On Wed, May 01, 2019 at 10:56:10PM -0700, Dan Williams wrote: > The libnvdimm sub-system has suffered a series of hacks and broken > workarounds for the memory-hotplug implementation's awkward > section-aligned (128MB) granularity. For example the following backtrace > is emitted when attempting arch_add_memory() with physical address > ranges that intersect 'System RAM' (RAM) with 'Persistent Memory' (PMEM) > within a given section: > > WARNING: CPU: 0 PID: 558 at kernel/memremap.c:300 devm_memremap_pages+0x3b5/0x4c0 > devm_memremap_pages attempted on mixed region [mem 0x200000000-0x2fbffffff flags 0x200] > [..] > Call Trace: > dump_stack+0x86/0xc3 > __warn+0xcb/0xf0 > warn_slowpath_fmt+0x5f/0x80 > devm_memremap_pages+0x3b5/0x4c0 > __wrap_devm_memremap_pages+0x58/0x70 [nfit_test_iomap] > pmem_attach_disk+0x19a/0x440 [nd_pmem] > > Recently it was discovered that the problem goes beyond RAM vs PMEM > collisions as some platform produce PMEM vs PMEM collisions within a > given section. The libnvdimm workaround for that case revealed that the > libnvdimm section-alignment-padding implementation has been broken for a > long while. A fix for that long-standing breakage introduces as many > problems as it solves as it would require a backward-incompatible change > to the namespace metadata interpretation. Instead of that dubious route > [1], address the root problem in the memory-hotplug implementation. > > [1]: https://lore.kernel.org/r/155000671719.348031.2347363160141119237.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> > --- > mm/sparse.c | 223 ++++++++++++++++++++++++++++++++++++++++------------------- > 1 file changed, 150 insertions(+), 73 deletions(-) > > diff --git a/mm/sparse.c b/mm/sparse.c > index 198371e5fc87..419a3620af6e 100644 > --- a/mm/sparse.c > +++ b/mm/sparse.c > @@ -83,8 +83,15 @@ static int __meminit sparse_index_init(unsigned long section_nr, int nid) > unsigned long root = SECTION_NR_TO_ROOT(section_nr); > struct mem_section *section; > > + /* > + * An existing section is possible in the sub-section hotplug > + * case. First hot-add instantiates, follow-on hot-add reuses > + * the existing section. > + * > + * The mem_hotplug_lock resolves the apparent race below. > + */ > if (mem_section[root]) > - return -EEXIST; > + return 0; Just a sidenote: we do not bail out on -EEXIST, so it should be fine if we stick with it. But if not, I would then clean up sparse_add_section: --- a/mm/sparse.c +++ b/mm/sparse.c @@ -901,13 +901,12 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn, int ret; ret = sparse_index_init(section_nr, nid); - if (ret < 0 && ret != -EEXIST) + if (ret < 0) return ret; memmap = section_activate(nid, start_pfn, nr_pages, altmap); if (IS_ERR(memmap)) return PTR_ERR(memmap); - ret = 0; > + > + if (!mask) > + rc = -EINVAL; > + else if (mask & ms->usage->map_active) else if (ms->usage->map_active) should be enough? > + rc = -EEXIST; > + else > + ms->usage->map_active |= mask; > + > + if (rc) { > + if (usage) > + ms->usage = NULL; > + kfree(usage); > + return ERR_PTR(rc); > + } > + > + /* > + * The early init code does not consider partially populated > + * initial sections, it simply assumes that memory will never be > + * referenced. If we hot-add memory into such a section then we > + * do not need to populate the memmap and can simply reuse what > + * is already there. > + */ This puzzles me a bit. I think we cannot have partially populated early sections, can we? And how we even come to hot-add memory into those? Could you please elaborate a bit here? > + ms = __pfn_to_section(start_pfn); > section_mark_present(ms); > - sparse_init_one_section(ms, section_nr, memmap, usage); > + sparse_init_one_section(ms, section_nr, memmap, ms->usage); > > -out: > - if (ret < 0) { > - kfree(usage); > - depopulate_section_memmap(start_pfn, PAGES_PER_SECTION, altmap); > - } > + if (ret < 0) > + section_deactivate(start_pfn, nr_pages, nid, altmap); Uhm, if my eyes do not trick me, ret is only used for the return value from sparse_index_init(), so this is not needed. Can we get rid of it? Unfortunately I am running out of time, but I plan to keep reviewing this patch in the next few days. -- Oscar Salvador SUSE L3
