linux-kernel - Re: [PATCH 4/5] x86/mm: Provide helpers for unaccepted memory

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Message-ID: <20210812203153.uoa7nx7w5zdsmzck@box.shutemov.name>
Date:   Thu, 12 Aug 2021 23:31:53 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 4/5] x86/mm: Provide helpers for unaccepted memory

On Tue, Aug 10, 2021 at 11:16:26AM -0700, Dave Hansen wrote:
> On 8/9/21 11:26 PM, Kirill A. Shutemov wrote:
> > +void accept_memory(phys_addr_t start, phys_addr_t end)
> > +{
> > +	if (!boot_params.unaccepted_memory)
> > +		return;
> > +
> > +	spin_lock(&unaccepted_memory_lock);
> > +	__accept_memory(start, end);
> > +	spin_unlock(&unaccepted_memory_lock);
> > +}
> 
> Isn't this taken in the:
> 
> 	del_page_from_free_list()->
> 	clear_page_offline()->
> 	accept_memory()
> 
> call path?
> 
> That's underneath:
> 
> 	spin_lock_irqsave(&zone->lock, flags);
> 
> Which means that accept_memory() can happen from interrupt context.  Is
> it always covered by another spin_lock_irqsave() which means that it can
> use a plain spin_lock()?

I didn't give it enough thought yet, but we always run under zone lock
which has to use spin_lock_irqsave() if it called from interrupt context.

Having said that I think it is good idea to move clear_page_offline() out
zone lock. It should help with allocation latency. Not sure how messy it
gets. Merging/splitting path looks complex and I'm not an expert in the
page allocator.

> If so, it would be nice to call out that logic.  It *looks* like a
> spinlock that we would want to be spin_lock_irqsave().

-- 
 Kirill A. Shutemov