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

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Message-ID:
 <SA6PR21MB4231F82E3042311244A8A101CEBA2@SA6PR21MB4231.namprd21.prod.outlook.com>
Date: Wed, 23 Apr 2025 18:39:44 +0000
From: Long Li <longli@...rosoft.com>
To: Michael Kelley <mhklinux@...look.com>, "longli@...uxonhyperv.com"
	<longli@...uxonhyperv.com>, KY 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: "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

> Subject: [EXTERNAL] 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.

There is a BUILD_BUG_ON(PAGE_SIZE < HV_HYP_PAGE_SIZE) in those functions, but it probably doesn't do anything.

If there is no objection, I can remove these functions.

Long

> 
> 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
> >