Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings

[David Hildenbrand <david@redhat.com>]

On 19.12.24 16:08, Zi Yan wrote:
> On 19 Dec 2024, at 9:19, Zi Yan wrote:
> 
>> On 19 Dec 2024, at 8:05, David Hildenbrand wrote:
>>
>>> On 23.11.24 00:23, Joanne Koong wrote:
>>>> For migrations called in MIGRATE_SYNC mode, skip migrating the folio if
>>>> it is under writeback and has the AS_WRITEBACK_INDETERMINATE flag set on its
>>>> mapping. If the AS_WRITEBACK_INDETERMINATE flag is set on the mapping, the
>>>> writeback may take an indeterminate amount of time to complete, and
>>>> waits may get stuck.
>>>>
>>>> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
>>>> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
>>>> ---
>>>>    mm/migrate.c | 5 ++++-
>>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index df91248755e4..fe73284e5246 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1260,7 +1260,10 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>>>    		 */
>>>>    		switch (mode) {
>>>>    		case MIGRATE_SYNC:
>>>> -			break;
>>>> +			if (!src->mapping ||
>>>> +			    !mapping_writeback_indeterminate(src->mapping))
>>>> +				break;
>>>> +			fallthrough;
>>>>    		default:
>>>>    			rc = -EBUSY;
>>>>    			goto out;
>>>
>>> Ehm, doesn't this mean that any fuse user can essentially completely block CMA allocations, memory compaction, memory hotunplug, memory poisoning... ?!
>>>
>>> That sounds very bad.
>>
>> Yeah, these writeback folios become unmovable. It makes memory fragmentation
>> unrecoverable. I do not know why AS_WRITEBACK_INDETERMINATE is allowed, since
>> it is essentially a forever pin to writeback folios. Why not introduce a
>> retry and timeout mechanism instead of waiting for the writeback forever?
> 
> If there is no way around such indeterminate writebacks, to avoid fragment memory,
> these to-be-written-back folios should be migrated to a physically contiguous region. Either you have a preallocated region or get free pages from MIGRATE_UNMOVABLE.

But at what point?

We surely don't want to make fuse consume only effectively-unmovable memory.

-- 
Cheers,

David / dhildenb