[PATCH v2] vfio/type1: avoid walking reserved-only mappings while unpinning

[PATCH v2] vfio/type1: avoid walking reserved-only mappings while unpinning

[Samuel Crossley]

Tearing down a large device-passthrough DMA mapping can process tens of
millions of reserved pfns in a single VFIO_IOMMU_UNMAP_DMA.  A 128 GiB
reserved mapping holds about 33.6 million 4KiB pfns, and
vfio_unpin_pages_remote() walks every one of them whenever dma->has_rsvd
is set, because reserved and non-reserved pfns might be mixed.  For a
mapping that contains only reserved pfns every put_pfn() is a no-op, but
each one still pays for the pfn_valid()/PageReserved() classification.

Seen on GPU-passthrough hosts running 6.13 with the v6.18 type1 series
backported, where a single VFIO_IOMMU_UNMAP_DMA held a CPU past the
soft-lockup watchdog (host identification, register dump and module list
trimmed):

 watchdog: BUG: soft lockup - CPU#261 stuck for 22s! [qemu-system-x86:134503]
 RIP: 0010:vfio_unpin_pages_remote+0x139/0x2a0 [vfio_iommu_type1]
 Code: 90 4c 89 e1 48 c1 e9 34 75 74 4c 89 e1 48 c1 e9 22 75 6b 48 8b 0d f7 6f c2 e3 48 85 c9 74 5f 4c 89 e2 48 c1 ea 16 48 8b 0c d1 <48> 85 c9 74 4f 41 8b 55 30 4c 89 e6 48 c1 ee 0f 83 e6 7f c1 e6 05
 Call Trace:
  <IRQ>
  ? watchdog_timer_fn+0x3d6/0x440
  ? running_clock+0x10/0x10
  ? hrtimer_interrupt+0x185/0x580
  ? __sysvec_apic_timer_interrupt+0x44/0xe0
  ? sysvec_apic_timer_interrupt+0x6b/0x80
  </IRQ>
  <TASK>
  ? asm_sysvec_apic_timer_interrupt+0x16/0x20
  ? vfio_unpin_pages_remote+0x139/0x2a0 [vfio_iommu_type1]
  vfio_sync_unpin+0x96/0xf0 [vfio_iommu_type1]
  vfio_unmap_unpin+0x324/0x3d0 [vfio_iommu_type1]
  vfio_remove_dma+0x25/0xa0 [vfio_iommu_type1]
  vfio_iommu_type1_ioctl+0xcd5/0x1760 [vfio_iommu_type1]
  ? amd_pmu_v2_enable_all+0xa/0x30
  ? perf_pmu_sched_task+0xbf/0xf0
  x64_sys_call+0x282/0x1ac0
  ? syscall_trace_enter+0x1e5/0x1f0
  do_syscall_64+0x68/0x130
  entry_SYSCALL_64_after_hwframe+0x4b/0x53

RIP is the mem_section root test in pfn_valid(), inlined into put_pfn()
through is_invalid_reserved_pfn(), so the stall is in the dma->has_rsvd
per-page loop.  mem_section[] has no root for that pfn, so pfn_valid()
returns false and put_pfn() does nothing - for every pfn in the mapping.

Track whether the mapping also contains non-reserved pfns and skip the
walk entirely for reserved-only mappings.  Mixed mappings keep the
existing per-pfn walk, and ordinary mappings keep the existing batched
unpin path.

Tested on an affected host: reserved-only mappings up to 128 GiB take the
new path and complete in microseconds.  The ordinary-page path was
exercised unchanged.

Suggested-by: Alex Williamson <alex@shazbot.org>
Link: https://lore.kernel.org/all/20260803154707.71cc0d6b@shazbot.org/
Signed-off-by: Samuel Crossley <samuelcrossley@gmail.com>
---
This is the reserved-only fast path you suggested on v1, tested on
the affected hosts. There is indeed no evidence that the non-reserved
VFIO path needs chunking, and agree any such change to
unpin_user_page_range_dirty_lock() would belong in mm, so not
including it in this patch.
---
Changes in v2:
- Replace v1's cond_resched() chunking with the reserved-only fast path.
- Tested on an affected host; reserved-only mappings up to 128 GiB now
  complete in microseconds instead of walking ~33.6M pfns.
- Link to v1: https://patch.msgid.link/20260723-vfio-v1-1-3b59579916c6@gmail.com
---
 drivers/vfio/vfio_iommu_type1.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index c8151ba54de3..f7addfbe1aab 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -94,6 +94,7 @@ struct vfio_dma {
 	bool			lock_cap;	/* capable(CAP_IPC_LOCK) */
 	bool			vaddr_invalid;
 	bool			has_rsvd;	/* has 1 or more rsvd pfns */
+	bool			has_non_rsvd;	/* has 1 or more !rsvd pfns */
 	struct task_struct	*task;
 	struct rb_root		pfn_list;	/* Ex-user pinned pfn list */
 	unsigned long		*bitmap;
@@ -791,6 +792,7 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
 
 out:
 	dma->has_rsvd |= rsvd;
+	dma->has_non_rsvd |= !rsvd;
 	ret = vfio_lock_acct(dma, lock_acct, false);
 
 unpin_out:
@@ -821,11 +823,13 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
 	long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
 
 	if (dma->has_rsvd) {
-		unsigned long i;
+		if (dma->has_non_rsvd) {
+			unsigned long i;
 
-		for (i = 0; i < npage; i++)
-			if (put_pfn(pfn++, dma->prot))
-				unlocked++;
+			for (i = 0; i < npage; i++)
+				if (put_pfn(pfn++, dma->prot))
+					unlocked++;
+		}
 	} else {
 		put_valid_unreserved_pfns(pfn, npage, dma->prot);
 		unlocked = npage;

---
base-commit: 4e3c1fc8abcb8eff062150b4340fa4569696d645
change-id: 20260723-vfio-decd86bf3cf6

Best regards,
--  
Samuel Crossley <samuelcrossley@gmail.com>