On 19 Dec 2024, at 11:09, Bernd Schubert wrote:
> On 12/19/24 17:02, Zi Yan wrote:
>> On 19 Dec 2024, at 11:00, Zi Yan wrote:
>>> On 19 Dec 2024, at 10:56, Bernd Schubert wrote:
>>>
>>>> On 12/19/24 16:55, Zi Yan wrote:
>>>>> On 19 Dec 2024, at 10:53, Shakeel Butt wrote:
>>>>>
>>>>>> On Thu, Dec 19, 2024 at 04:47:18PM +0100, David Hildenbrand wrote:
>>>>>>> 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@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.
>>>>>>>>
>>>>>>>> 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?
>>>>>>
>>>>>> Oh sorry, my mistake, yes this will become an isolation issue with the
>>>>>> removal of the temp page in-between which this series is doing. I think
>>>>>> the tradeoff is between extra memory plus slow write performance versus
>>>>>> temporary unmovable memory.
>>>>>
>>>>> No, the tradeoff is slow FUSE performance vs whole system slowdown due to
>>>>> memory fragmentation. AS_WRITEBACK_INDETERMINATE indicates it is not
>>>>> temporary.
>>>>
>>>> Is there is a difference between FUSE TMP page being unmovable and
>>>> AS_WRITEBACK_INDETERMINATE folios/pages being unmovable?
>>
>> (Fix my response location)
>>
>> Both are unmovable, but you can control where FUSE TMP page
>> can come from to avoid spread across the entire memory space. For example,
>> allocate a contiguous region as a TMP page pool.
>
> Wouldn't it make sense to have that for fuse writeback pages as well?
> Fuse tries to limit dirty pages anyway.
Can fuse constraint the location of writeback pages? Something like what
I proposed[1], migrating pages to a location before their writeback? Will
that be a performance concern?
In terms of the number of dirty pages, you only need one page out of 512
pages to prevent 2MB THP from allocation. For CMA allocation, one unmovable
page can kill one contiguous range. What is the limit of fuse dirty pages?
[1] https://lore.kernel.org/linux-mm/90C41581-179F-40B6-9801-9C9DBBEB1AF4@xxxxxxxxxx/
--
Best Regards,
Yan, Zi
