linux-kernel - Re: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache

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Message-ID: <20190412072401.GA69620@maya190131.isni1t2eisqetojrdim5hhf1se.xx.internal.cloudapp.net>
Date:   Fri, 12 Apr 2019 07:24:03 +0000
From:   Maya Nakamura <m.maya.nakamura@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     mikelley@...rosoft.com, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, sashal@...nel.org, x86@...nel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with
 kmem_cache_alloc()

On Fri, Apr 05, 2019 at 01:31:02PM +0200, Vitaly Kuznetsov wrote:
> Maya Nakamura <m.maya.nakamura@...il.com> writes:
> 
> > @@ -98,18 +99,20 @@ EXPORT_SYMBOL_GPL(hyperv_pcpu_input_arg);
> >  u32 hv_max_vp_index;
> >  EXPORT_SYMBOL_GPL(hv_max_vp_index);
> >  
> > +struct kmem_cache *cachep;
> > +EXPORT_SYMBOL_GPL(cachep);
> > +
> >  static int hv_cpu_init(unsigned int cpu)
> >  {
> >  	u64 msr_vp_index;
> >  	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
> >  	void **input_arg;
> > -	struct page *pg;
> >  
> >  	input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
> > -	pg = alloc_page(GFP_KERNEL);
> > -	if (unlikely(!pg))
> > +	*input_arg = kmem_cache_alloc(cachep, GFP_KERNEL);
> 
> I'm not sure use of kmem_cache is justified here: pages we allocate are
> not cache-line and all these allocations are supposed to persist for the
> lifetime of the guest. In case you think that even on x86 it will be
> possible to see PAGE_SIZE != HV_HYP_PAGE_SIZE you can use alloc_pages()
> instead.
> 
Thank you for your feedback, Vitaly!

Will you please tell me how cache-line relates to kmem_cache?

I understand that alloc_pages() would work when PAGE_SIZE <=
HV_HYP_PAGE_SIZE, but I think that it would not work if PAGE_SIZE >
HV_HYP_PAGE_SIZE.

> Also, in case the idea is to generalize stuff, what will happen if
> PAGE_SIZE > HV_HYP_PAGE_SIZE? Who will guarantee proper alignment?
> 
> I think we can leave hypercall arguments, vp_assist and similar pages
> alone for now: the code is not going to be shared among architectures
> anyways.
> 
About the alignment, kmem_cache_create() aligns memory with its third
parameter, offset.

> > @@ -338,7 +349,10 @@ void __init hyperv_init(void)
> >  	guest_id = generate_guest_id(0, LINUX_VERSION_CODE, 0);
> >  	wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);
> >  
> > -	hv_hypercall_pg  = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL_RX);
> > +	hv_hypercall_pg = kmem_cache_alloc(cachep, GFP_KERNEL);
> > +	if (hv_hypercall_pg)
> > +		set_memory_x((unsigned long)hv_hypercall_pg, 1);
> 
> _RX is not writeable, right?
> 
Yes, you are correct. I should use set_memory_ro() in addition to
set_memory_x().

> > @@ -416,6 +431,7 @@ void hyperv_cleanup(void)
> >  	 * let hypercall operations fail safely rather than
> >  	 * panic the kernel for using invalid hypercall page
> >  	 */
> > +	kmem_cache_free(cachep, hv_hypercall_pg);
> 
> Please don't do that: hyperv_cleanup() is called on kexec/kdump and
> we're trying to do the bare minimum to allow next kernel to boot. Doing
> excessive work here will likely lead to consequent problems (we're
> already crashing the case it's kdump!).
> 
Thank you for the explanation! I will remove that.