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Subject[PATCH 3.16 60/87] coredump: fix race condition between collapse_huge_page() and core dumping
3.16.75-rc1 review patch.  If anyone has any objections, please let me know.


From: Andrea Arcangeli <>

commit 59ea6d06cfa9247b586a695c21f94afa7183af74 upstream.

When fixing the race conditions between the coredump and the mmap_sem
holders outside the context of the process, we focused on
mmget_not_zero()/get_task_mm() callers in 04f5866e41fb70 ("coredump: fix
race condition between mmget_not_zero()/get_task_mm() and core
dumping"), but those aren't the only cases where the mmap_sem can be
taken outside of the context of the process as Michal Hocko noticed
while backporting that commit to older -stable kernels.

If mmgrab() is called in the context of the process, but then the
mm_count reference is transferred outside the context of the process,
that can also be a problem if the mmap_sem has to be taken for writing
through that mm_count reference.

khugepaged registration calls mmgrab() in the context of the process,
but the mmap_sem for writing is taken later in the context of the
khugepaged kernel thread.

collapse_huge_page() after taking the mmap_sem for writing doesn't
modify any vma, so it's not obvious that it could cause a problem to the
coredump, but it happens to modify the pmd in a way that breaks an
invariant that pmd_trans_huge_lock() relies upon. collapse_huge_page()
needs the mmap_sem for writing just to block concurrent page faults that
call pmd_trans_huge_lock().

Specifically the invariant that "!pmd_trans_huge()" cannot become a
"pmd_trans_huge()" doesn't hold while collapse_huge_page() runs.

The coredump will call __get_user_pages() without mmap_sem for reading,
which eventually can invoke a lockless page fault which will need a
functional pmd_trans_huge_lock().

So collapse_huge_page() needs to use mmget_still_valid() to check it's
not running concurrently with the coredump... as long as the coredump
can invoke page faults without holding the mmap_sem for reading.

This has "Fixes: khugepaged" to facilitate backporting, but in my view
it's more a bug in the coredump code that will eventually have to be
rewritten to stop invoking page faults without the mmap_sem for reading.
So the long term plan is still to drop all mmget_still_valid().

Fixes: ba76149f47d8 ("thp: khugepaged")
Signed-off-by: Andrea Arcangeli <>
Reported-by: Michal Hocko <>
Acked-by: Michal Hocko <>
Acked-by: Kirill A. Shutemov <>
Cc: Oleg Nesterov <>
Cc: Jann Horn <>
Cc: Hugh Dickins <>
Cc: Mike Rapoport <>
Cc: Mike Kravetz <>
Cc: Peter Xu <>
Cc: Jason Gunthorpe <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
[bwh: Backported to 3.16:
- Don't set result variable; collapse_huge_range() returns void
- Adjust filenames]
Signed-off-by: Ben Hutchings <>
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2439,6 +2439,10 @@ static inline void mmdrop(struct mm_stru
* followed by taking the mmap_sem for writing before modifying the
* vmas or anything the coredump pretends not to change from under it.
+ * It also has to be called when mmgrab() is used in the context of
+ * the process, but then the mm_count refcount is transferred outside
+ * the context of the process to run down_write() on that pinned mm.
+ *
* NOTE: find_extend_vma() called from GUP context is the only place
* that can modify the "mm" (notably the vm_start/end) under mmap_sem
* for reading and outside the context of the process, so it is also
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2428,6 +2428,8 @@ static void collapse_huge_page(struct mm
* handled by the anon_vma lock + PG_lock.
+ if (!mmget_still_valid(mm))
+ goto out;
if (unlikely(khugepaged_test_exit(mm)))
goto out;

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