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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Date:   Mon, 9 Jul 2018 10:20:37 -0700
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     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 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?