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SubjectRe: [PATCH v3] ksm: fix conflict between mmput and scan_get_next_rmap_item
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_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.
>
> Signed-off-by: Zhou Chengming<zhouchengming1@huawei.com>
> Suggested-by: Andrea Arcangeli<aarcange@redhat.com>
> Reviewed-by: Andrea Arcangeli<aarcange@redhat.com>
> ---
> 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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