On Sat, Jul 27, 2024 at 09:38:54AM GMT, Jens Axboe wrote:
> On 7/27/24 9:30 AM, Jens Axboe wrote:
> > On 7/26/24 4:48 PM, Linus Torvalds wrote:
> >> I didn't even look at what the issue was with the
> >> bio_for_each_segment() expansion, in the hope that Jens will make that
> >> one look better.
> >
> > I did take a quick look, pretty obviously bvec_iter_bvec() which makes
> > it horrible, which came from Kent's immutable work quite a while ago.
> > Not sure yet what to do about it, will spend some time on this next
> > week.
>
> Maybe something like this, totally untested...
>
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index f41c7f0ef91e..9ccccddadde2 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -130,12 +130,15 @@ struct bvec_iter_all {
> (mp_bvec_iter_page((bvec), (iter)) + \
> mp_bvec_iter_page_idx((bvec), (iter)))
>
> -#define bvec_iter_bvec(bvec, iter) \
> -((struct bio_vec) { \
> - .bv_page = bvec_iter_page((bvec), (iter)), \
> - .bv_len = bvec_iter_len((bvec), (iter)), \
> - .bv_offset = bvec_iter_offset((bvec), (iter)), \
> -})
> +static inline struct bio_vec bvec_iter_bvec(struct bio_vec *bv,
> + struct bvec_iter iter)
> +{
> + return (struct bio_vec) {
> + .bv_page = bvec_iter_page(bv, iter),
> + .bv_len = bvec_iter_len(bv, iter),
> + .bv_offset = bvec_iter_offset(bv, iter)
> + };
> +}
>
> static inline bool bvec_iter_advance(const struct bio_vec *bv,
> struct bvec_iter *iter, unsigned bytes)
>
> --
> Jens Axboe
>
I tried this patch, doesn't seem to make a huge difference, going from
3,958,564 bytes with longest line of 82 kB to 3,943,824 bytes with a
longest line of 77kB.
It seems that the .bv_len = ... expansion is what's doing it, so I tried
patching mp_bvec_iter_len() as well to do a silly ?: thing (sorry), which
takes us down to 3,880,309 with longest line of 20kB.
This is starting to feel like whack-a-mole isn't it? I looked at the next
longest line, which originates from include/linux/pid_namespace.h believe
it or not where some compiler cleverness + a loop is resulting in _another_
combinatorial explosion.
Whacking that gets us down to 3,860,780 with longest line of 16kB and
(credit to Willy for telling me about the make <file basename>.i thing):
awk '{print length}' mm/compaction.i | sort -n
Tells me we have... 18 lines >10k remaining in that file.
Obviously dropping from 82 kB to 16 kB max size is handy but I think all
this points to the need for a fundamental fix for this (I vote for
fundamental simplification of core min()/max() macros, keep something
complicated around for true const cases, e.g. MIN()/MAX()).
Patch attached including Jens's change + mine.
----8<----
