Re: [RFC 4/4] Sparse initialization of struct page array.

Ingo Molnar
2013-07-23T08:32:16Z

* H. Peter Anvin <hpa@zytor.com> wrote:

> On 07/15/2013 11:26 AM, Robin Holt wrote:
>
> > Is there a fairly cheap way to determine definitively that the struct
> > page is not initialized?
>
> By definition I would assume no. The only way I can think of would be
> to unmap the memory associated with the struct page in the TLB and
> initialize the struct pages at trap time.

But ... the only fastpath impact I can see of delayed initialization right
now is this piece of logic in prep_new_page():

@@ -903,6 +964,10 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)

for (i = 0; i < (1 << order); i++) {
struct page *p = page + i;
+
+ if (PageUninitialized2Mib(p))
+ expand_page_initialization(page);
+
if (unlikely(check_new_page(p)))
return 1;

That is where I think it can be made zero overhead in the
already-initialized case, because page-flags are already used in
check_new_page():

static inline int check_new_page(struct page *page)
{
if (unlikely(page_mapcount(page) |
(page->mapping != NULL) |
(atomic_read(&page->_count) != 0) |
(page->flags & PAGE_FLAGS_CHECK_AT_PREP) |
(mem_cgroup_bad_page_check(page)))) {
bad_page(page);
return 1;

see that PAGE_FLAGS_CHECK_AT_PREP flag? That always gets checked for every
struct page on allocation.

We can micro-optimize that low overhead to zero-overhead, by integrating
the PageUninitialized2Mib() check into check_new_page(). This can be done
by adding PG_uninitialized2mib to PAGE_FLAGS_CHECK_AT_PREP and doing:

if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_PREP)) {
if (PageUninitialized2Mib(p))
expand_page_initialization(page);
...
}

if (unlikely(page_mapcount(page) |
(page->mapping != NULL) |
(atomic_read(&page->_count) != 0) |
(mem_cgroup_bad_page_check(page)))) {
bad_page(page);

return 1;

this will result in making it essentially zero-overhead, the
expand_page_initialization() logic is now in a slowpath.

Am I missing anything here?

Thanks,

Ingo