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

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Message-ID: <20171023180232.luayzqacnkepnm57@dhcp22.suse.cz>
Date:   Mon, 23 Oct 2017 20:02:32 +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 Mon 23-10-17 19:52:27, C.Wehrmeyer wrote:
[...]
> > or you can mmap a larger block and
> > munmap the initial unaligned part.
> 
> And how is that supposed to be transparent? When I hear "transparent" I
> think of a mechanism which I can put under a system so that it benefits from
> it, while the system does not notice or at least does not need to be aware
> of it. The system also does not need to be changed for it.

How do you expect to get a huge page when the mapping itself is not
properly aligned? Sure if you have a large mapping then you probably do
not care about first and last chunk being not 2MB aligned but in order
to get a THP you really need a 2MB aligned address.
-- 
Michal Hocko
SUSE Labs