On Tue, 15 Jul 2025 11:46:22 -0700
Keith Busch <kbusch@xxxxxxxx> wrote:
> From: Keith Busch <kbusch@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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);
Applied to vfio next branch for v6.17. Thanks,
Alex
