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?
--
Best Regards,
Yan, Zi
