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@xxxxxxxxx>
>>> Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
>>> ---
>>> 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.
--
Best Regards,
Yan, Zi
