Re: [PATCH v1 15/26] x86/sev: Introduce snp leaked pages list

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Hello Vlastimil,

On 1/8/2024 4:45 AM, Vlastimil Babka wrote:
On 12/30/23 17:19, Michael Roth wrote:
From: Ashish Kalra <ashish.kalra@xxxxxxx>

Pages are unsafe to be released back to the page-allocator, if they
have been transitioned to firmware/guest state and can't be reclaimed
or transitioned back to hypervisor/shared state. In this case add
them to an internal leaked pages list to ensure that they are not freed
or touched/accessed to cause fatal page faults.

Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
[mdr: relocate to arch/x86/virt/svm/sev.c]
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
Hi, sorry I didn't respond in time to the last mail discussing previous
version in
https://lore.kernel.org/all/8c1fd8da-912a-a9ce-9547-107ba8a450fc@xxxxxxx/
due to upcoming holidays.

I would rather avoid the approach of allocating container objects:
- it's allocating memory when effectively losing memory, a dangerous thing
- are all the callers and their context ok with GFP_KERNEL?
- GFP_KERNEL_ACCOUNT seems wrong, why would we be charging this to the
current process, it's probably not its fault the pages are leaked? Also the
charging can fail?
- given the benefit of having leaked pages on a list is basically just
debugging (i.e. crash dump or drgn inspection) this seems too heavy

I think it would be better and sufficient to use page->lru for order-0 and
head pages, and simply skip tail pages (possibly with adjusted warning
message for that case).

Vlastimil

<snip

Considering the above thoughts, this is updated version of snp_leak_pages(), looking forward to any review comments/feedback you have on the same:

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.
                 */
                if ((likely(!PageCompound(page))) || (PageCompound(page) &&
                    !PageTail(page) && compound_head(page) == page))
                        /*
                         * Skip inserting tail pages of compound page as
                         * page->buddy_list of tail pages is not usable.
                         */
                        list_add_tail(&page->buddy_list, &snp_leaked_pages_list);
                sev_dump_rmpentry(pfn);
                snp_nr_leaked_pages++;
                pfn++;
                page++;
        }
        spin_unlock(&snp_leaked_pages_list_lock);
}

Thanks, Ashish





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