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Date:   Mon, 3 Apr 2023 16:43:08 +0200
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Subject: Re: [PATCHv9 03/14] mm/page_alloc: Fake unaccepted memory
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Ard Biesheuvel <ardb@kernel.org>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Mike Rapoport <rppt@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        khalid.elmously@canonical.com, philip.cox@canonical.com,
        aarcange@redhat.com, peterx@redhat.com, x86@kernel.org,
        linux-mm@kvack.org, linux-coco@lists.linux.dev,
        linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20230330114956.20342-1-kirill.shutemov@linux.intel.com>
 <20230330114956.20342-4-kirill.shutemov@linux.intel.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230330114956.20342-4-kirill.shutemov@linux.intel.com>
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On 30.03.23 13:49, Kirill A. Shutemov wrote:
> For testing purposes, it is useful to fake unaccepted memory in the
> system. It helps to understand unaccepted memory overhead to the page
> allocator.
> 
> The patch allows to treat memory above the specified physical memory
> address as unaccepted.
> 
> The change only fakes unaccepted memory for page allocator. Memblock is
> not affected.
> 
> It also assumes that arch-provided accept_memory() on already accepted
> memory is a nop.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>   mm/page_alloc.c | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d62fcb2f28bd..509a93b7e5af 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7213,6 +7213,8 @@ static DEFINE_STATIC_KEY_FALSE(zones_with_unaccepted_pages);
>   
>   static bool lazy_accept = true;
>   
> +static unsigned long fake_unaccepted_start = -1UL;
> +
>   static int __init accept_memory_parse(char *p)
>   {
>   	if (!strcmp(p, "lazy")) {
> @@ -7227,11 +7229,30 @@ static int __init accept_memory_parse(char *p)
>   }
>   early_param("accept_memory", accept_memory_parse);
>   
> +static int __init fake_unaccepted_start_parse(char *p)
> +{
> +	if (!p)
> +		return -EINVAL;
> +
> +	fake_unaccepted_start = memparse(p, &p);
> +
> +	if (*p != '\0') {
> +		fake_unaccepted_start = -1UL;
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +early_param("fake_unaccepted_start", fake_unaccepted_start_parse);
> +
>   static bool page_contains_unaccepted(struct page *page, unsigned int order)
>   {
>   	phys_addr_t start = page_to_phys(page);
>   	phys_addr_t end = start + (PAGE_SIZE << order);
>   
> +	if (start >= fake_unaccepted_start)
> +		return true;
> +
>   	return range_contains_unaccepted_memory(start, end);
>   }
>   

The "unpleasant" thing about this is, that page_contains_unaccepted() 
could not be used for sanity checks because the result is static.

For example, something like

if (page_contains_unaccepted(page, 0))
	accept_memory(page, 0);
BUG_ON(!page_contains_unaccepted(page, 0));

Would work on real hardware, however, not for the fake variant.

-- 
Thanks,

David / dhildenb