On 21.05.24 14:57, Brendan Jackman wrote:
These fields are written by memory hotplug under mem_hotplug_lock but read without any lock. It seems like reader code is robust against the value being stale or "from the future", but we also need to account for: 1. Load/store tearing (according to Linus[1], this really happens, even when everything is aligned as you would hope). 2. Invented loads[2] - the compiler can spill and re-read these fields ([2] calls this "invented loads") and assume that they have not changed. Note we don't need READ_ONCE in paths that have the mem_hotplug_lock for write, but we still need WRITE_ONCE to prevent store-tearing. [1] https://lore.kernel.org/all/CAHk-=wj2t+GK+DGQ7Xy6U7zMf72e7Jkxn4_-kGyfH3WFEoH+YQ@xxxxxxxxxxxxxx/T/#u As discovered via the original big-bad article[2] [2] https://lwn.net/Articles/793253/ Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx> --- include/linux/mmzone.h | 14 ++++++++++---- mm/compaction.c | 2 +- mm/memory_hotplug.c | 20 ++++++++++++-------- mm/mm_init.c | 2 +- mm/page_alloc.c | 2 +- mm/show_mem.c | 8 ++++---- mm/vmstat.c | 4 ++-- 7 files changed, 31 insertions(+), 21 deletions(-) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 194ef7fed9d6..bdb3be76d10c 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -1018,11 +1018,13 @@ static inline unsigned long zone_cma_pages(struct zone *zone) #endif }+/* This is unstable unless you hold mem_hotplug_lock. */static inline unsigned long zone_end_pfn(const struct zone *zone) { - return zone->zone_start_pfn + zone->spanned_pages; + return zone->zone_start_pfn + READ_ONCE(zone->spanned_pages);
It's weird to apply that logic only to spanned_pages, whereby zone_start_pfn can (and will) similarly change when onlining/offlining memory.
-- Cheers, David / dhildenb
