Hello, Recently Ye Bin has reported an ext4 crash which he tracked tracked down to a problem that several places (block_write_full_page(), fallocate(2) on blkdev, etc.) can invalidate buffers under a live filesystem - block_invalidatepage() will clear (among other) BH_Mapped flag and following lookup of the buffer_head will reinitialize it (init_page_buffers()) which among other things clears bh->b_private fields which then makes jbd2 crash. I was thinking how to best fix this. block_write_full_page() is easy to deal with as the invalidation there is just a relict from the past and we don't need to invalidate pages there at all (patch 1/2). Other cases are more questionable. In patch 2/2, I have made fallocate(2) on the block device and discard ioctls bail with EBUSY if there's filesystem mounted because it seems very weird and problematic to mess with a block device like that under a filesystem. What do people think? Is anyone aware of a user that would be broken by this? There are also other possibilities of fixing this like making block_invalidatepage() (or rather new ->invalidatepage callback for the block device) less aggressive so that it does not discard that much state from buffer_heads. But details of that are not yet clear to me. Or other possibilities people see to fix this? Honza
