Patch "xfs: Fix missed holes in SEEK_HOLE implementation" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

xfs: Fix missed holes in SEEK_HOLE implementation

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
xfs-fix-missed-holes-in-seek_hole-implementation.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jan Kara <[email protected]>
Date: Thu, 18 May 2017 16:36:22 -0700
Subject: xfs: Fix missed holes in SEEK_HOLE implementation

From: Jan Kara <[email protected]>

commit 5375023ae1266553a7baa0845e82917d8803f48c upstream.

XFS SEEK_HOLE implementation could miss a hole in an unwritten extent as can be seen by the following command:

xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "pwrite 128k 8k"

       -c "seek -h 0" file

wrote 57344/57344 bytes at offset 0
56 KiB, 14 ops; 0.0000 sec (49.312 MiB/sec and 12623.9856 ops/sec) wrote 8192/8192 bytes at offset 131072
8 KiB, 2 ops; 0.0000 sec (70.383 MiB/sec and 18018.0180 ops/sec)
Whence Result
HOLE 139264

Where we can see that hole at offset 56k was just ignored by SEEK_HOLE implementation. The bug is in xfs_find_get_desired_pgoff() which does not properly detect the case when pages are not contiguous.

Fix the problem by properly detecting when found page has larger offset than expected.

---
 fs/xfs/xfs_file.c |   29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

@@ -1076,17 +1076,6 @@ xfs_find_get_desired_pgoff(
 			break;
 		}
 
-		/*
-		 * At lease we found one page.  If this is the first time we
-		 * step into the loop, and if the first page index offset is
-		 * greater than the given search offset, a hole was found.
-		 */
-		if (type == HOLE_OFF && lastoff == startoff &&
-		    lastoff < page_offset(pvec.pages[0])) {
-			found = true;
-			break;
-		}
-
 		for (i = 0; i < nr_pages; i++) {
 			struct page	*page = pvec.pages[i];
 			loff_t		b_offset;

@@ -1098,18 +1087,18 @@ xfs_find_get_desired_pgoff(
 			 * file mapping. However, page->index will not change
 			 * because we have a reference on the page.
 			 *
-			 * Searching done if the page index is out of range.
-			 * If the current offset is not reaches the end of
-			 * the specified search range, there should be a hole
-			 * between them.
+			 * If current page offset is beyond where we've ended,
+			 * we've found a hole.
 			 */
-			if (page->index > end) {
-				if (type == HOLE_OFF && lastoff < endoff) {
-					*offset = lastoff;
-					found = true;
-				}
+			if (type == HOLE_OFF && lastoff < endoff &&
+			    lastoff < page_offset(pvec.pages[i])) {
+				found = true;
+				*offset = lastoff;
 				goto out;
 			}
+			/* Searching done if the page index is out of range. */
+			if (page->index > end)
+				goto out;
 
 			lock_page(page);
 			/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/xfs-fix-missed-holes-in-seek_hole-implementation.patch queue-4.11/mm-avoid-spurious-bad-pmd-warning-messages.patch queue-4.11/xfs-fix-off-by-one-on-max-nr_pages-in-xfs_find_get_desired_pgoff.patch queue-4.11/dax-fix-race-between-colliding-pmd-pte-entries.patch