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Date:   Thu, 26 Jan 2023 11:54:55 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Jan Kara <jack@suse.cz>
Cc:     <linux-fsdevel@vger.kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] fs: gracefully handle ->get_block not mapping bh in
 __mpage_writepage
Message-Id: <20230126115455.296681b67273410e729309b0@linux-foundation.org>
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On Thu, 26 Jan 2023 09:51:55 +0100 Jan Kara <jack@suse.cz> 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

"the function" being __mpage_writepage(), not ->get_block()...

> bogus bdev and block number to bio allocation routines which leads to
> crashes sooner or later.

Crashes are unwelcome.  How is this bug triggered?  Should we backport
the fix?  I assume this is a longstanding thing and that any Fixes:
target would be ancient?  If ancient, why did it take so long to
discover?

> 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.
>
> ...