linux-kernel - Re: [PATCH 1/1] Drivers: hv: vmbus: Don't release fb_mmio resource in vmbus_free

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Message-ID: <Z84pLHrwB-iTt2_V@liuwe-devbox-ubuntu-v2.lamzopl0uupeniq2etz1fddiyg.xx.internal.cloudapp.net>
Date: Sun, 9 Mar 2025 23:50:04 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
Cc: mhklinux@...look.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, kys@...rosoft.com, jakeo@...rosoft.com,
	linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 1/1] Drivers: hv: vmbus: Don't release fb_mmio resource
 in vmbus_free_mmio()

On Sat, Mar 08, 2025 at 08:50:12PM -0800, Saurabh Singh Sengar wrote:
> On Mon, Feb 10, 2025 at 09:01:14PM -0800, mhkelley58@...il.com wrote:
> > From: Michael Kelley <mhklinux@...look.com>
> > 
> > The VMBus driver manages the MMIO space it owns via the hyperv_mmio
> > resource tree. Because the synthetic video framebuffer portion of the
> > MMIO space is initially setup by the Hyper-V host for each guest, the
> > VMBus driver does an early reserve of that portion of MMIO space in the
> > hyperv_mmio resource tree. It saves a pointer to that resource in
> > fb_mmio. When a VMBus driver requests MMIO space and passes "true"
> > for the "fb_overlap_ok" argument, the reserved framebuffer space is
> > used if possible. In that case it's not necessary to do another request
> > against the "shadow" hyperv_mmio resource tree because that resource
> > was already requested in the early reserve steps.
> > 
> > However, the vmbus_free_mmio() function currently does no special
> > handling for the fb_mmio resource. When a framebuffer device is
> > removed, or the driver is unbound, the current code for
> > vmbus_free_mmio() releases the reserved resource, leaving fb_mmio
> > pointing to memory that has been freed. If the same or another
> > driver is subsequently bound to the device, vmbus_allocate_mmio()
> > checks against fb_mmio, and potentially gets garbage. Furthermore
> > a second unbind operation produces this "nonexistent resource" error
> > because of the unbalanced behavior between vmbus_allocate_mmio() and
> > vmbus_free_mmio():
> > 
> > [   55.499643] resource: Trying to free nonexistent
> > 			resource <0x00000000f0000000-0x00000000f07fffff>
> > 
> > Fix this by adding logic to vmbus_free_mmio() to recognize when
> > MMIO space in the fb_mmio reserved area would be released, and don't
> > release it. This filtering ensures the fb_mmio resource always exists,
> > and makes vmbus_free_mmio() more parallel with vmbus_allocate_mmio().
> > 
> > Fixes: be000f93e5d7 ("drivers:hv: Track allocations of children of hv_vmbus in private resource tree")
> > Signed-off-by: Michael Kelley <mhklinux@...look.com>
> > ---
> >  drivers/hv/vmbus_drv.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index 2892b8da20a5..7507b3641ebd 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -2262,12 +2262,25 @@ void vmbus_free_mmio(resource_size_t start, resource_size_t size)
> >  	struct resource *iter;
> >  
> >  	mutex_lock(&hyperv_mmio_lock);
> > +
> > +	/*
> > +	 * If all bytes of the MMIO range to be released are within the
> > +	 * special case fb_mmio shadow region, skip releasing the shadow
> > +	 * region since no corresponding __request_region() was done
> > +	 * in vmbus_allocate_mmio().
> > +	 */
> > +	if (fb_mmio && (start >= fb_mmio->start) &&
> > +				(start + size - 1 <= fb_mmio->end))
> > +		goto skip_shadow_release;
> > +
> >  	for (iter = hyperv_mmio; iter; iter = iter->sibling) {
> >  		if ((iter->start >= start + size) || (iter->end <= start))
> >  			continue;
> >  
> >  		__release_region(iter, start, size);
> >  	}
> > +
> > +skip_shadow_release:
> >  	release_mem_region(start, size);
> >  	mutex_unlock(&hyperv_mmio_lock);
> >  
> > -- 
> > 2.25.1
> > 
> 
> Thanks for the fix.
> There are couple of checkpatch.pl --strict CHECK, post fixing that:
> 
> Tested-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> Reviewed-by: Saurabh Sengar <ssengar@...ux.microsoft.com>

I will wait for a new version with the checkpatch.pl issues fixed.

Wei.