linux-kernel - Re: [PATCH v2] x86/mm: use max memory block size on bare metal

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Message-ID: <20200612032959.yo43ydg273zu35lx@ca-dmjordan1.us.oracle.com>
Date:   Thu, 11 Jun 2020 23:29:59 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH v2] x86/mm: use max memory block size on bare metal

On Thu, Jun 11, 2020 at 10:05:38AM -0700, Dave Hansen wrote:
> One other nit for this.  We *do* have actual hardware hotplug, and I'm
> pretty sure the alignment guarantees for hardware hotplug are pretty
> weak.  For instance, the alignment guarantees for persistent memory are
> still only 64MB even on modern platforms.
>
> Let's say we're on bare metal and we see an SRAT table that has some
> areas that show that hotplug might happen there.  Is this patch still
> ideal there?

Well, not if there's concern about hardware hotplug.

My assumption going in was that this wasn't a problem in practice.
078eb6aa50dc50 ("x86/mm/memory_hotplug: determine block size based on the end
of boot memory") was merged in 2018 to address qemu hotplug failures and >64G
systems have used a 2G block since 2014 with no complaints about alignment
issues, to my knowledge anyway.