iomap: fix warning in iomap_write_delalloc_release()

While fsstress + drop cache test, we get following warning:

 ------------[ cut here ]------------
 WARNING: CPU: 2 PID: 1003 at fs/iomap/buffered-io.c:1182 iomap_file_buffered_write_punch_delalloc+0x691/0x730
 Modules linked in:
 CPU: 2 PID: 1003 Comm: fsstress Not tainted 6.7.0-rc5-06945-g3ba9b31d6bf3-dirty torvalds#256
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
 RIP: 0010:iomap_file_buffered_write_punch_delalloc+0x691/0x730
 Code: d1 0b 01 0f 0b 48 83 05 14 a2 d1 0b 01 48 89 05 35 a1 d1 0b 49 39 ec 0f 8c 09 fb ff ff e9 b6 fd ff ff 48 83 05 df a1 d1 0b 01 <0f> 0b 48 83 05 dd a1 d1 0b 01 48 39 6c 24 10 7c c0 48 89 05 07 a1
 RSP: 0018:ffffc900005b7b08 EFLAGS: 00010202
 RAX: 0000000000000001 RBX: ffff888102363d40 RCX: 0000000000000001
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff888108080000
 RBP: 0000000000050000 R08: ffff888108084eb8 R09: ffff888108084eb8
 R10: 000000000000005c R11: 0000000000000059 R12: 0000000000050000
 R13: ffffffff8c978ef0 R14: 0000000000050000 R15: 000000000005a000
 FS:  00007efc04c63b40(0000) GS:ffff88813bd00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007efc0375c000 CR3: 0000000105a4d000 CR4: 00000000000006f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  <TASK>
  xfs_buffered_write_iomap_end+0x40/0xb0
  iomap_iter+0x8e/0x5f0
  iomap_file_buffered_write+0xa4/0x460
  xfs_file_buffered_write+0x156/0x3d0
  xfs_file_write_iter+0xb2/0x1c0
  do_iter_readv_writev+0x19b/0x1f0
  vfs_writev+0x114/0x4f0
  do_writev+0x7f/0x1c0
  __x64_sys_writev+0x24/0x30
  do_syscall_64+0x3f/0xe0
  entry_SYSCALL_64_after_hwframe+0x62/0x6a
 RIP: 0033:0x7efc03b06610
 Code: 73 01 c3 48 8b 0d 78 88 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d d9 e0 2c 00 00 75 10 b8 14 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 5e 8e 01 00 48 89 04 24
 RSP: 002b:00007ffdf8f426d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000014
 RAX: ffffffffffffffda RBX: 000000000000007a RCX: 00007efc03b06610
 RDX: 00000000000002c4 RSI: 00000000012f5580 RDI: 0000000000000003
 RBP: 0000000000000003 R08: 00000000012f53a0 R09: 0000000000000077
 R10: 000000000000007c R11: 0000000000000246 R12: 00000000000002c4
 R13: 00000000012dba50 R14: 00000000012f5580 R15: 0000000000000094

The warning occurred in the following code of iomap_write_delalloc_release().
After analyzing vmcore, I found that the reason for the warning is that
data_end was equal to start_byte.

        WARN_ON_ONCE(data_end <= start_byte);

If some delay is added between seeking for data and seeking for hole
in iomap_write_delalloc_release(), the problem can be reproduced quickly.
The root cause of the problem is that clean data page was dropped between
two seeking in the page cache. As a result, data_end may be equal to
start_byte.

buffered write                        drop cache
---------------------------           ---------------------------
xfs_buffered_write_iomap_end
  iomap_file_buffered_write_punch_delalloc
    iomap_write_delalloc_release
      start_byte = mapping_seek_hole_data(SEEK_DATA)

                                      drop_pagecache_sb
                                        invalidate_mapping_pages
                                          mapping_try_invalidate
                                            mapping_evict_folio
                                              remove_mapping

      data_end = mapping_seek_hole_data(SEEK_HOLE)
      WARN_ON_ONCE(data_end <= start_byte)

In my investigation, I found that clean data pages was alloced and added
to page cache when reading the file's hole. After that, while buffered
write and goes into delalloc release, we seek for data, it will find
the start offset of the clean data pages. If the clean data pages was
dropped, when we seek for hole, it will find the same offset as the
previous seek.

During delalloc release, we punch out all the delalloc blocks in the range
given except for those that have dirty data still pending in the page cache.
If the start_byte is equal to data_end after seeking data and hole, it will
be returned directly in the delalloc scan, and we can continue to find the
next data, and perform delalloc scan. It does not affect the range of
delalloc block that need to be punched out.

Therefore, if start_byte equal to data_end, just let it seek for data
again in the loop.

Fixes: f43dc4d ("iomap: buffered write failure should not truncate the page cache")
Signed-off-by: Long Li <[email protected]>

fs/iomap/buffered-io.c

@@ -1189,7 +1189,17 @@ static int iomap_write_delalloc_release(struct inode *inode,
 			error = data_end;
 			goto out_unlock;
 		}
-		WARN_ON_ONCE(data_end <= start_byte);
+
+		/*
+		 * Seek for data/hole in the page cache can race with drop
+		 * cache, if data page was dropped between seek for data and
+		 * hole, data_end may be equal to start_byte, just let it keep
+		 * seeking.
+		 */
+		if (data_end == start_byte)
+			continue;
+
+		WARN_ON_ONCE(data_end < start_byte);
 		WARN_ON_ONCE(data_end > scan_end_byte);
 
 		error = iomap_write_delalloc_scan(inode, &punch_start_byte,