linux-kernel - Re: [PATCH -mm -v4 14/21] mm, cgroup, THP, swap: Support to move swap account for PMD swap mapping

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Message-ID: <20180710224923.lm5n7d2jot6pggw2@ca-dmjordan1.us.oracle.com>
Date:   Tue, 10 Jul 2018 15:49:23 -0700
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shaohua Li <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <zi.yan@...rutgers.edu>
Subject: Re: [PATCH -mm -v4 14/21] mm, cgroup, THP, swap: Support to move
 swap account for PMD swap mapping

On Tue, Jul 10, 2018 at 03:49:58PM +0800, Huang, Ying wrote:
> Daniel Jordan <daniel.m.jordan@...cle.com> writes:
> 
> > On Fri, Jun 22, 2018 at 11:51:44AM +0800, Huang, Ying wrote:
> >> Because there is no way to prevent a huge swap cluster from being
> >> split except when it has SWAP_HAS_CACHE flag set.
> >
> > What about making get_mctgt_type_thp take the cluster lock?  That function
> > would be the first lock_cluster user outside of swapfile.c, but it would
> > serialize with split_swap_cluster.
> >
> >> It is possible for
> >> the huge swap cluster to be split and the charge for the swap slots
> >> inside to be changed, after we check the PMD swap mapping and the huge
> >> swap cluster before we commit the charge moving.  But the race window
> >> is so small, that we will just ignore the race.
> >
> > Moving the charges is a slow path, so can't we just be correct here and not
> > leak?
> 
> Check the code and thought about this again, found the race may not
> exist.  Because the PMD is locked when get_mctgt_type_thp() is called
> until charge is completed for the PMD.  So the charge of the huge swap
> cluster cannot be changed at the same time even if the huge swap cluster
> is split by other processes.

That's true, the PMD lock does prevent the swap charge from going stale between
the time mem_cgroup_move_charge_pte_range identifies a huge swap entry in
get_mctgt_type_thp and the time it moves the charge in mem_cgroup_move_account,
at least for some events like swapping in pages.

> Right?

I'm not sure the PMD lock covers everything, but after looking a while longer
at this, the charge moving seems to be a best-effort feature in other respects,
so the accounting doesn't need to be completely accurate.