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

[Roman Gushchin]

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@xxxxxxxxxxxxx>
> 

Thank you!