Re: [PATCH 1/2] vfio/type1: Periodically try rescheduling when unmapping

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The use of `BIT(PUD_ORDER)` to rate-limit `cond_resched()` evaluates to incorrectly large values on architectures with large page sizes, failing to prevent the scheduler stall.
--

--- Patch [1]: [PATCH 1/2] vfio/type1: Periodically try rescheduling when unmapping ---
commit 2d6c55cffed789bb39f187cfc28ac1de6edd70b2
Author: Aaron Lewis <aaronlewis@google.com>

vfio/type1: Periodically try rescheduling when unmapping

This commit adds a cond_resched() to the inner loop of vfio_unmap_unpin()
to prevent CPU stalls during the unmapping of large DMA regions. It
attempts to rate limit the reschedule check to once per 1GB using
BIT(PUD_ORDER).

> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
[ ... ]
> @@ -1196,6 +1197,8 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
>  			next = iommu_iova_to_phys(domain->domain, iova + len);
>  			if (next != phys + len)
>  				break;
> +			if ((++i % BIT(PUD_ORDER)) == 0)
> +				cond_resched();

[Severity: Medium]
Will this rate limit fail to prevent scheduler stalls on architectures with
large page sizes?

On an ARM64 system configured with 64KB pages, PUD_SHIFT is 42 and PAGE_SHIFT
is 16, resulting in a PUD_ORDER of 26. BIT(26) evaluates to 67,108,864 loop
iterations.

Since each iteration processes PAGE_SIZE (64KB), the loop would process 4TB
of contiguous memory before calling cond_resched(). As noted in the commit
message, 67 million loop iterations can take over 1 second, which vastly
exceeds the 100ms scheduler tick limit.

Could this use a constant size limit instead, such as (SZ_1G >> PAGE_SHIFT),
to ensure the interval remains consistent across different architectures and
page sizes?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260714210303.3967981-1-aaronlewis@google.com?part=1