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Date:   Thu, 11 Mar 2021 17:26:53 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Zhou Guanghui <zhouguanghui1@huawei.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, hughd@google.com,
        kirill.shutemov@linux.intel.com, npiggin@gmail.com, ziy@nvidia.com,
        wangkefeng.wang@huawei.com, guohanjun@huawei.com,
        dingtianhong@huawei.com, chenweilong@huawei.com,
        rui.xiang@huawei.com
Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page
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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