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

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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




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