linux-kernel - Re: [PATCH 1/2] mm: add private lock to serialize memory hotplug operations

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Date:   Thu, 09 Mar 2017 23:43:42 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Ben Hutchings <ben@...adent.org.uk>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Sebastian Ott <sebott@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] mm: add private lock to serialize memory hotplug operations

On Thursday, March 09, 2017 02:37:55 PM Dan Williams wrote:
> On Thu, Mar 9, 2017 at 2:22 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > On Thursday, March 09, 2017 11:15:47 PM Rafael J. Wysocki wrote:
> >> On Thursday, March 09, 2017 10:10:31 AM Dan Williams wrote:
> >> > On Thu, Mar 9, 2017 at 5:39 AM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> [..]
> >> > I *think* we're ok in this case because unplugging the CPU package
> >> > that contains a persistent memory device will trigger
> >> > devm_memremap_pages() to call arch_remove_memory(). Removing a pmem
> >> > device can't fail. It may be held off while pages are pinned for DMA
> >> > memory, but it will eventually complete.
> >>
> >> What about the offlining, though?  Is it guaranteed that no memory from those
> >> ranges will go back online after the acpi_scan_try_to_offline() call in
> >> acpi_scan_hot_remove()?
> >
> > My point is that after the acpi_evaluate_ej0() in acpi_scan_hot_remove() the
> > hardware is physically gone, so if anything is still doing DMA to that memory at
> > that point, then the user is going to be unhappy.
> 
> Hmm, ACPI 6.1 does not have any text about what _EJ0 means for ACPI0012.

ACPI0012 is exceptional, but in general _EJ0 does not have to be present under
a particular device for it to be affected.  It can be under the device's parent, for
example, in which case the entire subtree under a device with _EJ0 goes away in
one go.  And that very well may mean disconnect at the physical level (voltage
goes away IOW).

Thanks,
Rafael