Please ignore this patch v3. I forgot to change the function unmerge_and_remove_all_rmap_items(). Patch v4 will be the final version, I think.. Sorry for my carelessness. Thanks! On 2016/5/8 14:56, Zhou Chengming wrote:
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.From the suggestion of Andrea Arcangeli, unmerge_and_remove_all_rmap_itemshas 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. Signed-off-by: Zhou Chengming<zhouchengming1@xxxxxxxxxx> Suggested-by: Andrea Arcangeli<aarcange@xxxxxxxxxx> Reviewed-by: Andrea Arcangeli<aarcange@xxxxxxxxxx> --- mm/ksm.c | 16 ++++++++++------ 1 files changed, 10 insertions(+), 6 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index ca6d2a0..b6dc387 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -777,6 +777,7 @@ 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, @@ -784,16 +785,12 @@ static int unmerge_and_remove_all_rmap_items(void) if (ksm_test_exit(mm)) { hash_del(&mm_slot->link); list_del(&mm_slot->mm_list); - spin_unlock(&ksm_mmlist_lock); free_mm_slot(mm_slot); clear_bit(MMF_VM_MERGEABLE,&mm->flags); - up_read(&mm->mmap_sem); mmdrop(mm); - } else { - spin_unlock(&ksm_mmlist_lock); - up_read(&mm->mmap_sem); } + spin_unlock(&ksm_mmlist_lock); } /* Clean up stable nodes, but don't worry if some are still busy */ @@ -1657,8 +1654,15 @@ 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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