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Date:   Tue, 1 Sep 2020 11:01:49 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Michal Hocko <mhocko@suse.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Shakeel Butt <shakeelb@google.com>,
        Matthew Wilcox <willy@infradead.org>, Qian Cai <cai@lca.pw>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 1/5] ksm: reinstate memcg charge on copied pages
Message-ID: <20200901150149.GA45118@cmpxchg.org>
References: <alpine.LSU.2.11.2008301343270.5954@eggly.anvils>
 <alpine.LSU.2.11.2008301358020.5954@eggly.anvils>
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On Sun, Aug 30, 2020 at 01:59:35PM -0700, Hugh Dickins wrote:
> In 5.8 some instances of memcg charging in do_swap_page() and unuse_pte()
> were removed, on the understanding that swap cache is now already charged
> at those points; but a case was missed, when ksm_might_need_to_copy() has
> decided it must allocate a substitute page: such pages were never charged.
> Fix it inside ksm_might_need_to_copy().
> 
> This was discovered by Alex Shi's prospective commit "mm/memcg: warning
> on !memcg after readahead page charged".
> 
> But there is a another surprise: this also fixes some rarer uncharged
> PageAnon cases, when KSM is configured in, but has never been activated.
> ksm_might_need_to_copy()'s anon_vma->root and linear_page_index() check
> sometimes catches a case which would need to have been copied if KSM
> were turned on.  Or that's my optimistic interpretation (of my own old
> code), but it leaves some doubt as to whether everything is working as
> intended there - might it hint at rare anon ptes which rmap cannot find?
> A question not easily answered: put in the fix for missed memcg charges.
> 
> Fixes: 4c6355b25e8b ("mm: memcontrol: charge swapin pages on instantiation")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: stable@vger.kernel.org # v5.8

Acked-by: Johannes Weiner <hannes@cmpxchg.org>