linux-kernel - Re: [PATCH v10 16/50] x86/sev: Introduce snp leaked pages list

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Message-ID: <b54fdac3-9bdf-184e-f3fc-4790a328837c@amd.com>
Date:   Fri, 8 Dec 2023 16:10:25 -0600
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc:     linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
        rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com,
        bp@...en8.de, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        jarkko@...nel.org, nikunj.dadhania@....com, pankaj.gupta@....com,
        liam.merwick@...cle.com, zhi.a.wang@...el.com
Subject: Re: [PATCH v10 16/50] x86/sev: Introduce snp leaked pages list

Hello Vlastimil,

On 12/7/2023 10:20 AM, Vlastimil Babka wrote:

>> +
>> +void snp_leak_pages(u64 pfn, unsigned int npages)
>> +{
>> +	struct page *page = pfn_to_page(pfn);
>> +
>> +	pr_debug("%s: leaking PFN range 0x%llx-0x%llx\n", __func__, pfn, pfn + npages);
>> +
>> +	spin_lock(&snp_leaked_pages_list_lock);
>> +	while (npages--) {
>> +		/*
>> +		 * Reuse the page's buddy list for chaining into the leaked
>> +		 * pages list. This page should not be on a free list currently
>> +		 * and is also unsafe to be added to a free list.
>> +		 */
>> +		list_add_tail(&page->buddy_list, &snp_leaked_pages_list);
>> +		sev_dump_rmpentry(pfn);
>> +		pfn++;
> 
> You increment pfn, but not page, which is always pointing to the page of the
> initial pfn, so need to do page++ too.

Yes, that is a bug and needs to be fixed.

> But that assumes it's all order-0 pages (hard to tell for me whether that's
> true as we start with a pfn), if there can be compound pages, it would be
> best to only add the head page and skip the tail pages - it's not expected
> to use page->buddy_list of tail pages.

Can't we use PageCompound() to check if the page is a compound page and 
then use page->compound_head to get and add the head page to leaked 
pages list. I understand the tail pages for compound pages are really 
limited for usage.

Thanks,
Ashish