When filesystem's ->get_block function does not map the buffer head when called from __mpage_writepage(), __mpage_writepage() will happily go and pass bogus bdev and block number to bio allocation routines which leads to crashes sooner or later. E.g. UDF can do this because it doesn't want to allocate blocks from ->writepages callbacks. It allocates blocks on write or page fault but writeback can still spot dirty buffers without underlying blocks allocated e.g. if blocksize < pagesize, the tail page is dirtied (which means all its buffers are dirtied), and truncate extends the file so that some buffer starts to be within i_size.
Unified · Split
| @@ -532,6 +532,8 @@ static int __mpage_writepage(struct page *page, struct writeback_control *wbc, · 531 unmodified lines | |||
| 532 | map_bh.b_size = 1 << blkbits; | 532 | map_bh.b_size = 1 << blkbits; |
| 533 | if (mpd->get_block(inode, block_in_file, &map_bh, 1)) | 533 | if (mpd->get_block(inode, block_in_file, &map_bh, 1)) |
| 534 | goto confused; | 534 | goto confused; |
| 535 | if (!buffer_mapped(&map_bh)) | ||
| 536 | goto confused; | ||
| 535 | if (buffer_new(&map_bh)) | 537 | if (buffer_new(&map_bh)) |
| 536 | clean_bdev_bh_alias(&map_bh); | 538 | clean_bdev_bh_alias(&map_bh); |
| 537 | if (buffer_boundary(&map_bh)) { | 539 | if (buffer_boundary(&map_bh)) { |