Re: fuse: kernel BUG at mm/truncate.c:763!

[Luis Henriques]

On Fri, Mar 12, 2021 at 01:11:23PM +0000, Matthew Wilcox wrote:
> On Fri, Mar 12, 2021 at 12:21:59PM +0000, Luis Henriques wrote:
> > > > I've seen a bug report (5.10.16 kernel splat below) that seems to be
> > > > reproducible in kernels as early as 5.4.
> 
> If this is reproducible, can you turn this BUG_ON into a VM_BUG_ON_PAGE()
> so we know what kind of problem we're dealing with?  Assuming the SUSE
> tumbleweed kernels enable CONFIG_DEBUG_VM, which I'm sure they do.

Just to make sure I got this right, you want to test something like this:

 				}
 			}
-			BUG_ON(page_mapped(page));
+			VM_BUG_ON_PAGE(page_mapped(page), page);
 			ret2 = do_launder_page(mapping, page);
 			if (ret2 == 0) {
 				if (!invalidate_complete_page2(mapping, page))

Cheers,
--
Luís

> 
> > > Page fault locks the page before installing a new pte, at least
> > > AFAICS, so the BUG looks impossible.  The referenced commits only
> > > touch very high level control of writeback, so they may well increase
> > > the chance of a bug triggering, but very unlikely to be the actual
> > > cause of the bug.   I'm guessing this to be an MM issue.
> > 
> > Ok, thank you for having a look at it.
> > 
> > Interestingly, there's a single commit to mm/truncate.c in 5.4:
> > ef18a1ca847b ("mm/thp: allow dropping THP from page cache").  I'm Cc'ing
> > Andrew and Kirill, maybe they have some ideas.
> 
> That's probably not it; unless FUSE has developed the ability to insert
> compound pages into the page cache without me noticing.
> 
> (if it had, that would absolutely explain it -- i have a fix in my thp
> tree for this case, but it doesn't affect any existing filesystem
> because only shmem uses compound pages and it doesn't call
> invalidate_inode_pages2_range)