On Thu, Mar 04, 2021 at 09:12:31AM +0100, David Hildenbrand wrote:

On 04.03.21 04:31, Anshuman Khandual wrote:

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On 3/4/21 2:54 AM, Will Deacon wrote:

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On Wed, Mar 03, 2021 at 07:04:33PM +0000, Catalin Marinas wrote:

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On Thu, Feb 11, 2021 at 01:35:56PM +0100, David Hildenbrand wrote:

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On 11.02.21 13:10, Anshuman Khandual wrote:

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On 2/11/21 5:23 PM, Will Deacon wrote:

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... and dropped. These patches appear to be responsible for a boot
regression reported by CKI:

Ahh, boot regression ? These patches only change the behaviour
for non boot memory only.

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https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@redhat.com (local)

Will look into the logs and see if there is something pointing to
the problem.

It's strange. One thing I can imagine is a mis-detection of early sections.
However, I don't see that happening:

In sparse_init_nid(), we:
1. Initialize the memmap
2. Set SECTION_IS_EARLY | SECTION_HAS_MEM_MAP via
    sparse_init_one_section()

Only hotplugged sections (DIMMs, dax/kmem) set SECTION_HAS_MEM_MAP without
SECTION_IS_EARLY - which is correct, because these are not early.

So once we know that we have valid_section() -- SECTION_HAS_MEM_MAP is set
-- early_section() should be correct.

Even if someone would be doing a pfn_valid() after
memblocks_present()->memory_present() but before
sparse_init_nid(), we should be fine (!valid_section() -> return 0).

I couldn't figure out how this could fail with Anshuman's patches.
Will's suspicion is that some invalid/null pointer gets dereferenced
before being initialised but the only case I see is somewhere in
pfn_section_valid() (ms->usage) if valid_section() && !early_section().

Assuming that we do get a valid_section(ms) && !early_section(ms), is
there a case where ms->usage is not initialised? I guess races with
section_deactivate() are not possible this early.

Another situation could be that pfn_valid() returns true when no memory
is mapped for that pfn.

The case I wondered about was __pfn_to_section() with a bogus pfn, since
with patch 2/2 we call that *before* checking that pfn_to_section_nr() is
sane.

Right, that is problematic. __pfn_to_section() should not be called without
first validating pfn_to_section_nr(), as it could cause out-of-bound access
on mem_section buffer. Will fix that order but as there is no test scenario
which is definitive for this reported regression, how should we ensure that
it fixes the problem ?

Oh, right, I missed that in patch #2. (and when comparing to generic
pfn_valid()).

I thought bisecting pointed at patch #1, that's why I didn't even have
another look at patch #2. Makes sense.

I don't think we ever bisected it beyond these two patches, so it could
be either of them. Anshuman -- please work with Veronika on this, as she
has access to the problematic machine and was really helpful in debugging
this last time.

Will