Re: generic/068 crash on 5.18-rc2?

[Brian Foster <bfoster@redhat.com>]

On Sat, Apr 30, 2022 at 10:40:31PM +0100, Matthew Wilcox wrote:
> On Sat, Apr 30, 2022 at 04:44:07AM +0100, Matthew Wilcox wrote:
> > (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).
> 
> Does anyone have a preference for fixing this between Option A:
> 

After seeing the trace in my previous mail and several thousand
successful iterations of the test hack, I had reworked it into this
(which survived weekend testing until it ran into some other XFS problem
that looks unrelated):

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 278cc81cc1e7..aa820e09978e 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -269,6 +269,7 @@ struct folio_iter {
 	size_t offset;
 	size_t length;
 	/* private: for use by the iterator */
+	struct folio *_next;
 	size_t _seg_count;
 	int _i;
 };
@@ -279,6 +280,7 @@ static inline void bio_first_folio(struct folio_iter *fi, struct bio *bio,
 	struct bio_vec *bvec = bio_first_bvec_all(bio) + i;
 
 	fi->folio = page_folio(bvec->bv_page);
+	fi->_next = folio_next(fi->folio);
 	fi->offset = bvec->bv_offset +
 			PAGE_SIZE * (bvec->bv_page - &fi->folio->page);
 	fi->_seg_count = bvec->bv_len;
@@ -290,13 +292,15 @@ 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 = fi->_next;
+		fi->_next = folio_next(fi->folio);
 		fi->offset = 0;
 		fi->length = min(folio_size(fi->folio), fi->_seg_count);
 	} else if (fi->_i + 1 < bio->bi_vcnt) {
 		bio_first_folio(fi, bio, fi->_i + 1);
 	} else {
 		fi->folio = NULL;
+		fi->_next = NULL;
 	}
 }

So FWIW, that is just to say that I find option A to be cleaner and more
readable.

Brian

> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 49eff01fb829..55e2499beff6 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -269,6 +269,7 @@ struct folio_iter {
>         size_t offset;
>         size_t length;
>         /* private: for use by the iterator */
> +       struct folio *_next;
>         size_t _seg_count;
>         int _i;
>  };
> @@ -280,19 +281,23 @@ static inline void bio_first_folio(struct folio_iter *fi,
> struct bio *bio,
> 
>         fi->folio = page_folio(bvec->bv_page);
>         fi->offset = bvec->bv_offset +
> -                       PAGE_SIZE * (bvec->bv_page - &fi->folio->page);
> +                       PAGE_SIZE * folio_page_idx(fi->folio, bvec->bv_page);
>         fi->_seg_count = bvec->bv_len;
>         fi->length = min(folio_size(fi->folio) - fi->offset, fi->_seg_count);
>         fi->_i = i;
> +       if (fi->_seg_count > fi->length)
> +               fi->_next = folio_next(fi->folio);
>  }
> 
>  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 = fi->_next;
>                 fi->offset = 0;
>                 fi->length = min(folio_size(fi->folio), fi->_seg_count);
> +               if (fi->_seg_count > fi->length)
> +                       fi->_next = folio_next(fi->folio);
>         } else if (fi->_i + 1 < bio->bi_vcnt) {
>                 bio_first_folio(fi, bio, fi->_i + 1);
>         } else {
> 
> 
> and Option B:
> 
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 49eff01fb829..554f5fce060c 100644
> --- a/include/linux/bio.h
> +++ 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 = __folio_next(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) {
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index de32c0383387..9c5547af8d0e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1642,6 +1642,12 @@ static inline long folio_nr_pages(struct folio *folio)
>         return compound_nr(&folio->page);
>  }
> 
> +static inline struct folio *__folio_next(struct folio *folio,
> +               unsigned long nr_pages)
> +{
> +       return (struct folio *)folio_page(folio, nr_pages);
> +}
> +
>  /**
>   * folio_next - Move to the next physical folio.
>   * @folio: The folio we're currently operating on.
> @@ -1658,7 +1664,7 @@ static inline long folio_nr_pages(struct folio *folio)
>   */
>  static inline struct folio *folio_next(struct folio *folio)
>  {
> -       return (struct folio *)folio_page(folio, folio_nr_pages(folio));
> +       return __folio_next(folio, folio_nr_pages(folio));
>  }
> 
>  /**
> 
> 
> Currently running Option A through its paces.
>