[PATCH] khugepaged: collapse_pte_mapped_thp() protect the pmd lock

[Hugh Dickins]

When retract_page_tables() removes a page table to make way for a huge
pmd, it holds huge page lock, i_mmap_lock_write, mmap_write_trylock and
pmd lock; but when collapse_pte_mapped_thp() does the same (to handle
the case when the original mmap_write_trylock had failed), only
mmap_write_trylock and pmd lock are held.

That's not enough.  One machine has twice crashed under load, with
"BUG: spinlock bad magic" and GPF on 6b6b6b6b6b6b6b6b.  Examining the
second crash, page_vma_mapped_walk_done()'s spin_unlock of pvmw->ptl
(serving page_referenced() on a file THP, that had found a page table
at *pmd) discovers that the page table page and its lock have already
been freed by the time it comes to unlock.

Follow the example of retract_page_tables(), but we only need one of
huge page lock or i_mmap_lock_write to secure against this: because it's
the narrower lock, and because it simplifies collapse_pte_mapped_thp()
to know the hpage earlier, choose to rely on huge page lock here.

Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.4+
---

 mm/khugepaged.c |   44 +++++++++++++++++++-------------------------
 1 file changed, 19 insertions(+), 25 deletions(-)

--- 5.8-rc7/mm/khugepaged.c	2020-07-26 16:58:02.189038680 -0700
+++ linux/mm/khugepaged.c	2020-08-02 10:51:02.127688808 -0700
@@ -1412,7 +1412,7 @@ void collapse_pte_mapped_thp(struct mm_s
 {
 	unsigned long haddr = addr & HPAGE_PMD_MASK;
 	struct vm_area_struct *vma = find_vma(mm, haddr);
-	struct page *hpage = NULL;
+	struct page *hpage;
 	pte_t *start_pte, *pte;
 	pmd_t *pmd, _pmd;
 	spinlock_t *ptl;
@@ -1432,9 +1432,17 @@ void collapse_pte_mapped_thp(struct mm_s
 	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
 		return;
 
+	hpage = find_lock_page(vma->vm_file->f_mapping,
+			       linear_page_index(vma, haddr));
+	if (!hpage)
+		return;
+
+	if (!PageHead(hpage))
+		goto drop_hpage;
+
 	pmd = mm_find_pmd(mm, haddr);
 	if (!pmd)
-		return;
+		goto drop_hpage;
 
 	start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
 
@@ -1453,30 +1461,11 @@ void collapse_pte_mapped_thp(struct mm_s
 
 		page = vm_normal_page(vma, addr, *pte);
 
-		if (!page || !PageCompound(page))
-			goto abort;
-
-		if (!hpage) {
-			hpage = compound_head(page);
-			/*
-			 * The mapping of the THP should not change.
-			 *
-			 * Note that uprobe, debugger, or MAP_PRIVATE may
-			 * change the page table, but the new page will
-			 * not pass PageCompound() check.
-			 */
-			if (WARN_ON(hpage->mapping != vma->vm_file->f_mapping))
-				goto abort;
-		}
-
 		/*
-		 * Confirm the page maps to the correct subpage.
-		 *
-		 * Note that uprobe, debugger, or MAP_PRIVATE may change
-		 * the page table, but the new page will not pass
-		 * PageCompound() check.
+		 * Note that uprobe, debugger, or MAP_PRIVATE may change the
+		 * page table, but the new page will not be a subpage of hpage.
 		 */
-		if (WARN_ON(hpage + i != page))
+		if (hpage + i != page)
 			goto abort;
 		count++;
 	}
@@ -1495,7 +1484,7 @@ void collapse_pte_mapped_thp(struct mm_s
 	pte_unmap_unlock(start_pte, ptl);
 
 	/* step 3: set proper refcount and mm_counters. */
-	if (hpage) {
+	if (count) {
 		page_ref_sub(hpage, count);
 		add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count);
 	}
@@ -1506,10 +1495,15 @@ void collapse_pte_mapped_thp(struct mm_s
 	spin_unlock(ptl);
 	mm_dec_nr_ptes(mm);
 	pte_free(mm, pmd_pgtable(_pmd));
+
+drop_hpage:
+	unlock_page(hpage);
+	put_page(hpage);
 	return;
 
 abort:
 	pte_unmap_unlock(start_pte, ptl);
+	goto drop_hpage;
 }
 
 static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)