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SubjectRe: e827091cb1 "block: merge: get the 1st and last bvec via helpers" broken
On Sat, Mar 12, 2016 at 10:25:48PM +0800, Ming Lei wrote:
> On Sat, 12 Mar 2016 05:02:56 -0900
> Kent Overstreet <kent.overstreet@gmail.com> wrote:
>
> > Here's the output of the patch below:
> >
> > generic/036 11s ...run fstests generic/036 at 2016-03-12 13:58:21
> > end 4096 0 ffffea0001d611c0 end2 1024 0 ffffea0001d611c0
> > len 1024 offset 0 page ffffea0001d611c0
> > KGDB: Waiting for remote debugger
> >
> > Your code gives a biovec with bv_len of 4096, the old code gives a biovec with
> > bv_len of 1024 (and then we dump every biovec, we see that the bio had only a
> > single biovec that did indeed have bv_len == 1024).
>
> I guess we shouldn't have optimized for the case of non-cloned bio, could you
> try the following patch?
>
> --
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 1e7248f..4abc129 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -267,11 +267,6 @@ static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
> struct bvec_iter iter = bio->bi_iter;
> int idx;
>
> - if (!bio_flagged(bio, BIO_CLONED)) {
> - *bv = bio->bi_io_vec[bio->bi_vcnt - 1];
> - return;
> - }
> -
> if (unlikely(!bio_multiple_segments(bio))) {
> *bv = bio_iovec(bio);
> return;
>
> Thanks,
> Ming

Yes, that's it.

!BIO_CLONED is _not_ a guarantee that bi_size doesn't straddle the middle of a
bvec - bcachefs was hitting this by bouncing a bio that had already been split
(which can happen elsewhere in the kernel...) but there's other (perfectly
legal) ways it can happen.

I would still strongly suggest reverting the patch for 4.5 and resubmitting
during the next merge window.

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