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

[Matthew Wilcox]

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@xxxxxxxxxxxxx>