Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

[Michal Hocko <mhocko@kernel.org>]

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