Re: [fuse-devel] fuse: trying to steal weird page

[Matthew Wilcox <willy@infradead.org>]

On Sun, May 03, 2020 at 02:28:34PM -0400, Gabriel Krisman Bertazi wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> > mapcount is 0, mapping is NULL, refcount is 1, so that's all fine.
> > flags has 'waiters' set, which is not in the allowed list.  I don't
> > know the internals of FUSE, so I don't know why that is.
> 
> Hi
> 
> On the first message, Nikolaus sent the following line:
> 
> >> [ 2333.009937] fuse: page=00000000dd1750e3 index=2022240 flags=17ffffc0000097, count=1,
> >> mapcount=0, mapping=00000000125079ad
> 
> It should be noted that on the second run, where we got the dump_page
> log, it indeed had a null mapping, which is similar to what Nikolaus
> asked on the previous thread he linked to, but looks like this wasn't
> the case on at least some of the reproductions of the issue.  On the
> line above, the condition that triggered the warning was page->mapping
> != NULL.  I don't know what to do with this information, though.

I don't see anything in upstream which will print "NULL" for a null
pointer passed to pointer().  ptr_to_id() doesn't check for NULL, nor
does __ptr_to_hashval().

I think we _should_, along with the ERR_PTR values and values less
than 4096.  But that patch isn't upstream.