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Date:   Tue, 2 Mar 2021 15:49:38 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
cc:     Hugh Dickins <hughd@google.com>,
        Zhou Guanghui <zhouguanghui1@huawei.com>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org,
        wangkefeng.wang@huawei.com, guohanjun@huawei.com,
        dingtianhong@huawei.com, chenweilong@huawei.com,
        rui.xiang@huawei.com, Nicholas Piggin <npiggin@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH] mm/memcg: set memcg when split pages
In-Reply-To: <YD7Ch/8QebzmneCR@cmpxchg.org>
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On Tue, 2 Mar 2021, Johannes Weiner wrote:
> On Tue, Mar 02, 2021 at 12:24:41PM -0800, Hugh Dickins wrote:
> > On Tue, 2 Mar 2021, Michal Hocko wrote:
> > > [Cc Johannes for awareness and fixup Nick's email]
> > > 
> > > On Tue 02-03-21 01:34:51, Zhou Guanghui wrote:
> > > > When split page, the memory cgroup info recorded in first page is
> > > > not copied to tail pages. In this case, when the tail pages are
> > > > freed, the uncharge operation is not performed. As a result, the
> > > > usage of this memcg keeps increasing, and the OOM may occur.
> > > > 
> > > > So, the copying of first page's memory cgroup info to tail pages
> > > > is needed when split page.
> > > 
> > > I was not aware that alloc_pages_exact is used for accounted allocations
> > > but git grep told me otherwise so this is not a theoretical one. Both
> > > users (arm64 and s390 kvm) are quite recent AFAICS. split_page is also
> > > used in dma allocator but I got lost in indirection so I have no idea
> > > whether there are any users there.
> > 
> > Yes, it's a bit worrying that such a low-level thing as split_page()
> > can now get caught up in memcg accounting, but I suppose that's okay.
> > 
> > I feel rather strongly that whichever way it is done, THP splitting
> > and split_page() should use the same interface to memcg.
> > 
> > And a look at mem_cgroup_split_huge_fixup() suggests that nowadays
> > there need to be css_get()s too - or better, a css_get_many().
> > 
> > Its #ifdef CONFIG_TRANSPARENT_HUGEPAGE should be removed, rename
> > it mem_cgroup_split_page_fixup(), and take order from caller.
> 
> +1
> 
> There is already a split_page_owner() in both these places as well
> which does a similar thing. Mabye we can match that by calling it
> split_page_memcg() and having it take a nr of pages?

Agreed on both counts :) "fixup" was not an inspiring name.

> 
> > Though I've never much liked that separate pass: would it be
> > better page by page, like this copy_page_memcg() does?  Though
> > mem_cgroup_disabled() and css_getting make that less appealing.
> 
> Agreed on both counts. mem_cgroup_disabled() is a jump label and would
> be okay, IMO, but the refcounting - though it is (usually) per-cpu -
> adds at least two branches and rcu read locking.