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

From: David Hildenbrand
Date: Tue Nov 19 2019 - 05:04:08 EST

On 19.11.19 10:26, Michal Hocko wrote:
From: Michal Hocko <mhocko@xxxxxxxx>

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.

Reported-and-debugged-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
Fixes: 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations")
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
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,
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(),

Wow, that alignment/layout gives me nightmares ^

None of your business, though :)

sparsemap_buf_end = sparsemap_buf + size;

Acked-by: David Hildenbrand <david@xxxxxxxxxx>



David / dhildenb