From: Hugh Dickins <hughd@google.com>
Date: Fri, 30 Nov 2018 14:10:25 -0800
Subject: mm/khugepaged: collapse_shmem() stop if punched or truncated
Git-commit: 701270fa193aadf00bdcf607738f64997275d4c7
Patch-mainline: v4.20-rc5
References: VM Functionality, bsc#1121599

[ vbabka@suse.cz: use the 4.14 pre-xarray stable backport ]

Huge tmpfs testing showed that although collapse_shmem() recognizes a
concurrently truncated or hole-punched page correctly, its handling of
holes was liable to refill an emptied extent.  Add check to stop that.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261522040.2275@eggly.anvils
Fixes: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: <stable@vger.kernel.org>	[4.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/khugepaged.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1348,6 +1348,16 @@ static void collapse_shmem(struct mm_str
 		int n = min(iter.index, end) - index;
 
 		/*
+		 * Stop if extent has been hole-punched, and is now completely
+		 * empty (the more obvious i_size_read() check would take an
+		 * irq-unsafe seqlock on 32-bit).
+		 */
+		if (n >= HPAGE_PMD_NR) {
+			result = SCAN_TRUNCATED;
+			goto tree_locked;
+		}
+
+		/*
 		 * Handle holes in the radix tree: charge it from shmem and
 		 * insert relevant subpage of new_page into the radix-tree.
 		 */
@@ -1458,6 +1468,11 @@ out_unlock:
 	if (result == SCAN_SUCCEED && index < end) {
 		int n = end - index;
 
+		/* Stop if extent has been truncated, and is now empty */
+		if (n >= HPAGE_PMD_NR) {
+			result = SCAN_TRUNCATED;
+			goto tree_locked;
+		}
 		if (!shmem_charge(mapping->host, n)) {
 			result = SCAN_FAIL;
 			goto tree_locked;