Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

[Hugh Dickins <hughd@google.com>]

On Thu, 11 Mar 2021, Michal Hocko wrote:
> 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.

I agree: I do like Johannes' suggestion best, now that we already
have split_page_memcg().  Not too worried about contrived use of
free_unref_page() here; and whether non-compound high-order pages
should be perpetuated is a different discussion.

Hugh