Re: [PATCH v1 09/10] mm/memory_hotplug: teach offline_pages() to not try forever

[David Hildenbrand]

On 24.05.2018 16:39, Michal Hocko wrote:
> [I didn't really go through other patch but this one caught my eyes just
>  because of the similar request proposed yesterday]
> 
> On Wed 23-05-18 17:11:50, David Hildenbrand wrote:
> [...]
>> @@ -1686,6 +1686,10 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>  	pfn = scan_movable_pages(start_pfn, end_pfn);
>>  	if (pfn) { /* We have movable pages */
>>  		ret = do_migrate_range(pfn, end_pfn);
>> +		if (ret && !retry_forever) {
>> +			ret = -EBUSY;
>> +			goto failed_removal;
>> +		}
>>  		goto repeat;
>>  	}
>>  
> 
> Btw. this will not work in practice. Even a single temporary pin on a page
> will fail way too easily. If you really need to control this then make
> it a retry counter with default -1UL.

Interestingly, this will work for the one specific use case that I am
using this interface for right now.

The reason is that I don't want to offline a specific chunk, I want to
find some chunks to offline (in contrast to e.g. DIMMs where you rely
try to offline a very specific one).

If I get a failure on that chunk (e.g. temporary pin) I will retry the
next chunk. At one point, I will eventually retry this chunk and then it
succeeds.

> 
> We really do need a better error reporting from do_migrate_range and
> distinguish transient from permanent failures. In general we shouldn't
> even get here for pages which are not migrateable...

I totally agree, I also want to know if an error is permanent or
transient - and I want the posibility to "fail fast" (e.g. -EAGAIN)
instead of looping forever.


-- 

Thanks,

David / dhildenb