This Linux kernel change "mm, page_alloc: fix a division by zero error when boosting watermarks v2" is included in the Linux 5.0 release. This change is authored by Mel Gorman <mgorman [at] techsingularity.net> on Wed Feb 20 22:19:49 2019 -0800. The commit for this change in Linux stable tree is 94b3334 (patch).
mm, page_alloc: fix a division by zero error when boosting watermarks v2 Yury Norov reported that an arm64 KVM instance could not boot since after v5.0-rc1 and could addressed by reverting the patches 1c30844d2dfe272d58c ("mm: reclaim small amounts of memory when an external 73444bc4d8f92e46a20 ("mm, page_alloc: do not wake kswapd with zone lock held") The problem is that a division by zero error is possible if boosting occurs very early in boot if the system has very little memory. This patch avoids the division by zero error. Link: http://lkml.kernel.org/r/[email protected] Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Signed-off-by: Mel Gorman <[email protected]> Reported-by: Yury Norov <[email protected]> Tested-by: Yury Norov <[email protected]> Tested-by: Will Deacon <[email protected]> Acked-by: Vlastimil Babka <[email protected]> Cc: Andrea Arcangeli <[email protected]> Cc: David Rientjes <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Catalin Marinas <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
There are 12 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.
mm/page_alloc.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 7f79b78..0b9f577 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2170,6 +2170,18 @@ static inline void boost_watermark(struct zone *zone) max_boost = mult_frac(zone->_watermark[WMARK_HIGH], watermark_boost_factor, 10000); + + /* + * high watermark may be uninitialised if fragmentation occurs + * very early in boot so do not boost. We do not fall + * through and boost by pageblock_nr_pages as failing + * allocations that early means that reclaim is not going + * to help and it may even be impossible to reclaim the + * boosted watermark resulting in a hang. + */ + if (!max_boost) + return; + max_boost = max(pageblock_nr_pages, max_boost); zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages,