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Date:   Tue, 14 Jul 2020 10:33:06 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
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Subject: Re: WARNING: at mm/mremap.c:211 move_page_tables in i386
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On Sun, Jul 12, 2020 at 03:58:06PM -0700, Linus Torvalds wrote:
> Anybody else have any opinions?

Maybe we just shouldn't allow move_normal_pmd() if ranges overlap?

Other option: pass 'overlaps' down to move_normal_pmd() and only WARN() if
see establised PMD without overlaps being true.

Untested patch:

diff --git a/mm/mremap.c b/mm/mremap.c
index 5dd572d57ca9..e33fcee541fe 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -245,6 +245,18 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 	unsigned long extent, next, old_end;
 	struct mmu_notifier_range range;
 	pmd_t *old_pmd, *new_pmd;
+	bool overlaps;
+
+	/*
+	 * shift_arg_pages() can call move_page_tables() on overlapping ranges.
+	 * In this case we cannot use move_normal_pmd() because destination pmd
+	 * might be established page table: move_ptes() doesn't free page
+	 * table.
+	 */
+	if (old_addr > new_addr)
+		overlaps = old_addr - new_addr < len;
+	else
+		overlaps = new_addr - old_addr < len;
 
 	old_end = old_addr + len;
 	flush_cache_range(vma, old_addr, old_end);
@@ -282,7 +294,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 			split_huge_pmd(vma, old_pmd, old_addr);
 			if (pmd_trans_unstable(old_pmd))
 				continue;
-		} else if (extent == PMD_SIZE) {
+		} else if (!overlaps && extent == PMD_SIZE) {
 #ifdef CONFIG_HAVE_MOVE_PMD
 			/*
 			 * If the extent is PMD-sized, try to speed the move by
-- 
 Kirill A. Shutemov