Woops, looks like my phone doesn't send plain text emails :/ On Tue, Jul 2, 2019 at 6:52 PM Rashmica Gupta <rashmica.g@xxxxxxxxx> wrote: > > On Tue, Jul 2, 2019 at 5:48 PM Oscar Salvador <osalvador@xxxxxxx> wrote: >> >> 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: >> > > Sure! > >> >> 1) How memtrace works > > > You write the size of the chunk of memory you want into the debugfs file > and memtrace will attempt to find a contiguous section of memory of that size > that can be offlined. If it finds that, then the memory is removed from the > kernel's mappings. If you want a different size, then you write that to the > debugsfs file and memtrace will re-add the memory it first removed and then > try to offline and remove the a chunk of the new size. > > >> >> 2) Why it was designed, what is the goal of the interface? >> 3) When it is supposed to be used? >> > > There is a hardware debugging facility (htm) on some power chips. To use > this you need a contiguous portion of memory for the output to be dumped > to - and we obviously don't want this memory to be simultaneously used by > the kernel. > > At boot time we can portion off a section of memory for this (and not tell the > kernel about it), but sometimes you want to be able to use the hardware > debugging facilities and you haven't done this and you don't want to reboot > your machine - and memtrace is the solution for this. > > If you're curious one tool that uses this debugging facility is here: > https://github.com/open-power/pdbg. Relevant files are libpdbg/htm.c and src/htm.c. > > >> 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. >> > > Is that enough detail? > >> >> Thanks >> >> -- >> Oscar Salvador >> SUSE L3
