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.st...@dwillia2-desk3.amr.corp.intel.com
> Cc: Michal Hocko <mho...@suse.com>
> Cc: Vlastimil Babka <vba...@suse.cz>
> Cc: Logan Gunthorpe <log...@deltatee.com>
> Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
> ---
> 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
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