RE: [PATCH v2] mshv_vtl: Fix vmemmap_shift exceeding MAX_FOLIO_ORDER

[Michael Kelley]

From: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx> Sent: Monday, April 6, 2026 2:25 AM
> 
> When registering VTL0 memory via MSHV_ADD_VTL0_MEMORY, the kernel
> computes pgmap->vmemmap_shift as the number of trailing zeros in the
> OR of start_pfn and last_pfn, intending to use the largest compound
> page order both endpoints are aligned to.
> 
> However, this value is not clamped to MAX_FOLIO_ORDER, so a
> sufficiently aligned range (e.g. physical range
> [0x800000000000, 0x800080000000), corresponding to start_pfn=0x800000000
> with 35 trailing zeros) can produce a shift larger than what
> memremap_pages() accepts, triggering a WARN and returning -EINVAL:
> 
>   WARNING: ... memremap_pages+0x512/0x650
>   requested folio size unsupported
> 
> The MAX_FOLIO_ORDER check was added by
> commit 646b67d57589 ("mm/memremap: reject unreasonable folio/compound
> page sizes in memremap_pages()").
> 
> Fix this by clamping vmemmap_shift to MAX_FOLIO_ORDER so we always
> request the largest order the kernel supports, in those cases, rather
> than an out-of-range value.
> 
> Also fix the error path to propagate the actual error code from
> devm_memremap_pages() instead of hard-coding -EFAULT, which was
> masking the real -EINVAL return.
> 
> Fixes: 7bfe3b8ea6e3 ("Drivers: hv: Introduce mshv_vtl driver")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx>
> ---
> Changes since v1:
> https://lore.kernel.org/all/20260401054005.1532381-1-
> namjain@xxxxxxxxxxxxxxxxxxx/
> Addressed Michael's comments:
> * remove MAX_FOLIO_ORDER value related text in commit msg
> * Change change summary to keep prefix "mshv_vtl:"
> * Add comments regarding last_pfn to avoid confusion
> * use min instead of min_t
> ---
>  drivers/hv/mshv_vtl_main.c | 12 +++++++++---
>  include/uapi/linux/mshv.h  |  2 +-
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hv/mshv_vtl_main.c b/drivers/hv/mshv_vtl_main.c
> index 5856975f32e1..c19400701467 100644
> --- a/drivers/hv/mshv_vtl_main.c
> +++ b/drivers/hv/mshv_vtl_main.c
> @@ -386,7 +386,6 @@ static int mshv_vtl_ioctl_add_vtl0_mem(struct mshv_vtl *vtl, void __user *arg)
> 
>  	if (copy_from_user(&vtl0_mem, arg, sizeof(vtl0_mem)))
>  		return -EFAULT;
> -	/* vtl0_mem.last_pfn is excluded in the pagemap range for VTL0 as per design */
>  	if (vtl0_mem.last_pfn <= vtl0_mem.start_pfn) {
>  		dev_err(vtl->module_dev, "range start pfn (%llx) > end pfn (%llx)\n",
>  			vtl0_mem.start_pfn, vtl0_mem.last_pfn);
> @@ -397,6 +396,10 @@ static int mshv_vtl_ioctl_add_vtl0_mem(struct mshv_vtl *vtl, void __user *arg)
>  	if (!pgmap)
>  		return -ENOMEM;
> 
> +	/*
> +	 * vtl0_mem.last_pfn is excluded in the pagemap range for VTL0 as per design.
> +	 * last_pfn is not reserved or wasted, and reflects 'start_pfn + size' of pagemap range.
> +	 */
>  	pgmap->ranges[0].start = PFN_PHYS(vtl0_mem.start_pfn);
>  	pgmap->ranges[0].end = PFN_PHYS(vtl0_mem.last_pfn) - 1;
>  	pgmap->nr_range = 1;
> @@ -405,8 +408,11 @@ static int mshv_vtl_ioctl_add_vtl0_mem(struct mshv_vtl *vtl, void __user *arg)
>  	/*
>  	 * Determine the highest page order that can be used for the given memory range.
>  	 * This works best when the range is aligned; i.e. both the start and the length.
> +	 * Clamp to MAX_FOLIO_ORDER to avoid a WARN in memremap_pages() when the range
> +	 * alignment exceeds the maximum supported folio order for this kernel config.
>  	 */
> -	pgmap->vmemmap_shift = count_trailing_zeros(vtl0_mem.start_pfn | vtl0_mem.last_pfn);
> +	pgmap->vmemmap_shift = min(count_trailing_zeros(vtl0_mem.start_pfn | vtl0_mem.last_pfn),
> +				   MAX_FOLIO_ORDER);
>  	dev_dbg(vtl->module_dev,
>  		"Add VTL0 memory: start: 0x%llx, end_pfn: 0x%llx, page order: %lu\n",
>  		vtl0_mem.start_pfn, vtl0_mem.last_pfn, pgmap->vmemmap_shift);
> @@ -415,7 +421,7 @@ static int mshv_vtl_ioctl_add_vtl0_mem(struct mshv_vtl *vtl, void __user *arg)
>  	if (IS_ERR(addr)) {
>  		dev_err(vtl->module_dev, "devm_memremap_pages error: %ld\n", PTR_ERR(addr));
>  		kfree(pgmap);
> -		return -EFAULT;
> +		return PTR_ERR(addr);
>  	}
> 
>  	/* Don't free pgmap, since it has to stick around until the memory
> diff --git a/include/uapi/linux/mshv.h b/include/uapi/linux/mshv.h
> index e0645a34b55b..32ff92b6342b 100644
> --- a/include/uapi/linux/mshv.h
> +++ b/include/uapi/linux/mshv.h
> @@ -357,7 +357,7 @@ struct mshv_vtl_sint_post_msg {
> 
>  struct mshv_vtl_ram_disposition {
>  	__u64 start_pfn;
> -	__u64 last_pfn;
> +	__u64 last_pfn; /* last_pfn is excluded from the range [start_pfn, last_pfn) */
>  };
> 
>  struct mshv_vtl_set_poll_file {
> --
> 2.43.0
> 

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>