[PATCH v4 0/2] arm64/mm: prevent panic on -ENOMEM in arch_add_memory()

[Linu Cherian <linu.cherian@arm.com>]

arch_add_memory() acts as a means to hotplug memory into a system. It
invokes __create_pgd_mapping() which further unwinds to call
pgtable_alloc(). Initially, this path was only invoked during early boot
and therefore it made sense to BUG_ON() in case pgtable_alloc() failed.
Now however, we risk running into a kernel crash if we try to hotplug
memory into a system that is already extremely tight on available
memory. This is undesirable and hence __create_pgd_mapping() and it's
helpers are reworked to be able to propagate the error from
pgtable_alloc() allowing the system to fail gracefully.

Keeping in mind that it is still essential to BUG_ON()/panic if
pgtable_alloc() encounters failure at the time of boot, a wrapper is
created around __create_pgd_mapping() which is designed to panic() if
it encounters a non-zero return value. This wrapper is then invoked from
the init functions instead of __create_pgd_mapping(), thereby keeping the
original functionality intact.

This theoretical bug was identified by Ryan Roberts<ryan.roberts@arm.com>
as a part of code review of the following series[1].

[1] https://lore.kernel.org/linux-arm-kernel/20250304222018.615808-4-yang@os.amperecomputing.com/


Changelog

v4:
Few trivial code readability improvements 

v3:
* Added a Fixes tag to patch 1 and CCed to stable
* Fixed a maybe-uninitialized case in alloc_init_pud
* Update  pgd_pgtable_alloc_init_mm to make use of
  pgd_pgtable_alloc_init_mm_gfp
  * Few other trivial cleanups

v2:
* With cleanup merged as part of, "arm64: mm: Move KPTI helpers to mmu.c"
  changes in patch 2(v1) got much simplified and squashed to patch 1 itself.
* Patch 2 now does a trivial renaming for better readability 
* Make use of INVALID_PHYS_ADDR for error checks instead of 0.  
* Do early function return where we do not have any
  common cleanup in return path
* Remove redundant variable initialization
* Changed BUG_ON to panic
* Renamed ___create_pgd_mapping to early_create_pgd_mapping  

This series is now rebased to linux-6.18-rc1.



Chaitanya S Prakash (1):
  arm64/mm: Allow __create_pgd_mapping() to propagate pgtable_alloc()
    errors

Linu Cherian (1):
  arm64/mm: Rename try_pgd_pgtable_alloc_init_mm

 arch/arm64/mm/mmu.c | 220 ++++++++++++++++++++++++++++----------------
 1 file changed, 139 insertions(+), 81 deletions(-)

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2.43.0