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

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Message-ID: <YPXHokLyedlmdrZ8@carbon.lan>
Date:   Mon, 19 Jul 2021 11:42:42 -0700
From:   Roman Gushchin <guro@...com>
To:     Matthew Wilcox <willy@...radead.org>
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 07:13:26PM +0100, Matthew Wilcox wrote:
> 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.

Indeed! A good note.

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

Thank you!