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

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Message-ID: <20210308210225.GF3479805@casper.infradead.org>
Date:   Mon, 8 Mar 2021 21:02:25 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Zhou Guanghui <zhouguanghui1@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, mhocko@...e.com, hannes@...xchg.org,
        hughd@...gle.com, kirill.shutemov@...ux.intel.com,
        npiggin@...il.com, ziy@...dia.com, wangkefeng.wang@...wei.com,
        guohanjun@...wei.com, dingtianhong@...wei.com,
        chenweilong@...wei.com, rui.xiang@...wei.com
Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

On Thu, Mar 04, 2021 at 07:40:53AM +0000, Zhou Guanghui wrote:
> As described in the split_page function comment, for the non-compound
> high order page, the sub-pages must be freed individually. If the
> memcg of the fisrt page is valid, the tail pages cannot be uncharged
> when be freed.
> 
> For example, when alloc_pages_exact is used to allocate 1MB continuous
> physical memory, 2MB is charged(kmemcg is enabled and __GFP_ACCOUNT is
> set). When make_alloc_exact free the unused 1MB and free_pages_exact
> free the applied 1MB, actually, only 4KB(one page) is uncharged.
> 
> Therefore, the memcg of the tail page needs to be set when split page.

There's another place we need to do this to ...

+++ b/mm/page_alloc.c
@@ -5081,9 +5081,15 @@ void __free_pages(struct page *page, unsigned int order)
 {
        if (put_page_testzero(page))
                free_the_page(page, order);
-       else if (!PageHead(page))
-               while (order-- > 0)
-                       free_the_page(page + (1 << order), order);
+       else if (!PageHead(page)) {
+               while (order-- > 0) {
+                       struct page *tail = page + (1 << order);
+#ifdef CONFIG_MEMCG
+                       tail->memcg_data = page->memcg_data;
+#endif
+                       free_the_page(tail, order);
+               }
+       }
 }
 EXPORT_SYMBOL(__free_pages);
 

I wonder if we shouldn't initialise memcg_data on all subsequent pages
of non-compound allocations instead?  Because I'm not sure this is the
only place that needs to be fixed.