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.

[1] http://lkml.kernel.org/r/20191030131122.8256-1-vincent.whitchurch@axis.com

[akpm@linux-foundation.org: tweak layout, per David]
Link: http://lkml.kernel.org/r/20191119092642.31799-1-mhocko@kernel.org
Fixes: 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations")
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Debugged-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Oscar Salvador <OSalvador@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/sparse.c b/mm/sparse.c
index 163b4d59cf6c976e3a61df2239b296dde0a46d52..8526d3bf1e4ea499c858d5895b0fe6bd23f75e23 100644 (file)
--- 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_raw(size,
-                                         PAGE_SIZE, addr,
+       map = memblock_alloc_try_nid_raw(size, size, addr,
                                          MEMBLOCK_ALLOC_ACCESSIBLE, nid);
        if (!map)
                panic("%s: Failed to allocate %lu bytes align=0x%lx nid=%d from=%pa\n",
@@ -482,10 +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()? */
-       sparsemap_buf =
-               memblock_alloc_try_nid_raw(size, PAGE_SIZE,
-                                               addr,
-                                               MEMBLOCK_ALLOC_ACCESSIBLE, nid);
+       /*
+        * 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(),
+                                       addr, MEMBLOCK_ALLOC_ACCESSIBLE, nid);
        sparsemap_buf_end = sparsemap_buf + size;
 }