Commit 7496fea9 authored by Zhou Chengming's avatar Zhou Chengming Committed by Linus Torvalds

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.comSigned-off-by: default avatarZhou Chengming <zhouchengming1@huawei.com>
Suggested-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
Reviewed-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Geliang Tang <geliangtang@163.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Ding Tianhong <dingtianhong@huawei.com>
Cc: Li Bin <huawei.libin@huawei.com>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent c25a1e06
......@@ -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 */
......
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