should not be retried.

> But no biggie either way

So will leave this as for now at least.

> Thanks,
> Jason