On Thu 11-03-21 10:21:39, Johannes Weiner wrote:
> On Thu, Mar 11, 2021 at 09:37:02AM +0100, Michal Hocko wrote:
> > Johannes, Hugh,
> >
> > what do you think about this approach? If we want to stick with
> > split_page approach then we need to update the missing place Matthew has
> > pointed out.
>
> I find the __free_pages() code quite tricky as well. But for that
> reason I would actually prefer to initiate the splitting in there,
> since that's the place where we actually split the page, rather than
> spread the handling of this situation further out.
>
> The race condition shouldn't be hot, so I don't think we need to be as
> efficient about setting page->memcg_data only on the higher-order
> buddies as in Willy's scratch patch. We can call split_page_memcg(),
> which IMO should actually help document what's happening to the page.
>
> I think that function could also benefit a bit more from step-by-step
> documentation about what's going on. The kerneldoc is helpful, but I
> don't think it does justice to how tricky this race condition is.
>
> Something like this?
>
> void __free_pages(struct page *page, unsigned int order)
> {
> /*
> * Drop the base reference from __alloc_pages and free. In
> * case there is an outstanding speculative reference, from
> * e.g. the page cache, it will put and free the page later.
> */
> if (likely(put_page_testzero(page))) {
> free_the_page(page, order);
> return;
> }
>
> /*
> * The speculative reference will put and free the page.
> *
> * However, if the speculation was into a higher-order page
> * that isn't marked compound, the other side will know
> * nothing about our buddy pages and only free the order-0
> * page at the start of our chunk! We must split off and free
> * the buddy pages here.
> *
> * The buddy pages aren't individually refcounted, so they
> * can't have any pending speculative references themselves.
> */
> if (!PageHead(page) && order > 0) {
> split_page_memcg(page, 1 << order);
> while (order-- > 0)
> free_the_page(page + (1 << order), order);
> }
> }
Fine with me. Mathew was concerned about more places that do something
similar but I would say that if we find out more places we might
reconsider and currently stay with a reasonably clear model that it is
only head patch that carries the memcg information and split_page_memcg
is necessary to break such page into smaller pieces.
--
Michal Hocko
SUSE Labs
