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Date:   Fri, 2 Oct 2020 10:10:51 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Zi Yan <ziy@nvidia.com>, linux-mm@kvack.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Shakeel Butt <shakeelb@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        William Kucharski <william.kucharski@oracle.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        David Nellans <dnellans@nvidia.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86_64
Message-ID: <20201002081023.GA4555@dhcp22.suse.cz>
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On Fri 02-10-20 09:50:02, David Hildenbrand wrote:
> >>> - huge page sizes controllable by the userspace?
> >>
> >> It might be good to allow advanced users to choose the page sizes, so they
> >> have better control of their applications.
> > 
> > Could you elaborate more? Those advanced users can use hugetlb, right?
> > They get a very good control over page size and pool preallocation etc.
> > So they can get what they need - assuming there is enough memory.
> > 
> 
> I am still not convinced that 1G THP (TGP :) ) are really what we want
> to support. I can understand that there are some use cases that might
> benefit from it, especially:

Well, I would say that internal support for larger huge pages (e.g. 1GB)
that can transparently split under memory pressure is a useful
funtionality. I cannot really judge how complex that would be
consideting that 2MB THP have turned out to be quite a pain but
situation has settled over time. Maybe our current code base is prepared
for that much better.

Exposing that interface to the userspace is a different story of course.
I do agree that we likely do not want to be very explicit about that.
E.g. an interface for address space defragmentation without any more
specifics sounds like a useful feature to me. It will be up to the
kernel to decide which huge pages to use.
-- 
Michal Hocko
SUSE Labs