Re: [PATCH] mm: thp: fix MADV_REMOVE deadlock on shmem THP

[Hillf Danton <hdanton@sina.com>]

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@gmail.com>
> Fixes: c444eb564fb1 ("mm: thp: make the THP mapcount atomic against __split_huge_pmd_locked()")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> Cc: stable@vger.kernel.org
> ---
> 
> 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.
>