Commit e80d6a24 authored by Mel Gorman's avatar Mel Gorman Committed by Russell King

[ARM] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo

Ordinarily, memory holes in flatmem still have a valid memmap and is safe
to use. However, an architecture (ARM) frees up the memmap backing memory
holes on the assumption it is never used. /proc/pagetypeinfo reads the
whole range of pages in a zone believing that the memmap is valid and that
pfn_valid will return false if it is not. On ARM, freeing the memmap breaks
the page->zone linkages even though pfn_valid() returns true and the kernel
can oops shortly afterwards due to accessing a bogus struct zone *.

This patch lets architectures say when FLATMEM can have holes in the
memmap. Rather than an expensive check for valid memory, /proc/pagetypeinfo
will confirm that the page linkages are still valid by checking page->zone
is still the expected zone. The lookup of page_zone is safe as there is a
limited range of memory that is accessed when calling page_zone.  Even if
page_zone happens to return the correct zone, the impact is that the counters
in /proc/pagetypeinfo are slightly off but fragmentation monitoring is
unlikely to be relevant on an embedded system.
Reported-by: default avatarH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
Tested-by: default avatarH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent f1bcf7e3
......@@ -810,6 +810,11 @@ config OABI_COMPAT
UNPREDICTABLE (in fact it can be predicted that it won't work
at all). If in doubt say Y.
config ARCH_FLATMEM_HAS_HOLES
bool
default y
depends on FLATMEM
config ARCH_DISCONTIGMEM_ENABLE
bool
default (ARCH_LH7A40X && !LH7A40X_CONTIGMEM)
......
......@@ -516,9 +516,26 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
continue;
page = pfn_to_page(pfn);
#ifdef CONFIG_ARCH_FLATMEM_HAS_HOLES
/*
* Ordinarily, memory holes in flatmem still have a valid
* memmap for the PFN range. However, an architecture for
* embedded systems (e.g. ARM) can free up the memmap backing
* holes to save memory on the assumption the memmap is
* never used. The page_zone linkages are then broken even
* though pfn_valid() returns true. Skip the page if the
* linkages are broken. Even if this test passed, the impact
* is that the counters for the movable type are off but
* fragmentation monitoring is likely meaningless on small
* systems.
*/
if (page_zone(page) != zone)
continue;
#endif
mtype = get_pageblock_migratetype(page);
count[mtype]++;
if (mtype < MIGRATE_TYPES)
count[mtype]++;
}
/* Print counts */
......
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