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

[Dan Williams]

On Fri, Jan 17, 2020 at 7:30 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Fri 17-01-20 15:58:26, David Hildenbrand wrote:
> > On 17.01.20 15:52, Michal Hocko wrote:
> > > On Fri 17-01-20 14:08:06, David Hildenbrand wrote:
> > >> On 17.01.20 12:33, Michal Hocko wrote:
> > >>> On Fri 17-01-20 11:57:59, David Hildenbrand wrote:
> > >>>> 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.
> >
> > It's a pure speedup. And for that, the interface has been working
> > perfectly fine for years?
> >
> > >
> > >> 4. lsmem displays/groups by "removable".
> > >
> > > Is anybody really using that?
> >
> > Well at least I am using that when testing to identify which
> > (ZONE_NORMAL!) block I can easily offline/re-online (e.g., to validate
> > all the zone shrinking stuff I have been fixing)
> >
> > So there is at least one user ;)
>
> Fair enough. But I would argue that there are better ways to do the same
> solely for testing purposes. Rather than having a subtly broken code to
> maintain.
>
> > >
> > >>> 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.
> >
> > I can see that, but I can perfectly well understand why - especially
> > powerpc - wants a fast way to sense which blocks actually sense to try
> > to online.
> >
> > The original patch correctly states
> >    "which sections of
> >     memory are likely to be removable before attempting the potentially
> >     expensive operation."
> >
> > It works as designed I would say.
>
> Then I would just keep it crippled the same way it has been for years
> without anybody noticing.

I tend to agree. At least the kmem driver that wants to unplug memory
could not use an interface that does not give stable answers. It just
relies on remove_memory() to return a definitive error.