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Date:   Sat, 16 Jan 2021 14:16:24 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Jann Horn <jannh@...gle.com>, Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: thp: fix MADV_REMOVE deadlock on shmem THP

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.

Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Fixes: c444eb564fb1 ("mm: thp: make the THP mapcount atomic against __split_huge_pmd_locked()")
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>
Cc: stable@...r.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.

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