Re: kernel BUG when mounting large block xfs backed by 9p (folio ref count bug)

Dominique Martinet
2025-12-07T13:49:53Z

Chris,

I'm not sure why but I can't reproduce with your .config either :/
If you can still reproduce this reliably, could you try with the
following diff applied (which is basically the same as what Christian
suggested a couple of days ago with also ubuf, whatever that is)
------------
diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 10c2dd486438..f7ee1f864b03 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -318,7 +318,7 @@ static int p9_get_mapped_pages(struct virtio_chan *chan,
if (!iov_iter_count(data))
return 0;

- if (!iov_iter_is_kvec(data)) {
+ if (user_backed_iter(data)) {
int n;
/*
* We allow only p9_max_pages pinned. We wait for the
-----------

Willy,

Matthew Wilcox wrote on Sun, Dec 07, 2025 at 07:18:02AM +0000:
> In readahead, we allocate a folio, lock it and add it to the page cache.
> We then submit it to the filesystem for read. It cannot be truncated
> from the page cache until the filesystem unlocks it (generally by calling
> folio_end_read() but some filesystems explicitly call folio_unlock()
> instead). So you don't need to take an extra reference to it.

Thanks.

My main problem with this all is that trans_virtio adds the buffers to
the virtio virtqueue but does nothing to take it off if
wait_event_killable() in virtio_request() gets killed, but looking at it
even in the code path that gets a ref the code will happily drop the ref
even before the flush is over so I guess there's no reason to actively
try to pin kernel pages...

I'd sleep better if there was a way to remove (detach?) the buffer from
the virtqueue but I can't see how to do that without breaking something
else, so I guess we'll have to live with that behavior unless someone
knows better.

--
Dominique Martinet | Asmadeus