xfs: fix infinite loop at xfs_vm_writepage on 32bit system

From: Jie Liu <[email protected]>

Write to a file with an offset greater than 16TB on 32-bit system and then trigger page write-back via sync(1) will cause task hang.

# block_size=4096
# offset=$(((2**32 - 1) * $block_size))

# xfs_io -f -c "pwrite $offset $block_size" /storage/test_file
# sync

INFO: task sync:2590 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
sync            D c1064a28     0  2590   2097 0x00000000
.....
Call Trace:
[<c1064a28>] ? ttwu_do_wakeup+0x18/0x130
[<c1066d0e>] ? try_to_wake_up+0x1ce/0x220
[<c1066dbf>] ? wake_up_process+0x1f/0x40
[<c104fc2e>] ? wake_up_worker+0x1e/0x30
[<c15b6083>] schedule+0x23/0x60
[<c15b3c2d>] schedule_timeout+0x18d/0x1f0
[<c12a143e>] ? do_raw_spin_unlock+0x4e/0x90
[<c10515f1>] ? __queue_delayed_work+0x91/0x150
[<c12a12ef>] ? do_raw_spin_lock+0x3f/0x100
[<c12a143e>] ? do_raw_spin_unlock+0x4e/0x90
[<c15b5b5d>] wait_for_completion+0x7d/0xc0
[<c1066d60>] ? try_to_wake_up+0x220/0x220
[<c116a4d2>] sync_inodes_sb+0x92/0x180
[<c116fb05>] sync_inodes_one_sb+0x15/0x20
[<c114a8f8>] iterate_supers+0xb8/0xc0
[<c116faf0>] ? fdatawrite_one_bdev+0x20/0x20
[<c116fc21>] sys_sync+0x31/0x80
[<c15be18d>] sysenter_do_call+0x12/0x28

This issue can be triggered via xfstests/generic/308.

The reason is that the end_index is unsigned long with maximum value '2^32-1=4294967295' on 32-bit platform, and the given offset cause it wrapped to 0, so that the following codes will repeat again and again until the task schedule time out:

end_index = offset >> PAGE_CACHE_SHIFT;
last_index = (offset - 1) >> PAGE_CACHE_SHIFT;

if (page->index >= end_index) {
	unsigned offset_into_page = offset & (PAGE_CACHE_SIZE - 1);
        /*
         * Just skip the page if it is fully outside i_size, e.g. due
         * to a truncate operation that is in progress.
         */
        if (page->index >= end_index + 1 || offset_into_page == 0) {
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
		unlock_page(page);
		return 0;
	}

In order to check if a page is fully outsids i_size or not, we can fix the code logic as below:

	if (page->index > end_index ||
	    (page->index == end_index && offset_into_page == 0))

Secondly, there still has another similar issue when calculating the end offset for mapping the filesystem blocks to the file blocks for delalloc. With the same tests to above, run unmount(8) will cause kernel panic if CONFIG_XFS_DEBUG is enabled:

XFS: Assertion failed: XFS_FORCED_SHUTDOWN(ip->i_mount) || \
	ip->i_delayed_blks == 0, file: fs/xfs/xfs_super.c, line: 964

kernel BUG at fs/xfs/xfs_message.c:108!
invalid opcode: 0000 [#1] SMP
task: edddc100 ti: ec6ee000 task.ti: ec6ee000
EIP: 0060:[<f83d87cb>] EFLAGS: 00010296 CPU: 1
EIP is at assfail+0x2b/0x30 [xfs]
..............

Call Trace:
[<f83d9cd4>] xfs_fs_destroy_inode+0x74/0x120 [xfs]
[<c115ddf1>] destroy_inode+0x31/0x50
[<c115deff>] evict+0xef/0x170
[<c115dfb2>] dispose_list+0x32/0x40
[<c115ea3a>] evict_inodes+0xca/0xe0
[<c1149706>] generic_shutdown_super+0x46/0xd0
[<c11497b9>] kill_block_super+0x29/0x70
[<c1149a14>] deactivate_locked_super+0x44/0x70
[<c114a427>] deactivate_super+0x47/0x60
[<c1161c3d>] mntput_no_expire+0xcd/0x120
[<c1162ae8>] SyS_umount+0xa8/0x370
[<c1162dce>] SyS_oldumount+0x1e/0x20
[<c15be18d>] sysenter_do_call+0x12/0x28

That because the end_offset is evaluated to 0 which is the same reason to above, hence the mapping and covertion for dealloc file blocks to file system blocks did not happened.

This patch just fixed both issues.

---
v4: s/dead loop/infinite loop/, as the latter is more meaningful
v3: Add code comments to reflect this change.

    http://oss.sgi.com/archives/xfs/2013-09/msg00688.html

v2: don't reset the s_max_bytes to MAX_LFS_FILESIZE, instead, revise the
page offset check up strategy to avoid the potential overflow.
http://oss.sgi.com/archives/xfs/2013-07/msg00339.html
v1: http://oss.sgi.com/archives/xfs/2013-07/msg00154.html

 fs/xfs/xfs_aops.c | 49 +++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 43 insertions(+), 6 deletions(-)

index 0479c32..8ec41dd 100644
@@ -982,7 +982,32 @@ xfs_vm_writepage(
 	offset = i_size_read(inode);
 	end_index = offset >> PAGE_CACHE_SHIFT;
 	last_index = (offset - 1) >> PAGE_CACHE_SHIFT;
-	if (page->index >= end_index) {
+
+	/*
+	 * The page index is less than the end_index, adjust the end_offset
+	 * to the highest offset that this page should represent.
+	 * -----------------------------------------------------
+	 * |			file mapping	       | <EOF> |
+	 * -----------------------------------------------------
+	 * | Page ... | Page N-2 | Page N-1 |  Page N  |       |
+	 * ^--------------------------------^----------|--------
+	 * |     desired writeback range    |      see else    |
+	 * ---------------------------------^------------------|
+	 */
+	if (page->index < end_index)
+		end_offset = (xfs_off_t)(page->index + 1) << PAGE_CACHE_SHIFT;
+	else {
+		/*
+		 * Check whether the page to write out is beyond or straddles
+		 * i_size or not.
+		 * -------------------------------------------------------
+		 * |		file mapping		        | <EOF>  |
+		 * -------------------------------------------------------
+		 * | Page ... | Page N-2 | Page N-1 |  Page N   | Beyond |
+		 * ^--------------------------------^-----------|---------
+		 * |				    |      Straddles     |
+		 * ---------------------------------^-----------|--------|
+		 */
 		unsigned offset_into_page = offset & (PAGE_CACHE_SIZE - 1);
 
 		/*

@@ -990,24 +1015,36 @@ xfs_vm_writepage(
 		 * truncate operation that is in progress. We must redirty the
 		 * page so that reclaim stops reclaiming it. Otherwise
 		 * xfs_vm_releasepage() is called on it and gets confused.
+		 *
+		 * Note that the end_index is unsigned long, it would overflow
+		 * if the given offset is greater than 16TB on 32-bit system
+		 * and if we do check the page is fully outside i_size or not
+		 * via "if (page->index >= end_index + 1)" as "end_index + 1"
+		 * will be evaluated to 0.  Hence this page will be redirtied
+		 * and be written out repeatedly which would result in an
+		 * infinite loop, the user program that perform this operation
+		 * will hang.  Instead, we can verify this situation by checking
+		 * if the page to write is totally beyond the i_size or if it's
+		 * offset is just equal to the EOF.
 		 */
-		if (page->index >= end_index + 1 || offset_into_page == 0)
+		if (page->index > end_index ||
+		    (page->index == end_index && offset_into_page == 0))
 			goto redirty;
 
 		/*
 		 * The page straddles i_size.  It must be zeroed out on each
 		 * and every writepage invocation because it may be mmapped.
 		 * "A file is mapped in multiples of the page size.  For a file
-		 * that is not a multiple of the  page size, the remaining
+		 * that is not a multiple of the page size, the remaining
 		 * memory is zeroed when mapped, and writes to that region are
 		 * not written out to the file."
 		 */
 		zero_user_segment(page, offset_into_page, PAGE_CACHE_SIZE);
+
+		/* Adjust the end_offset to the end of file */
+		end_offset = offset;
 	}
 
-	end_offset = min_t(unsigned long long,
-			(xfs_off_t)(page->index + 1) << PAGE_CACHE_SHIFT,
-			offset);
 	len = 1 << inode->i_blkbits;
 
 	bh = head = page_buffers(page);

1.8.3.2

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Fix looks good. Thanks for resending this, Jeff!

Cheers,

Dave.

Dave Chinner
[email protected]

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