linux-kernel - Re: [v5 2/3] mm/hotplug: make remove

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Message-ID: <CAPcyv4greisKBSorzQWebcVOf2AqUH6DwbvNKMW0MQ5bCwYZrw@mail.gmail.com>
Date:   Mon, 6 May 2019 11:01:14 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Pavel Tatashin <pasha.tatashin@...een.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>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [v5 2/3] mm/hotplug: make remove_memory() interface useable

On Mon, May 6, 2019 at 10:57 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> > -static inline void remove_memory(int nid, u64 start, u64 size) {}
> > +static inline bool remove_memory(int nid, u64 start, u64 size)
> > +{
> > +     return -EBUSY;
> > +}
>
> This seems like an appropriate place for a WARN_ONCE(), if someone
> manages to call remove_memory() with hotplug disabled.
>
> BTW, I looked and can't think of a better errno, but -EBUSY probably
> isn't the best error code, right?
>
> > -void remove_memory(int nid, u64 start, u64 size)
> > +/**
> > + * remove_memory
> > + * @nid: the node ID
> > + * @start: physical address of the region to remove
> > + * @size: size of the region to remove
> > + *
> > + * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
> > + * and online/offline operations before this call, as required by
> > + * try_offline_node().
> > + */
> > +void __remove_memory(int nid, u64 start, u64 size)
> >  {
> > +
> > +     /*
> > +      * trigger BUG() is some memory is not offlined prior to calling this
> > +      * function
> > +      */
> > +     if (try_remove_memory(nid, start, size))
> > +             BUG();
> > +}
>
> Could we call this remove_offline_memory()?  That way, it makes _some_
> sense why we would BUG() if the memory isn't offline.

Please WARN() instead of BUG() because failing to remove memory should
not be system fatal.