On Fri 08-01-21 17:01:03, Muchun Song wrote:

On Fri, Jan 8, 2021 at 4:43 PM Michal Hocko [off-list ref] wrote:

quoted

On Thu 07-01-21 23:11:22, Muchun Song wrote:

[..]

quoted

quoted

But I find a tricky problem to solve. See free_huge_page().
If we are in non-task context, we should schedule a work
to free the page. We reuse the page->mapping. If the page
is already freed by the dissolve path. We should not touch
the page->mapping. So we need to check PageHuge().
The check and llist_add() should be protected by
hugetlb_lock. But we cannot do that. Right? If dissolve
happens after it is linked to the list. We also should
remove it from the list (hpage_freelist). It seems to make
the thing more complex.

I am not sure I follow you here but yes PageHuge under hugetlb_lock
should be the reliable way to check for the race. I am not sure why we
really need to care about mapping or other state.

CPU0:                               CPU1:
free_huge_page(page)
  if (PageHuge(page))
                                    dissolve_free_huge_page(page)
                                      spin_lock(&hugetlb_lock)
                                      update_and_free_page(page)
                                      spin_unlock(&hugetlb_lock)
    llist_add(page->mapping)
    // the mapping is corrupted

The PageHuge(page) and llist_add() should be protected by
hugetlb_lock. Right? If so, we cannot hold hugetlb_lock
in free_huge_page() path.

OK, I see. I completely forgot about this snowflake. I thought that
free_huge_page was a typo missing initial __. Anyway you are right that
this path needs a check as well. But I don't see why we couldn't use the
lock here. The lock can be held only inside the !in_task branch.
Although it would be much more nicer if the lock was held at this layer
rather than both free_huge_page and __free_huge_page. But that clean up
can be done on top.

-- 
Michal Hocko
SUSE Labs