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Date:   Tue, 6 Apr 2021 14:02:51 +0200
From:   Gerald Schaefer <gerald.schaefer@linux.ibm.com>
To:     Yang Shi <shy828301@gmail.com>
Cc:     Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zi Yan <ziy@nvidia.com>, Michal Hocko <mhocko@suse.com>,
        Huang Ying <ying.huang@intel.com>,
        Hugh Dickins <hughd@google.com>, hca@linux.ibm.com,
        gor@linux.ibm.com, borntraeger@de.ibm.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>, linux-s390@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexander Gordeev <agordeev@linux.ibm.com>
Subject: Re: [RFC PATCH 0/6] mm: thp: use generic THP migration for NUMA
 hinting fault
Message-ID: <20210406140251.2779c400@thinkpad>
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On Thu, 1 Apr 2021 13:10:49 -0700
Yang Shi <shy828301@gmail.com> wrote:

[...]
> > >
> > > Yes, it could be. The old behavior of migration was to return -ENOMEM
> > > if THP migration is not supported then split THP. That behavior was
> > > not very friendly to some usecases, for example, memory policy and
> > > migration lieu of reclaim (the upcoming). But I don't mean we restore
> > > the old behavior. We could split THP if it returns -ENOSYS and the
> > > page is THP.
> >
> > OK, as long as we don't get any broken PMD migration entries established
> > for s390, some extra THP splitting would be acceptable I guess.
> 
> There will be no migration PMD installed. The current behavior is a
> no-op if THP migration is not supported.

Ok, just for completeness, since Mel also replied that the split
was not done on other architectures "because the loss from splitting
exceeded the gain of improved locality":

I did not mean to request extra splitting functionality for s390,
simply skipping / ignoring large PMDs would also be fine for s390,
no need to add extra complexity.