[PATCH] mm, sparse: do not waste pre allocated memmap space

[Michal Hocko <mhocko@kernel.org>]

From: Michal Hocko <mhocko@suse.com>

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.

[1] http://lkml.kernel.org/r/20191030131122.8256-1-vincent.whitchurch@axis.com
Reported-and-debugged-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Fixes: 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations")
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/sparse.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index f6891c1992b1..079f3e3c4cab 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -458,8 +458,7 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
 	if (map)
 		return map;
 
-	map = memblock_alloc_try_nid(size,
-					  PAGE_SIZE, addr,
+	map = memblock_alloc_try_nid(size, size, addr,
 					  MEMBLOCK_ALLOC_ACCESSIBLE, nid);
 	if (!map)
 		panic("%s: Failed to allocate %lu bytes align=0x%lx nid=%d from=%pa\n",
@@ -482,8 +481,13 @@ 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 =
-		memblock_alloc_try_nid_raw(size, PAGE_SIZE,
+		memblock_alloc_try_nid_raw(size, section_map_size(),
 						addr,
 						MEMBLOCK_ALLOC_ACCESSIBLE, nid);
 	sparsemap_buf_end = sparsemap_buf + size;
-- 
2.20.1