Re: [PATCH v7 09/12] mm/sparsemem: Support sub-section hotplug

From: Dan Williams
Date: Tue Jun 04 2019 - 00:21:51 EST


On Fri, May 3, 2019 at 5:56 AM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> 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;

Good catch, folded the cleanup.

>
>
> > +
> > + 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?

Yes, at boot memory need not be section aligned it has historically
been handled as a un-removable section of memory with holes.

> And how we even come to hot-add memory into those?
>
> Could you please elaborate a bit here?

Those sections are excluded from add_memory_resource() adding more
memory, but arch_add_memory() with sub-section support can fill in the
subsection holes in mem_map.

>
> > + 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?

Yes, these can go.

Apologies for the delay and missing these comments in the v8 posting.