mm/sparse: remove sparse buffer pre-allocation mechanism

Commit 9bdac9142407 ("sparsemem: Put mem map for one node together.")
introduced a mechanism to pre-allocate a large memory block to hold all
memmaps for a NUMA node upfront.

However, the original commit message did not clearly state the actual
benefits or the necessity of explicitly pre-allocating a single chunk for
all memmap areas of a given node.

One of the concerns about removing this pre-allocation is that the
subsequent per-section memmap allocations could become scattered around,
and might turn too many memory blocks/sections into an "un-offlinable"
state.  However, tests show that even without the explicit node-wide
pre-allocation, memblock still allocates memory closely and back-to-back. 
When tracing vmemmap_set_pmd allocations, the physical chunks allocated by
memblock are strictly adjacent to each other in a single contiguous
physical range (mapped top-down).  Because they are packed tightly
together naturally, they will at most consume or pollute the exact same
number of memory blocks as the explicit pre-allocation did.

Another concern is the boot performance impact of calling memmap_alloc()
multiple times compared to one large node-wide allocation.  Tests on a
256GB VM showed that memmap allocation time increased from 199,555 ns to
741,292 ns.  Even though it is 3.7x slower, on a 1TB machine, the entire
memory allocation time would only take a few milliseconds.  This boot
performance difference is completely negligible.

Since no negative impact on memory offlining behavior or noticeable boot
performance regression was found, this patch proposes removing the
explicit node-wide memmap pre-allocation mechanism to reduce the
maintenance burden.

Link: https://lore.kernel.org/20260410092419.2446420-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 0 insertions, 1 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e3b6112a8d79..8a0078a4dc78 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4855,7 +4855,6 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
 }
 #endif
 
-void *sparse_buffer_alloc(unsigned long size);
 unsigned long section_map_size(void);
 struct page * __populate_section_memmap(unsigned long pfn,
 		unsigned long nr_pages, int nid, struct vmem_altmap *altmap,