linux-kernel - [PATCH] mm/sparse: Consistently do not zero memmap

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Message-Id: <20191030131122.8256-1-vincent.whitchurch@axis.com>
Date:   Wed, 30 Oct 2019 14:11:22 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     akpm@...ux-foundation.org
Cc:     osalvador@...e.de, mhocko@...e.com, pasha.tatashin@...cle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vincent Whitchurch <rabinv@...s.com>
Subject: [PATCH] mm/sparse: Consistently do not zero memmap

sparsemem without VMEMMAP has two allocation paths to allocate the
memory needed for its memmap (done in sparse_mem_map_populate()).

In one allocation path (sparse_buffer_alloc() succeeds), the memory is
not zeroed (since it was previously allocated with
memblock_alloc_try_nid_raw()).

In the other allocation path (sparse_buffer_alloc() fails and
sparse_mem_map_populate() falls back to memblock_alloc_try_nid()), the
memory is zeroed.

AFAICS this difference does not appear to be on purpose.  If the code is
supposed to work with non-initialized memory (__init_single_page() takes
care of zeroing the struct pages which are actually used), we should
consistently not zero the memory, to avoid masking bugs.

(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.)

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
---
 mm/sparse.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index f6891c1992b1..01e467adc219 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -458,7 +458,7 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
 	if (map)
 		return map;

-	map = memblock_alloc_try_nid(size,
+	map = memblock_alloc_try_nid_raw(size,
 					  PAGE_SIZE, addr,
 					  MEMBLOCK_ALLOC_ACCESSIBLE, nid);
 	if (!map)
-- 
2.20.0