linux-kernel - Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM

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Date:   Wed, 24 Apr 2019 17:34:45 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     David Hildenbrand <david@...hat.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Keith Busch <keith.busch@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ross Zwisler <zwisler@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Takashi Iwai <tiwai@...e.de>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used
 like normal RAM

> > > +static int
> > > +offline_memblock_cb(struct memory_block *mem, void *arg)
> >
> > Function name suggests that you are actually trying to offline memory
> > here. Maybe check_memblocks_offline_cb(), just like we have in
> > mm/memory_hotplug.c.

Makes sense, I will rename to check_memblocks_offline_cb()

> > > +     lock_device_hotplug();
> > > +     rc = walk_memory_range(start_pfn, end_pfn, dev, offline_memblock_cb);
> > > +     unlock_device_hotplug();
> > > +
> > > +     /*
> > > +      * If admin has not offlined memory beforehand, we cannot hotremove dax.
> > > +      * Unfortunately, because unbind will still succeed there is no way for
> > > +      * user to hotremove dax after this.
> > > +      */
> > > +     if (rc)
> > > +             return rc;
> >
> > Can't it happen that there is a race between you checking if memory is
> > offline and an admin onlining memory again? maybe pull the
> > remove_memory() into the locked region, using __remove_memory() instead.
>
> I think the race is ok. The admin gets to keep the pieces of allowing
> racing updates to the state and the kernel will keep the range active
> until the next reboot.

Thank you for noticing this. I will pull it into locking region.
Because, __remove_memory() has this code:

1868   ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
1869   check_memblock_offlined_cb);
1870   if (ret)
1871      BUG();

Basically, panic if at least something is not offlined.

Pasha