linux-kernel - Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

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Date:   Tue, 14 Jan 2020 13:59:21 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Wei Yang <richardw.yang@...ux.intel.com>, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        yang.shi@...ux.alibaba.com, alexander.duyck@...il.com,
        rientjes@...gle.com
Subject: Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

On Tue, Jan 14, 2020 at 01:31:12PM +0300, Kirill A. Shutemov wrote:
> On Tue, Jan 14, 2020 at 10:31:22AM +0100, Michal Hocko wrote:
> > On Sat 11-01-20 03:03:52, Kirill A. Shutemov wrote:
> > > On Thu, Jan 09, 2020 at 10:30:54PM +0800, Wei Yang wrote:
> > > > As all the other places, we grab the lock before manipulate the defer list.
> > > > Current implementation may face a race condition.
> > > > 
> > > > For example, the potential race would be:
> > > > 
> > > >     CPU1                      CPU2
> > > >     mem_cgroup_move_account   split_huge_page_to_list
> > > >       !list_empty
> > > >                                 lock
> > > >                                 !list_empty
> > > >                                 list_del
> > > >                                 unlock
> > > >       lock
> > > >       # !list_empty might not hold anymore
> > > >       list_del_init
> > > >       unlock
> > > 
> > > I don't think this particular race is possible. Both parties take page
> > > lock before messing with deferred queue, but anytway:
> > > 
> > > Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > 
> > I am confused, if the above race is not possible then what would be a
> > real race? We really do not want to have a patch with a misleading
> > changelog, do we?
> 
> The alternative is to make sure that all page_deferred_list() called with
> page lock taken.
> 
> I'll look into it.

split_huge_page_to_list() has page lock taken.

free_transhuge_page() is in the free path and doesn't susceptible to the
race.

deferred_split_scan() is trickier. list_move() should be safe against
list_empty() as it will not produce false-positive list_empty().
list_del_init() *should* (correct me if I'm wrong) be safe because the page
is freeing and memcg will not touch the page anymore.

deferred_split_huge_page() is a problematic one. It called from
page_remove_rmap() path witch does require page lock. I don't see any
obvious way to exclude race with mem_cgroup_move_account() here.
Anybody else?

Wei, could you rewrite the commit message with deferred_split_huge_page()
as a race source instead of split_huge_page_to_list()?

-- 
 Kirill A. Shutemov