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

On Fri 17-01-20 14:08:06, David Hildenbrand wrote:

On 17.01.20 12:33, Michal Hocko wrote:

quoted

On Fri 17-01-20 11:57:59, David Hildenbrand wrote:

quoted

Let's refactor that code. We want to check if we can offline memory
blocks. Add a new function is_mem_section_offlineable() for that and
make it call is_mem_section_offlineable() for each contained section.
Within is_mem_section_offlineable(), add some more sanity checks and
directly bail out if the section contains holes or if it spans multiple
zones.

I didn't read the patch (yet) but I am wondering. If we want to touch
this code, can we simply always return true there? I mean whoever
depends on this check is racy and the failure can happen even after
the sysfs says good to go, right? The check is essentially as expensive
as calling the offlining code itself. So the only usecase I can think of
is a dumb driver to crawl over blocks and check which is removable and
try to hotremove it. But just trying to offline one block after another
is essentially going to achieve the same.

Some thoughts:

1. It allows you to check if memory is likely to be offlineable without
doing expensive locking and trying to isolate pages (meaning:
zone->lock, mem_hotplug_lock. but also, calling drain_all_pages()
when isolating)

2. There are use cases that want to identify a memory block/DIMM to
unplug. One example is PPC DLPAR code (see this patch). Going over all
memory block trying to offline them is an expensive operation.

3. powerpc-utils (https://github.com/ibm-power-utilities/powerpc-utils)
makes use of /sys/.../removable to speed up the search AFAIK.

Well, while I do see those points I am not really sure they are worth
having a broken (by-definition) interface.
 

4. lsmem displays/groups by "removable".

Is anybody really using that?

5. If "removable=false" then it usually really is not offlineable.
Of course, there could also be races (free the last unmovable page),
but it means "don't even try". OTOH, "removable=true" is more racy,
and gives less guarantees. ("looks okay, feel free to try")

Yeah, but you could be already pessimistic and try movable zones before
other kernel zones.

quoted

Or does anybody see any reasonable usecase that would break if we did
that unconditional behavior?

If we would return always "true", then the whole reason the
interface originally was introduced would be "broken" (meaning, less
performant as you would try to offline any memory block).

I would argue that the whole interface is broken ;). Not the first time
in the kernel development history and not the last time either. What I
am trying to say here is that unless there are _real_ usecases depending
on knowing that something surely is _not_ offlineable then I would just
try to drop the functionality while preserving the interface and see
what happens.
-- 
Michal Hocko
SUSE Labs