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* [patch 086/158] mm/sparse.c: do not waste pre allocated memmap space
@ 2019-12-01  1:54 akpm
  0 siblings, 0 replies; only message in thread
From: akpm @ 2019-12-01  1:54 UTC (permalink / raw)
  To: akpm, david, linux-mm, mhocko, mm-commits, OSalvador,
	pasha.tatashin, torvalds, vincent.whitchurch

From: Michal Hocko <>
Subject: mm/sparse.c: do not waste pre allocated memmap space

Vincent has noticed [1] that there is something unusual with the memmap
allocations going on on his platform

: I noticed this because on my ARM64 platform, with 1 GiB of memory the
: first [and only] section is allocated from the zeroing path while with
: 2 GiB of memory the first 1 GiB section is allocated from the
: non-zeroing path.

The underlying problem is that although sparse_buffer_init allocates
enough memory for all sections on the node sparse_buffer_alloc is not able
to consume them due to mismatch in the expected allocation alignement. 
While sparse_buffer_init preallocation uses the PAGE_SIZE alignment the
real memmap has to be aligned to section_map_size() this results in a
wasted initial chunk of the preallocated memmap and unnecessary fallback
allocation for a section.

While we are at it also change __populate_section_memmap to align to the
requested size because at least VMEMMAP has constrains to have memmap
properly aligned.


[ tweak layout, per David]
Fixes: 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations")
Signed-off-by: Michal Hocko <>
Reported-by: Vincent Whitchurch <>
Debugged-by: Vincent Whitchurch <>
Acked-by: David Hildenbrand <>
Cc: Pavel Tatashin <>
Cc: Oscar Salvador <>
Signed-off-by: Andrew Morton <>

 mm/sparse.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

--- a/mm/sparse.c~mm-sparse-do-not-waste-pre-allocated-memmap-space
+++ a/mm/sparse.c
@@ -458,8 +458,7 @@ struct page __init *__populate_section_m
 	if (map)
 		return map;
-	map = memblock_alloc_try_nid_raw(size,
-					  PAGE_SIZE, addr,
+	map = memblock_alloc_try_nid_raw(size, size, addr,
 	if (!map)
 		panic("%s: Failed to allocate %lu bytes align=0x%lx nid=%d from=%pa\n",
@@ -482,10 +481,13 @@ static void __init sparse_buffer_init(un
 	phys_addr_t addr = __pa(MAX_DMA_ADDRESS);
 	WARN_ON(sparsemap_buf);	/* forgot to call sparse_buffer_fini()? */
-	sparsemap_buf =
-		memblock_alloc_try_nid_raw(size, PAGE_SIZE,
-						addr,
+	/*
+	 * 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 = memblock_alloc_try_nid_raw(size, section_map_size(),
 	sparsemap_buf_end = sparsemap_buf + size;

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2019-12-01  1:54 [patch 086/158] mm/sparse.c: do not waste pre allocated memmap space akpm

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