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TL;DR: always call the PTE/PMD ctor, passing the appropriate mm to skip
ptlock_init() if unneeded.
__create_pgd_mapping() is used for creating different kinds of mappings,
and may allocate page table pages if passed an allocator callback. There
are currently three such cases:
1. create_pgd_mapping(), which is used to create the EFI mapping
2. arch_add_memory()
3. map_entry_trampoline()
1. uses pgd_pgtable_alloc() as allocator callback, which calls the
PTE/PMD ctor, while 2. and 3. use __pgd_pgtable_alloc(), which does not.
The rationale is most likely that pgtables associated with init_mm do not
make use of split page table locks, and it is therefore unnecessary to
initialise them by calling the ctor. 2. operates on swapper_pg_dir so
the allocated pgtables are clearly associated with init_mm, this is
arguably the case for 3. too (the trampoline mapping is never modified so
ptlocks are anyway irrelevant). 1. corresponds to efi_mm so ptlocks need
to be initialised in that case.
We are now moving towards calling the ctor for all page tables, even those
associated with init_mm. pagetable_{pte,pmd}_ctor() have become aware of
the associated mm so that the ptlock initialisation can be skipped for
init_mm. This patch therefore amends the allocator callbacks so that the
PTE/PMD ctor are always called, with an appropriate mm pointer to avoid
unnecessary ptlock overhead.
Modifying the prototype of the allocator callbacks to take the mm and
propagating that pointer all the way down would be pretty invasive.
Instead:
* __pgd_pgtable_alloc() (cases 2. and 3. above) is replaced with
pgd_pgtable_alloc_init_mm(), resulting in the ctors being called with
&init_mm. This is the main functional change in this patch; the ptlock
still isn't initialised, but other ctor actions (e.g.
accounting-related) are now carried out for those allocated pgtables.
* pgd_pgtable_alloc() (case 1. above) is replaced with
pgd_pgtable_alloc_special_mm(), resulting in the ctors being called with
NULL as mm. No functional change here; NULL essentially means "not
init_mm", and the ptlock is still initialised.
__pgd_pgtable_alloc() is now the common implementation of those two
helpers. While at it we switch it to using pagetable_alloc() like
standard pgtable allocator functions and remove the comment regarding ctor
calls (ctors are now always expected to be called).
Link: https://lkml.kernel.org/r/20250408095222.860601-10-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Linus Waleij <linus.walleij@linaro.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: <x86@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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