On Tue, Jul 02, 2019 at 04:42:34PM +1000, Rashmica Gupta wrote: > Hi David, > > Sorry for the late reply. > > On Wed, 2019-06-26 at 10:28 +0200, David Hildenbrand wrote: > > On 26.06.19 10:15, Oscar Salvador wrote: > > > On Wed, Jun 26, 2019 at 10:11:06AM +0200, David Hildenbrand wrote: > > > > Back then, I already mentioned that we might have some users that > > > > remove_memory() they never added in a granularity it wasn't > > > > added. My > > > > concerns back then were never fully sorted out. > > > > > > > > arch/powerpc/platforms/powernv/memtrace.c > > > > > > > > - Will remove memory in memory block size chunks it never added > > > > - What if that memory resides on a DIMM added via > > > > MHP_MEMMAP_DEVICE? > > > > > > > > Will it at least bail out? Or simply break? > > > > > > > > IOW: I am not yet 100% convinced that MHP_MEMMAP_DEVICE is save > > > > to be > > > > introduced. > > > > > > Uhm, I will take a closer look and see if I can clear your > > > concerns. > > > TBH, I did not try to use arch/powerpc/platforms/powernv/memtrace.c > > > yet. > > > > > > I will get back to you once I tried it out. > > > > > > > BTW, I consider the code in arch/powerpc/platforms/powernv/memtrace.c > > very ugly and dangerous. > > Yes it would be nice to clean this up. > > > We should never allow to manually > > offline/online pages / hack into memory block states. > > > > What I would want to see here is rather: > > > > 1. User space offlines the blocks to be used > > 2. memtrace installs a hotplug notifier and hinders the blocks it > > wants > > to use from getting onlined. > > 3. memory is not added/removed/onlined/offlined in memtrace code. > > > > I remember looking into doing it a similar way. I can't recall the > details but my issue was probably 'how does userspace indicate to > the kernel that this memory being offlined should be removed'? > > I don't know the mm code nor how the notifiers work very well so I > can't quite see how the above would work. I'm assuming memtrace would > register a hotplug notifier and when memory is offlined from userspace, > the callback func in memtrace would be called if the priority was high > enough? But how do we know that the memory being offlined is intended > for usto touch? Is there a way to offline memory from userspace not > using sysfs or have I missed something in the sysfs interface? > > On a second read, perhaps you are assuming that memtrace is used after > adding new memory at runtime? If so, that is not the case. If not, then > would you be able to clarify what I'm not seeing? Hi Rashmica, let us go the easy way here. Could you please explain: 1) How memtrace works 2) Why it was designed, what is the goal of the interface? 3) When it is supposed to be used? I have seen a couple of reports in the past from people running memtrace and failing to do so sometimes, and back then I could not grasp why people was using it, or under which circumstances was nice to have. So it would be nice to have a detailed explanation from the person who wrote it. Thanks -- Oscar Salvador SUSE L3
