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

[Matthew Wilcox]

On Thu, Mar 18, 2021 at 03:05:00PM +0100, Michal Hocko wrote:
> On Thu 11-03-21 12:37:20, Hugh Dickins wrote:
> > 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.
> 
> Matthew, are you planning to post a patch with suggested changes or
> should I do it?

I'm busy with the folio work; could you do it please?