linux-kernel - RE: [PATCH 1/2] Drivers: hv: Allocate interrupt and monitor pages aligned to system page boundary

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Message-ID:
 <BN7PR02MB414817742F4B15AB928450E6D4B92@BN7PR02MB4148.namprd02.prod.outlook.com>
Date: Sun, 20 Apr 2025 23:57:57 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "longli@...uxonhyperv.com" <longli@...uxonhyperv.com>, "K. Y. Srinivasan"
	<kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu
	<wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: Long Li <longli@...rosoft.com>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: RE: [PATCH 1/2] Drivers: hv: Allocate interrupt and monitor pages
 aligned to system page boundary

From: longli@...uxonhyperv.com <longli@...uxonhyperv.com> Sent: Thursday, April 17, 2025 5:43 PM
> 
> There are use cases that interrupt and monitor pages are mapped to
> user-mode through UIO, they need to be system page aligned. Some Hyper-V
> allocation APIs introduced earlier broke those requirements.
> 
> Fix those APIs by always allocating Hyper-V page at system page boundaries.

I'd suggest doing away with the hv_alloc/free_*() functions entirely since
they are now reduced to just being a wrapper around __get_free_pages(),
which doesn't add any value. Once all the arm64 support and CoCo VM
code settled out, it turned out that these functions to allocate Hyper-V
size pages had dwindling usage.

Allocation of the interrupt and monitor pages can use __get_free_pages()
directly, and that properly captures the need for those allocations to be a
full page. Just add a comment that this wastes space when PAGE_SIZE
> HV_HYP_PAGE_SIZE, but is necessary because the page may be mapped
into user space by uio_hv_generic.

The only other use is in hv_kmsg_dump_register(), and it can do
kzalloc(HV_HYP_PAGE_SIZE), since that case really is tied to the Hyper-V
page size, not PAGE_SIZE. There's no need to waste space by allocating
a full page.

Michael

> 
> Cc: stable@...r.kernel.org
> Fixes: ca48739e59df ("Drivers: hv: vmbus: Move Hyper-V page allocator to arch neutral
> code")
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  drivers/hv/hv_common.c | 29 +++++++----------------------
>  1 file changed, 7 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index a7d7494feaca..f426aaa9b8f9 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -106,41 +106,26 @@ void __init hv_common_free(void)
>  }
> 
>  /*
> - * Functions for allocating and freeing memory with size and
> - * alignment HV_HYP_PAGE_SIZE. These functions are needed because
> - * the guest page size may not be the same as the Hyper-V page
> - * size. We depend upon kmalloc() aligning power-of-two size
> - * allocations to the allocation size boundary, so that the
> - * allocated memory appears to Hyper-V as a page of the size
> - * it expects.
> + * A Hyper-V page can be used by UIO for mapping to user-space, it should
> + * always be allocated on system page boundaries.
>   */
> -
>  void *hv_alloc_hyperv_page(void)
>  {
> -	BUILD_BUG_ON(PAGE_SIZE <  HV_HYP_PAGE_SIZE);
> -
> -	if (PAGE_SIZE == HV_HYP_PAGE_SIZE)
> -		return (void *)__get_free_page(GFP_KERNEL);
> -	else
> -		return kmalloc(HV_HYP_PAGE_SIZE, GFP_KERNEL);
> +	BUILD_BUG_ON(PAGE_SIZE < HV_HYP_PAGE_SIZE);
> +	return (void *)__get_free_page(GFP_KERNEL);
>  }
>  EXPORT_SYMBOL_GPL(hv_alloc_hyperv_page);
> 
>  void *hv_alloc_hyperv_zeroed_page(void)
>  {
> -	if (PAGE_SIZE == HV_HYP_PAGE_SIZE)
> -		return (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> -	else
> -		return kzalloc(HV_HYP_PAGE_SIZE, GFP_KERNEL);
> +	BUILD_BUG_ON(PAGE_SIZE < HV_HYP_PAGE_SIZE);
> +	return (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
>  }
>  EXPORT_SYMBOL_GPL(hv_alloc_hyperv_zeroed_page);
> 
>  void hv_free_hyperv_page(void *addr)
>  {
> -	if (PAGE_SIZE == HV_HYP_PAGE_SIZE)
> -		free_page((unsigned long)addr);
> -	else
> -		kfree(addr);
> +	free_page((unsigned long)addr);
>  }
>  EXPORT_SYMBOL_GPL(hv_free_hyperv_page);
> 
> --
> 2.34.1
>