diff options
| author | Petr Tesarik <ptesarik@suse.com> | 2018-01-31 16:20:26 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2018-01-31 17:18:39 -0800 |
| commit | def9b71ee651a6fee93a10734b94f93a69cdb2d4 (patch) | |
| tree | facc9a7783035460106f12e2fc845e5d8225b6c5 /mm/sparse.c | |
| parent | 112d2d29fc087d3078f60db220c4f31f25e59cf0 (diff) | |
include/linux/mmzone.h: fix explanation of lower bits in the SPARSEMEM mem_map pointer
The comment is confusing. On the one hand, it refers to 32-bit
alignment (struct page alignment on 32-bit platforms), but this would
only guarantee that the 2 lowest bits must be zero. On the other hand,
it claims that at least 3 bits are available, and 3 bits are actually
used.
This is not broken, because there is a stronger alignment guarantee,
just less obvious. Let's fix the comment to make it clear how many bits
are available and why.
Although memmap arrays are allocated in various places, the resulting
pointer is encoded eventually, so I am adding a BUG_ON() here to enforce
at runtime that all expected bits are indeed available.
I have also added a BUILD_BUG_ON to check that PFN_SECTION_SHIFT is
sufficient, because this part of the calculation can be easily checked
at build time.
[ptesarik@suse.com: v2]
Link: http://lkml.kernel.org/r/20180125100516.589ea6af@ezekiel.suse.cz
Link: http://lkml.kernel.org/r/20180119080908.3a662e6f@ezekiel.suse.cz
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemi Wang <kemi.wang@intel.com>
Cc: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/sparse.c')
| -rw-r--r-- | mm/sparse.c | 6 |
1 files changed, 5 insertions, 1 deletions
diff --git a/mm/sparse.c b/mm/sparse.c index 2609aba121e8..6b8b5e91ceef 100644 --- a/mm/sparse.c +++ b/mm/sparse.c @@ -264,7 +264,11 @@ unsigned long __init node_memmap_size_bytes(int nid, unsigned long start_pfn, */ static unsigned long sparse_encode_mem_map(struct page *mem_map, unsigned long pnum) { - return (unsigned long)(mem_map - (section_nr_to_pfn(pnum))); + unsigned long coded_mem_map = + (unsigned long)(mem_map - (section_nr_to_pfn(pnum))); + BUILD_BUG_ON(SECTION_MAP_LAST_BIT > (1UL<<PFN_SECTION_SHIFT)); + BUG_ON(coded_mem_map & ~SECTION_MAP_MASK); + return coded_mem_map; } /* |