linux-kernel - Re: PROBLEM: Remapping hugepages mappings causes kernel to return EINVAL

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Message-ID: <20171024081232.6to62flr7h3qgxvv@dhcp22.suse.cz>
Date:   Tue, 24 Oct 2017 10:12:32 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "C.Wehrmeyer" <c.wehrmeyer@....de>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: PROBLEM: Remapping hugepages mappings causes kernel to return
 EINVAL

On Tue 24-10-17 09:41:46, C.Wehrmeyer wrote:
[...]
> 1. Provide mmap with some sort of flag (which would be redundant IMHO) in
> order to churn out properly aligned pages (not transparent, but the current
> MAP_HUGETLB flag isn't either).

You can easily implement such a thing in userspace. In fact glibc has
already done that for you.

> 2. Based on THP enabling status always churn out properly aligned pages, and
> just failsafe to smaller pages if hugepages couldn't be allocated (truly
> transparent).
> 3. Map in memory, then tell madvise to make as many hugepages out of it as
> possible while still keeping the initial mapping (not transparent, and not
> sure Linux can actually do that).

I think there is still some confusion here. Kernel will try to fault in
THP pages on properly aligned addresses. So if you create a larger
mapping than the THP size then you will get a THP (assuming the memory
is not fragmented). It is just the unaligned addresses will get regular
pages.
-- 
Michal Hocko
SUSE Labs