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

[Kalra, Ashish]

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