linux-kernel - Re: [Patch v3] mm: thp: grab the lock before manipulation defer list

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Date:   Fri, 17 Jan 2020 12:59:35 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>, g@...e.cz,
        kirill.shutemov@...ux.intel.com
Cc:     David Rientjes <rientjes@...gle.com>,
        Wei Yang <richardw.yang@...ux.intel.com>, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.or,
        yang.shi@...ux.alibaba.com, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        alexander.duyck@...il.com, stable@...r.kernel.org
Subject: Re: [Patch v3] mm: thp: grab the lock before manipulation defer list

On Fri 17-01-20 12:42:05, Kirill Tkhai wrote:
> On 17.01.2020 12:32, David Rientjes wrote:
> > On Fri, 17 Jan 2020, Kirill Tkhai wrote:
> > 
> >>>> I think that's a good point, especially considering that the current code 
> >>>> appears to unconditionally place any compound page on the deferred split 
> >>>> queue of the destination memcg.  The correct list that it should appear 
> >>>> on, I believe, depends on whether the pmd has been split for the process 
> >>>> being moved: note the MC_TARGET_PAGE caveat in 
> >>>> mem_cgroup_move_charge_pte_range() that does not move the charge for 
> >>>> compound pages with split pmds.  So when mem_cgroup_move_account() is 
> >>>> called with compound == true, we're moving the charge of the entire 
> >>>> compound page: why would it appear on that memcg's deferred split queue?
> >>>
> >>> I believe Kirill asked how do we know that the page should be actually
> >>> added to the deferred list just from the list_empty check. In other
> >>> words what if the page hasn't been split at all?
> >>
> >> Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
> >> huge page to the deferred list, while we need to do that only for pages,
> >> which are queued for splitting...
> >>
> > 
> > Yup, and that appears broken before Wei's patch.  Since we only migrate 
> > charges of entire compound pages (we have a mapping pmd, the underlying 
> > page cannot be split), it should not appear on the deferred split queue 
> > for any memcg, right?
> 
> Hm. Can't a huge page be mapped in two tasks:

It can but it will get charged to only of the initially. I haven't
checked the THP code in that aspect but from what I remember subpages
shouldn't refer to different memcgs. Kirill Shutemov?
-- 
Michal Hocko
SUSE Labs