on Sat, 16 Jan 2021 14:16:24 -0800 (PST) Hugh Dickins wrote:
>
> Sergey reported deadlock between kswapd correctly doing its usual
> lock_page(page) followed by down_read(page->mapping->i_mmap_rwsem),
> and madvise(MADV_REMOVE) on an madvise(MADV_HUGEPAGE) area doing
> down_write(page->mapping->i_mmap_rwsem) followed by lock_page(page).
>
> This happened when shmem_fallocate(punch hole)'s unmap_mapping_range()
> reaches zap_pmd_range()'s call to __split_huge_pmd(). The same deadlock
> could occur when partially truncating a mapped huge tmpfs file, or using
> fallocate(FALLOC_FL_PUNCH_HOLE) on it.
>
> __split_huge_pmd()'s page lock was added in 5.8, to make sure that any
> concurrent use of reuse_swap_page() (holding page lock) could not catch
> the anon THP's mapcounts and swapcounts while they were being split.
>
> Fortunately, reuse_swap_page() is never applied to a shmem or file THP
> (not even by khugepaged, which checks PageSwapCache before calling),
> and anonymous THPs are never created in shmem or file areas: so that
> __split_huge_pmd()'s page lock can only be necessary for anonymous THPs,
> on which there is no risk of deadlock with i_mmap_rwsem.
CPU0 CPU1
---- ----
kswapd madvise
lock_page down_write(i_mmap_rwsem)
down_read lock_page
Given nothing wrong on the reclaimer's side, it is the reverse locking
order on CPU1 that paves a brick for the peril to run another term, a
long one maybe, if I dont misread you.
> Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
> Fixes: c444eb564fb1 ("mm: thp: make the THP mapcount atomic against __split_huge_pmd_locked()")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>
> The status of reuse_swap_page(), and its use on THPs, is currently under
> discussion, and may need to be changed: but this patch is a simple fix
> to the reported deadlock, which can go in now, and be easily backported
> to whichever stable and longterm releases took in 5.8's c444eb564fb1.
>
> mm/huge_memory.c | 37 +++++++++++++++++++++++--------------
> 1 file changed, 23 insertions(+), 14 deletions(-)
>
> --- 5.11-rc3/mm/huge_memory.c 2020-12-27 20:39:37.667932292 -0800
> +++ linux/mm/huge_memory.c 2021-01-16 08:02:08.265551393 -0800
> @@ -2202,7 +2202,7 @@ void __split_huge_pmd(struct vm_area_str
> {
> spinlock_t *ptl;
> struct mmu_notifier_range range;
> - bool was_locked = false;
> + bool do_unlock_page = false;
> pmd_t _pmd;
>
> mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> @@ -2218,7 +2218,6 @@ void __split_huge_pmd(struct vm_area_str
> VM_BUG_ON(freeze && !page);
> if (page) {
> VM_WARN_ON_ONCE(!PageLocked(page));
> - was_locked = true;
> if (page != pmd_page(*pmd))
> goto out;
> }
> @@ -2227,19 +2226,29 @@ repeat:
> if (pmd_trans_huge(*pmd)) {
> if (!page) {
> page = pmd_page(*pmd);
> - if (unlikely(!trylock_page(page))) {
> - get_page(page);
> - _pmd = *pmd;
> - spin_unlock(ptl);
> - lock_page(page);
> - spin_lock(ptl);
> - if (unlikely(!pmd_same(*pmd, _pmd))) {
> - unlock_page(page);
> + /*
> + * An anonymous page must be locked, to ensure that a
> + * concurrent reuse_swap_page() sees stable mapcount;
> + * but reuse_swap_page() is not used on shmem or file,
> + * and page lock must not be taken when zap_pmd_range()
> + * calls __split_huge_pmd() while i_mmap_lock is held.
> + */
> + if (PageAnon(page)) {
> + if (unlikely(!trylock_page(page))) {
> + get_page(page);
> + _pmd = *pmd;
> + spin_unlock(ptl);
> + lock_page(page);
> + spin_lock(ptl);
> + if (unlikely(!pmd_same(*pmd, _pmd))) {
> + unlock_page(page);
> + put_page(page);
> + page = NULL;
> + goto repeat;
> + }
> put_page(page);
> - page = NULL;
> - goto repeat;
> }
> - put_page(page);
> + do_unlock_page = true;
> }
> }
> if (PageMlocked(page))
> @@ -2249,7 +2258,7 @@ repeat:
> __split_huge_pmd_locked(vma, pmd, range.start, freeze);
> out:
> spin_unlock(ptl);
> - if (!was_locked && page)
> + if (do_unlock_page)
> unlock_page(page);
> /*
> * No need to double call mmu_notifier->invalidate_range() callback.
>
