Messages in this thread | | | Date | Fri, 21 Oct 2022 14:15:26 +0800 | Subject | Re: [PATCH 2/2] mm: migrate: Try again if THP split is failed due to page refcnt | From | Baolin Wang <> |
| |
On 10/21/2022 3:21 AM, Yang Shi wrote: > On Thu, Oct 20, 2022 at 2:33 AM Baolin Wang > <baolin.wang@linux.alibaba.com> wrote: >> >> >> >> 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. > > You may try to trace all migrations across your fleet (or just pick > some sample machines, this should make data analysis easier) and > filter the migration by reasons, for example, MR_LONGTERM_PIN, then > compare the migration success rate before and after the patch. It > should be a good justification. But it may need some work on data > aggregation, process and analysis, not sure how feasible it is.
IMO the migration of MR_LONGTERM_PIN is very rare in this case, so we can obeserve the migraion failure of longterm pin, once obeserved, the application will be aborted. However like I said before, the problem is not easy to reproduce :(
Anyway we'll test this 2 patches on our products.
>>>> [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.
| |