[PATCH v2 0/2] Fix hugetlb free path race with memory errors

[Mike Kravetz]

In the discussion of Jiaqi Yan's series "Improve hugetlbfs read on
HWPOISON hugepages" the race window was discovered.
https://lore.kernel.org/linux-mm/20230616233447.GB7371@monkey/

Freeing a hugetlb page back to low level memory allocators is performed
in two steps.
1) Under hugetlb lock, remove page from hugetlb lists and clear destructor
2) Outside lock, allocate vmemmap if necessary and call low level free
Between these two steps, the hugetlb page will appear as a normal
compound page.  However, vmemmap for tail pages could be missing.
If a memory error occurs at this time, we could try to update page
flags non-existant page structs.

A much more detailed description is in the first patch.

The first patch addresses the race window.  However, it adds a
hugetlb_lock lock/unlock cycle to every vmemmap optimized hugetlb
page free operation.  This is sub-optimal but is hardly noticeable
on a mostly idle system (the normal case).

The second path optimizes the update_and_free_pages_bulk routine
to only take the lock once in bulk operations.

-> v2
  - Used the more definitive method of checking folio_test_hugetlb to
    determine if destructor must be cleared.
  - Added comment to clearly describe why and when we clear the
    destructor in __update_and_free_hugetlb_folio.
  - Clear destructor in hugetlb demote path.
  - Do not send second patch to stable releases.

Mike Kravetz (2):
  hugetlb: Do not clear hugetlb dtor until allocating vmemmap
  hugetlb: optimize update_and_free_pages_bulk to avoid lock cycles

 mm/hugetlb.c | 128 ++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 100 insertions(+), 28 deletions(-)

-- 
2.41.0