[PATCH] secretmem: optimize page_is_secretmem()

[Matthew Wilcox]

On Mon, Apr 19, 2021 at 11:42:18AM +0300, Mike Rapoport wrote:
> The perf profile of the test indicated that the regression is caused by
> page_is_secretmem() called from gup_pte_range() (inlined by gup_pgd_range):

Uhh ... you're calling it in the wrong place!

                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
                page = pte_page(pte);

                if (page_is_secretmem(page))
                        goto pte_unmap;

                head = try_grab_compound_head(page, 1, flags);
                if (!head)
                        goto pte_unmap;

So you're calling page_is_secretmem() on a struct page without having
a refcount on it.  That is definitely not allowed.  secretmem seems to
be full of these kinds of races; I know this isn't the first one I've
seen in it.  I don't think this patchset is ready for this merge window.

With that fixed, you'll have a head page that you can use for testing,
which means you don't need to test PageCompound() (because you know the
page isn't PageTail), you can just test PageHead().