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

[David Hildenbrand <david@redhat.com>]

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).

> 
> 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. 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?

-- 
Thanks,

David / dhildenb