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 16:43, Shakeel Butt wrote:
> On Thu, Dec 19, 2024 at 02:05:04PM +0100, 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.
> 
> The page under writeback are already unmovable while they are under
> writeback. This patch is only making potentially unrelated tasks to
> synchronously wait on writeback completion for such pages which in worst
> case can be indefinite. This actually is solving an isolation issue on a
> multi-tenant machine.
> 
Are you sure, because I read in the cover letter:

"In the current FUSE writeback design (see commit 3be5a52b30aa ("fuse: 
support writable mmap"))), a temp page is allocated for every dirty
page to be written back, the contents of the dirty page are copied over 
to the temp page, and the temp page gets handed to the server to write 
back. This is done so that writeback may be immediately cleared on the 
dirty page,"

Which to me means that they are immediately movable again?

-- 
Cheers,

David / dhildenb