Re: generic/068 crash on 5.18-rc2?

Matthew Wilcox
2022-04-30T03:44:19Z

On Thu, Apr 28, 2022 at 11:53:18AM -0400, Brian Foster wrote:
> The above is the variant of generic/068 failure I was reproducing and
> used to bisect [1]. With some additional tracing added to ioend
> completion, what I'm seeing is that the bio_for_each_folio_all() bvec
> iteration basically seems to go off the rails. What happens more
> specifically is that at some point during the loop, bio_next_folio()
> actually lands into the second page of the just processed folio instead
> of the actual next folio (i.e. as if it's walking to the next page from
> the head page of the folio instead of to the next 16k folio). I suspect
> completion is racing with some form of truncation/reclaim/invalidation
> here, what exactly I don't know, that perhaps breaks down the folio and
> renders the iteration (bio_next_folio() -> folio_next()) unsafe. To test
> that theory, I open coded and modified the loop to something like the
> following:
>
> for (bio_first_folio(&fi, bio, 0); fi.folio; ) {
> f = fi.folio;
> l = fi.length;
> bio_next_folio(&fi, bio);
> iomap_finish_folio_write(inode, f, l, error);
> folio_count++;
> }
>
> ... to avoid accessing folio metadata after writeback is cleared on it
> and this seems to make the problem disappear (so far, I'll need to let
> this spin for a while longer to be completely confident in that).

_Oh_.

It's not even a terribly weird race, then. It's just this:

CPU 0 CPU 1
truncate_inode_partial_folio()
folio_wait_writeback();
bio_next_folio(&fi, bio)
iomap_finish_folio_write(fi.folio)
folio_end_writeback(folio)
split_huge_page()
bio_next_folio()
... oops, now we only walked forward one page instead of the entire folio.

So ... I think we can fix this with:

+++ b/include/linux/bio.h
@@ -290,7 +290,8 @@ static inline void bio_next_folio(struct folio_iter *fi, struct bio *bio)
{
fi->_seg_count -= fi->length;
if (fi->_seg_count) {
- fi->folio = folio_next(fi->folio);
+ fi->folio = (struct folio *)folio_page(fi->folio,
+ (fi->offset + fi->length) / PAGE_SIZE);
fi->offset = 0;
fi->length = min(folio_size(fi->folio), fi->_seg_count);
} else if (fi->_i + 1 < bio->bi_vcnt) {

(I do not love this, have not even compiled it; it's late. We may be
better off just storing next_folio inside the folio_iter).