[PATCHv2] vfio/type1: conditional rescheduling while pinning

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

A large DMA mapping request can loop through dma address pinning for
many pages. In cases where THP can not be used, the repeated vmf_insert_pfn can
be costly, so let the task reschedule as need to prevent CPU stalls. Failure to
do so has potential harmful side effects, like increased memory pressure
as unrelated rcu tasks are unable to make their reclaim callbacks and
result in OOM conditions.

 rcu: INFO: rcu_sched self-detected stall on CPU
 rcu:   36-....: (20999 ticks this GP) idle=b01c/1/0x4000000000000000 softirq=35839/35839 fqs=3538
 rcu:            hardirqs   softirqs   csw/system
 rcu:    number:        0        107            0
 rcu:   cputime:       50          0        10446   ==> 10556(ms)
 rcu:   (t=21075 jiffies g=377761 q=204059 ncpus=384)
...
  <TASK>
  ? asm_sysvec_apic_timer_interrupt+0x16/0x20
  ? walk_system_ram_range+0x63/0x120
  ? walk_system_ram_range+0x46/0x120
  ? pgprot_writethrough+0x20/0x20
  lookup_memtype+0x67/0xf0
  track_pfn_insert+0x20/0x40
  vmf_insert_pfn_prot+0x88/0x140
  vfio_pci_mmap_huge_fault+0xf9/0x1b0 [vfio_pci_core]
  __do_fault+0x28/0x1b0
  handle_mm_fault+0xef1/0x2560
  fixup_user_fault+0xf5/0x270
  vaddr_get_pfns+0x169/0x2f0 [vfio_iommu_type1]
  vfio_pin_pages_remote+0x162/0x8e0 [vfio_iommu_type1]
  vfio_iommu_type1_ioctl+0x1121/0x1810 [vfio_iommu_type1]
  ? futex_wake+0x1c1/0x260
  x64_sys_call+0x234/0x17a0
  do_syscall_64+0x63/0x130
  ? exc_page_fault+0x63/0x130
  entry_SYSCALL_64_after_hwframe+0x4b/0x53

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
v1->v2:

  Merged up to vfio/next

  Moved the cond_resched() to a more appropriate place within the
  loop, and added a comment about why it's there.

  Update to change log describing one of the consequences of not doing
  this.

 drivers/vfio/vfio_iommu_type1.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 1136d7ac6b597..ad599b1601711 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -647,6 +647,13 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
 
 	while (npage) {
 		if (!batch->size) {
+			/*
+			 * Large mappings may take a while to repeatedly refill
+			 * the batch, so conditionally relinquish the CPU when
+			 * needed to avoid stalls.
+			 */
+			cond_resched();
+
 			/* Empty batch, so refill it. */
 			ret = vaddr_get_pfns(mm, vaddr, npage, dma->prot,
 					     &pfn, batch);
-- 
2.47.1