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mm/page_alloc: fix pcp->count race between drain_pages_zone() vs __rm…
…queue_pcplist() It's expected that no page should be left in pcp_list after calling zone_pcp_disable() in offline_pages(). Previously, it's observed that offline_pages() gets stuck [1] due to some pages remaining in pcp_list. Cause: There is a race condition between drain_pages_zone() and __rmqueue_pcplist() involving the pcp->count variable. See below scenario: CPU0 CPU1 ---------------- --------------- spin_lock(&pcp->lock); __rmqueue_pcplist() { zone_pcp_disable() { /* list is empty */ if (list_empty(list)) { /* add pages to pcp_list */ alloced = rmqueue_bulk() mutex_lock(&pcp_batch_high_lock) ... __drain_all_pages() { drain_pages_zone() { /* read pcp->count, it's 0 here */ count = READ_ONCE(pcp->count) /* 0 means nothing to drain */ /* update pcp->count */ pcp->count += alloced << order; ... ... spin_unlock(&pcp->lock); In this case, after calling zone_pcp_disable() though, there are still some pages in pcp_list. And these pages in pcp_list are neither movable nor isolated, offline_pages() gets stuck as a result. Solution: Expand the scope of the pcp->lock to also protect pcp->count in drain_pages_zone(), to ensure no pages are left in the pcp list after zone_pcp_disable() [1] https://lore.kernel.org/linux-mm/[email protected]/ Link: https://lkml.kernel.org/r/[email protected] Fixes: 4b23a68 ("mm/page_alloc: protect PCP lists with a spinlock") Signed-off-by: Li Zhijian <[email protected]> Reported-by: Yao Xingtao <[email protected]> Reviewed-by: Vlastimil Babka <[email protected]> Cc: David Hildenbrand <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]>
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