linux-kernel - Re: [PATCH 1/5] mm: Add support for unaccepted memory

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Date:   Tue, 10 Aug 2021 11:13:05 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        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>
Cc:     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 1/5] mm: Add support for unaccepted memory

> @@ -1001,6 +1004,9 @@ static inline void del_page_from_free_list(struct page *page, struct zone *zone,
>  	if (page_reported(page))
>  		__ClearPageReported(page);
>  
> +	if (PageOffline(page))
> +		clear_page_offline(page, order);
> +
>  	list_del(&page->lru);
>  	__ClearPageBuddy(page);
>  	set_page_private(page, 0);

So, this is right in the fast path of the page allocator.  It's a
one-time thing per 2M page, so it's not permanent.

*But* there's both a global spinlock and a firmware call hidden in
clear_page_offline().  That's *GOT* to hurt if you were, for instance,
running a benchmark while this code path is being tickled.  Not just to

That could be just downright catastrophic for scalability, albeit
temporarily.