From 7496fea9a6bf644afe360af795b121a77635b37d Mon Sep 17 00:00:00 2001 From: Zhou Chengming Date: Thu, 12 May 2016 15:42:21 -0700 Subject: [PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item A concurrency issue about KSM in the function scan_get_next_rmap_item. task A (ksmd): |task B (the mm's task): | mm = slot->mm; | down_read(&mm->mmap_sem); | | ... | | spin_lock(&ksm_mmlist_lock); | | ksm_scan.mm_slot go to the next slot; | | spin_unlock(&ksm_mmlist_lock); | |mmput() -> | ksm_exit(): | |spin_lock(&ksm_mmlist_lock); |if (mm_slot && ksm_scan.mm_slot != mm_slot) { | if (!mm_slot->rmap_list) { | easy_to_free = 1; | ... | |if (easy_to_free) { | mmdrop(mm); | ... | |So this mm_struct may be freed in the mmput(). | up_read(&mm->mmap_sem); | As we can see above, the ksmd thread may access a mm_struct that already been freed to the kmem_cache. Suppose a fork will get this mm_struct from the kmem_cache, the ksmd thread then call up_read(&mm->mmap_sem), will cause mmap_sem.count to become -1. As suggested by Andrea Arcangeli, unmerge_and_remove_all_rmap_items has the same SMP race condition, so fix it too. My prev fix in function scan_get_next_rmap_item will introduce a different SMP race condition, so just invert the up_read/spin_unlock order as Andrea Arcangeli said. Link: http://lkml.kernel.org/r/1462708815-31301-1-git-send-email-zhouchengming1@huawei.com Signed-off-by: Zhou Chengming Suggested-by: Andrea Arcangeli Reviewed-by: Andrea Arcangeli Cc: Hugh Dickins Cc: Kirill A. Shutemov Cc: Vlastimil Babka Cc: Geliang Tang Cc: Minchan Kim Cc: Hanjun Guo Cc: Ding Tianhong Cc: Li Bin Cc: Zhen Lei Cc: Xishi Qiu Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/ksm.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index b99e8281..4786b41 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -783,6 +783,7 @@ static int unmerge_and_remove_all_rmap_items(void) } remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list); + up_read(&mm->mmap_sem); spin_lock(&ksm_mmlist_lock); ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next, @@ -794,12 +795,9 @@ static int unmerge_and_remove_all_rmap_items(void) free_mm_slot(mm_slot); clear_bit(MMF_VM_MERGEABLE, &mm->flags); - up_read(&mm->mmap_sem); mmdrop(mm); - } else { + } else spin_unlock(&ksm_mmlist_lock); - up_read(&mm->mmap_sem); - } } /* Clean up stable nodes, but don't worry if some are still busy */ @@ -1663,8 +1661,15 @@ next_mm: up_read(&mm->mmap_sem); mmdrop(mm); } else { - spin_unlock(&ksm_mmlist_lock); up_read(&mm->mmap_sem); + /* + * up_read(&mm->mmap_sem) first because after + * spin_unlock(&ksm_mmlist_lock) run, the "mm" may + * already have been freed under us by __ksm_exit() + * because the "mm_slot" is still hashed and + * ksm_scan.mm_slot doesn't point to it anymore. + */ + spin_unlock(&ksm_mmlist_lock); } /* Repeat until we've completed scanning the whole list */ -- 2.7.4