linux-kernel - Re: [PATCH RFC v1] mm: is_mem_section

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Message-ID: <20200121120714.GJ29276@dhcp22.suse.cz>
Date:   Tue, 21 Jan 2020 13:07:14 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Leonardo Bras <leonardo@...ux.ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Anshuman Khandual <anshuman.khandual@....com>,
        lantianyu1986@...il.com,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

On Mon 20-01-20 10:14:44, David Hildenbrand wrote:
> On 20.01.20 08:48, Michal Hocko wrote:
> > On Fri 17-01-20 08:57:51, Dan Williams wrote:
> > [...]
> >> Unless the user is willing to hold the device_hotplug_lock over the
> >> evaluation then the result is unreliable.
> > 
> > Do we want to hold the device_hotplug_lock from this user readable file
> > in the first place? My book says that this just waits to become a
> > problem.
> 
> It was the "big hammer" solution for this RFC.
> 
> I think we could do with a try_lock() on the device_lock() paired with a
> device->removed flag. The latter is helpful for properly catching zombie
> devices on the onlining/offlining path either way (and on my todo list).

try_lock would be more considerate. It would at least make any potential
hammering a bit harder.

> > Really, the interface is flawed and should have never been merged in the
> > first place. We cannot simply remove it altogether I am afraid so let's
> > at least remove the bogus code and pretend that the world is a better
> > place where everything is removable except the reality sucks...
> 
> As I expressed already, the interface works as designed/documented and
> has been used like that for years.

It seems we do differ in the usefulness though. Using a crappy interface
for years doesn't make it less crappy. I do realize we cannot remove the
interface but we can remove issues with the implementation and I dare to
say that most existing users wouldn't really notice.

> I tend to agree that it never should have been merged like that.
> 
> We have (at least) two places that are racy (with concurrent memory
> hotplug):
> 
> 1. /sys/.../memoryX/removable
> - a) make it always return yes and make the interface useless
> - b) add proper locking and keep it running as is (e.g., so David can
>      identify offlineable memory blocks :) ).
> 
> 2. /sys/.../memoryX/valid_zones
> - a) always return "none" if the memory is online
> - b) add proper locking and keep it running as is
> - c) cache the result ("zone") when a block is onlined (e.g., in
> mem->zone. If it is NULL, either mixed zones or unknown)
> 
> At least 2. already scream for a proper device_lock() locking as the
> mem->state is not stable across the function call.
> 
> 1a and 2a are the easiest solutions but remove all ways to identify if a
> memory block could theoretically be offlined - without trying
> (especially, also to identify the MOVABLE zone).
> 
> I tend to prefer 1b) and 2c), paired with proper device_lock() locking.
> We don't affect existing use cases but are able to simplify the code +
> fix the races.
> 
> What's your opinion? Any alternatives?

1a) and 2c) if you ask me.
-- 
Michal Hocko
SUSE Labs