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mm/sparse: remove sparse buffer pre-allocation mechanism

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Commit 9bdac91424 ("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>
This commit is contained in:
Muchun Song
2026-04-10 17:24:19 +08:00
committed by Andrew Morton
parent 8c2c7df58b
commit ffe5539313
3 changed files with 2 additions and 64 deletions
Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/mm.h
View File

@@ -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,

7
mm/sparse-vmemmap.c
View File

@@ -87,15 +87,10 @@ static void * __meminit altmap_alloc_block_buf(unsigned long size,
void * __meminit vmemmap_alloc_block_buf(unsigned long size, int node,
struct vmem_altmap *altmap)
{
void *ptr;
if (altmap)
return altmap_alloc_block_buf(size, altmap);
ptr = sparse_buffer_alloc(size);
if (!ptr)
ptr = vmemmap_alloc_block(size, node);
return ptr;
return vmemmap_alloc_block(size, node);
}
static unsigned long __meminit vmem_altmap_next_pfn(struct vmem_altmap *altmap)

58
mm/sparse.c
View File

@@ -239,12 +239,9 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
struct dev_pagemap *pgmap)
{
unsigned long size = section_map_size();
struct page *map = sparse_buffer_alloc(size);
struct page *map;
phys_addr_t addr = __pa(MAX_DMA_ADDRESS);
if (map)
return map;
map = memmap_alloc(size, size, addr, nid, false);
if (!map)
panic("%s: Failed to allocate %lu bytes align=0x%lx nid=%d from=%pa\n",
@@ -254,55 +251,6 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
}
#endif /* !CONFIG_SPARSEMEM_VMEMMAP */
static void *sparsemap_buf __meminitdata;
static void *sparsemap_buf_end __meminitdata;
static inline void __meminit sparse_buffer_free(unsigned long size)
{
WARN_ON(!sparsemap_buf || size == 0);
memblock_free(sparsemap_buf, size);
}
static void __init sparse_buffer_init(unsigned long size, int nid)
{
phys_addr_t addr = __pa(MAX_DMA_ADDRESS);
WARN_ON(sparsemap_buf); /* forgot to call sparse_buffer_fini()? */
/*
* Pre-allocated buffer is mainly used by __populate_section_memmap
* and we want it to be properly aligned to the section size - this is
* especially the case for VMEMMAP which maps memmap to PMDs
*/
sparsemap_buf = memmap_alloc(size, section_map_size(), addr, nid, true);
sparsemap_buf_end = sparsemap_buf + size;
}
static void __init sparse_buffer_fini(void)
{
unsigned long size = sparsemap_buf_end - sparsemap_buf;
if (sparsemap_buf && size > 0)
sparse_buffer_free(size);
sparsemap_buf = NULL;
}
void * __meminit sparse_buffer_alloc(unsigned long size)
{
void *ptr = NULL;
if (sparsemap_buf) {
ptr = (void *) roundup((unsigned long)sparsemap_buf, size);
if (ptr + size > sparsemap_buf_end)
ptr = NULL;
else {
/* Free redundant aligned space */
if ((unsigned long)(ptr - sparsemap_buf) > 0)
sparse_buffer_free((unsigned long)(ptr - sparsemap_buf));
sparsemap_buf = ptr + size;
}
}
return ptr;
}
void __weak __meminit vmemmap_populate_print_last(void)
{
}
@@ -360,8 +308,6 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
goto failed;
}
sparse_buffer_init(map_count * section_map_size(), nid);
sparse_vmemmap_init_nid_early(nid);
for_each_present_section_nr(pnum_begin, pnum) {
@@ -379,7 +325,6 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
__func__, nid);
pnum_begin = pnum;
sparse_usage_fini();
sparse_buffer_fini();
goto failed;
}
memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
@@ -388,7 +333,6 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
}
}
sparse_usage_fini();
sparse_buffer_fini();
return;
failed:
/*