linux-kernel - Re: [PATCH] writeback, cgroup: do not reparent dax inodes

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Date:   Mon, 19 Jul 2021 19:13:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>, Dave Chinner <dchinner@...hat.com>,
        Murphy Zhou <jencce.kernel@...il.com>,
        "Darrick J . Wong" <djwong@...nel.org>
Subject: Re: [PATCH] writeback, cgroup: do not reparent dax inodes

On Mon, Jul 19, 2021 at 10:13:50AM -0700, Roman Gushchin wrote:
> The inode switching code is not suited for dax inodes. An attempt
> to switch a dax inode to a parent writeback structure (as a part
> of a writeback cleanup procedure) results in a panic like this:
[...]
> The crash happens on an attempt to iterate over attached pagecache
> pages and check the dirty flag: a dax inode's xarray contains pfn's
> instead of generic struct page pointers.

I wondered why this happens for DAX and not for other kinds of non-page
entries in the inodes.  The answer is that it's a tagged iteration, and
shadow/swap entries are never tagged; only DAX entries get tagged.

Acked-by: Matthew Wilcox (Oracle) <willy@...radead.org>