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Date:   Thu, 26 Jan 2023 07:15:20 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Jan Kara <jack@suse.cz>
Cc:     linux-fsdevel@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] fs: gracefully handle ->get_block not mapping bh in
 __mpage_writepage
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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>