Re: [PATCH 2/2] mm: migrate: Try again if THP split is failed due to page refcnt

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/20/2022 4:24 PM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> When creating a virtual machine, we will use memfd_create() to get
>> a file descriptor which can be used to create share memory mappings
>> using the mmap function, meanwhile the mmap() will set the MAP_POPULATE
>> flag to allocate physical pages for the virtual machine.
>>
>> When allocating physical pages for the guest, the host can fallback to
>> allocate some CMA pages for the guest when over half of the zone's free
>> memory is in the CMA area.
>>
>> In guest os, when the application wants to do some data transaction with
>> DMA, our QEMU will call VFIO_IOMMU_MAP_DMA ioctl to do longterm-pin and
>> create IOMMU mappings for the DMA pages. However, when calling
>> VFIO_IOMMU_MAP_DMA ioctl to pin the physical pages, we found it will be
>> failed to longterm-pin sometimes.
>>
>> After some invetigation, we found the pages used to do DMA mapping can
>> contain some CMA pages, and these CMA pages will cause a possible
>> failure of the longterm-pin, due to failed to migrate the CMA pages.
>> The reason of migration failure may be temporary reference count or
>> memory allocation failure. So that will cause the VFIO_IOMMU_MAP_DMA
>> ioctl returns error, which makes the application failed to start.
>>
>> I observed one migration failure case (which is not easy to reproduce) is
>> that, the 'thp_migration_fail' count is 1 and the 'thp_split_page_failed'
>> count is also 1.
>>
>> That means when migrating a THP which is in CMA area, but can not allocate
>> a new THP due to memory fragmentation, so it will split the THP. However
>> THP split is also failed, probably the reason is temporary reference count
>> of this THP. And the temporary reference count can be caused by dropping
>> page caches (I observed the drop caches operation in the system), but we
>> can not drop the shmem page caches due to they are already dirty at that time.
>>
>> Especially for THP split failure, which is caused by temporary reference
>> count, we can try again to mitigate the failure of migration in this case
>> according to previous discussion [1].
> 
> Does the patch solved your problem?

The problem is not easy to reproduce and I will test this patch on our 
products. However I think this is a likely case to fail the migration, 
which need to be addressed to mitigate the failure.

>> [1] https://lore.kernel.org/all/470dc638-a300-f261-94b4-e27250e42f96@redhat.com/
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/huge_memory.c |  4 ++--
>>   mm/migrate.c     | 18 +++++++++++++++---
>>   2 files changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ad17c8d..a79f03b 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2666,7 +2666,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>   	 * split PMDs
>>   	 */
>>   	if (!can_split_folio(folio, &extra_pins)) {
>> -		ret = -EBUSY;
>> +		ret = -EAGAIN;
>>   		goto out_unlock;
>>   	}
>>   
>> @@ -2716,7 +2716,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>   			xas_unlock(&xas);
>>   		local_irq_enable();
>>   		remap_page(folio, folio_nr_pages(folio));
>> -		ret = -EBUSY;
>> +		ret = -EAGAIN;
>>   	}
>>   
>>   out_unlock:
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 8e5eb6e..55c7855 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1506,9 +1506,21 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>   				if (is_thp) {
>>   					nr_thp_failed++;
>>   					/* THP NUMA faulting doesn't split THP to retry. */
>> -					if (!nosplit && !try_split_thp(page, &thp_split_pages)) {
>> -						nr_thp_split++;
>> -						break;
>> +					if (!nosplit) {
>> +						rc = try_split_thp(page, &thp_split_pages);
>> +						if (!rc) {
>> +							nr_thp_split++;
>> +							break;
>> +						} else if (reason == MR_LONGTERM_PIN &&
>> +							   rc == -EAGAIN) {
> 
> In case reason != MR_LONGTERM_PIN, you change the return value of
> migrate_pages().  So you need to use another variable for return value.

Good catch, will fix in next version. Thanks for your comments.