[LSF/MM TOPIC]: memory management bits in arch/*

[Mike Rapoport <rppt@linux.ibm.com>]

Hi,

There is a lot of similar and duplicated code in architecture specific
bits of memory management.

For instance, as it was recently discussed at [1], most architectures
have

	#define GFP_KERNEL | __GFP_ZERO

for allocating page table pages and many of them use similar, if not
identical, implementation of pte_alloc_one*().

But that's only the tip of the iceberg.

I've seen several early_alloc() or similarly called routines that do

	if (slab_is_available())
		return kazalloc()
	else
		return memblock_alloc()

Some other trivial examples are free_initmem(), free_initrd_mem() and,
to some extent, mem_init(), but more generally there are a lot of
similarities in arch/*/mm/.

More complex cases are per-cpu initialization, passing of memory topology
to the generic MM, reservation of crash kernel, mmap of vdso etc. They
are not really duplicated, but still are very similar in at least
several architectures.

While factoring out the common code is an obvious step to take, I
believe there is also room for refining arch <-> mm interface to avoid
adding extra HAVE_ARCH_NO_BOOTMEM^w^wWHAT_NOT and then searching for
ways to get rid of them.

This is particularly true for mm initialization. It evolved the way
it's evolved, but now we can step back to black/white board and
consider design that hopefully will avoid problems like [2].

As a side note, it might be also worth looking into dropping
DISCONTIGMEM, although Kconfig still recommends to prefer it over
SPARSEMEM [3].

[1] https://lore.kernel.org/lkml/1547619692-7946-1-git-send-email-anshuman.khandual@arm.com/
[2] https://lore.kernel.org/lkml/20190114082416.30939-1-mhocko@kernel.org/
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/Kconfig#n49

-- 
Sincerely yours,
Mike.