Re: [PATCH] fs: gracefully handle ->get_block not mapping bh in __mpage_writepage

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jan 26, 2023 at 09:51:55AM +0100, Jan Kara wrote:
> When filesystem's ->get_block function does not map the buffer head when
> called from __mpage_writepage(), the function 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.

Yes, this matches what the buffer.c helpers do, so:

Reviewed-by: Christoph Hellwig <hch@lst.de>