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.
