On Thu 26-03-20 15:26:22, Aneesh Kumar K.V wrote:

On 3/26/20 3:10 PM, Michal Hocko wrote:

quoted

On Wed 25-03-20 08:49:14, Aneesh Kumar K.V wrote:

quoted

Fixes the below crash

BUG: Kernel NULL pointer dereference on read at 0x00000000
Faulting instruction address: 0xc000000000c3447c
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
CPU: 11 PID: 7519 Comm: lt-ndctl Not tainted 5.6.0-rc7-autotest #1
...
NIP [c000000000c3447c] vmemmap_populated+0x98/0xc0
LR [c000000000088354] vmemmap_free+0x144/0x320
Call Trace:
  section_deactivate+0x220/0x240

It would be great to match this to the specific source code.

The crash is due to NULL dereference at

test_bit(idx, ms->usage->subsection_map); due to ms->usage = NULL;

It would be nice to call that out here as well

[...]

quoted

Why do we have to free usage before deactivaing section memmap? Now that
we have a late section_mem_map reset shouldn't we tear down the usage in
the same branch?

We still need to make the section invalid before we call into
depopulate_section_memmap(). Because architecture like powerpc can share
vmemmap area across sections (16MB mapping of vmemmap area) and we use
vmemmap_popluated() to make that decision.

This should be noted in a comment as well.

quoted

quoted

Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case")
Cc: Baoquan He <redacted>
Reported-by: Sachin Sant <redacted>
Signed-off-by: Aneesh Kumar K.V <redacted>
---
  mm/sparse.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/mm/sparse.c b/mm/sparse.c
index aadb7298dcef..3012d1f3771a 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c

@@ -781,6 +781,8 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
  			ms->usage = NULL;
  		}
  		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+		/* Mark the section invalid */
+		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;

Btw. this comment is not really helping at all.

That is marking the section invalid so that

static inline int valid_section(struct mem_section *section)
{
	return (section && (section->section_mem_map & SECTION_HAS_MEM_MAP));
}


returns false.

Yes that is obvious once you are clear where to look. I was really
hoping for a comment that would simply point you to the right
direcection without chasing SECTION_HAS_MEM_MAP usage. This code is
subtle and useful comments, even when they state something that is
obvious to you _right_now_, can be really helpful.

Thanks!
-- 
Michal Hocko
SUSE Labs