Re: [PATCH v2 2/2] mm: migrate: Try again if THP split is failed due to page refcnt

["Huang, Ying" <ying.huang@intel.com>]

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].
>
> [1] https://lore.kernel.org/all/470dc638-a300-f261-94b4-e27250e42f96@redhat.com/
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Best Regards,
Huang, Ying

> ---
> Changes from v1:
> - Use another variable to save the return value of THP split.
> ---
>  mm/huge_memory.c |  4 ++--
>  mm/migrate.c     | 19 ++++++++++++++++---
>  2 files changed, 18 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 1da0dbc..6d49a3e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1506,9 +1506,22 @@ 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) {
> +						int ret = try_split_thp(page, &thp_split_pages);
> +
> +						if (!ret) {
> +							nr_thp_split++;
> +							break;
> +						} else if (reason == MR_LONGTERM_PIN &&
> +							   ret == -EAGAIN) {
> +							/*
> +							 * Try again to split THP to mitigate
> +							 * the failure of longterm pinning.
> +							 */
> +							thp_retry++;
> +							nr_retry_pages += nr_subpages;
> +							break;
> +						}
>  					}
>  				} else if (!no_subpage_counting) {
>  					nr_failed++;