Re: [PATCH 0/7] minmax: reduce compilation time

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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<----
From 4303ddf7bd7c892f89c37d9b2cc4102860d8ef97 Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Date: Sat, 27 Jul 2024 17:25:03 +0100
Subject: [PATCH] Various attempts at whack-a-mole.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/bvec.h          | 26 +++++++++++++++++---------
 include/linux/pid_namespace.h |  8 ++++++--
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index f41c7f0ef91e..567522aec2f9 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -101,9 +101,14 @@ struct bvec_iter_all {
 #define mp_bvec_iter_page(bvec, iter)				\
 	(__bvec_iter_bvec((bvec), (iter))->bv_page)

-#define mp_bvec_iter_len(bvec, iter)				\
-	min((iter).bi_size,					\
-	    __bvec_iter_bvec((bvec), (iter))->bv_len - (iter).bi_bvec_done)
+static inline unsigned mp_bvec_iter_len(const struct bio_vec *bvec,
+					struct bvec_iter iter)
+{
+	unsigned remains = __bvec_iter_bvec(bvec, iter)->bv_len -
+		iter.bi_bvec_done;
+
+	return remains < iter.bi_size ? remains : iter.bi_size;
+}

 #define mp_bvec_iter_offset(bvec, iter)				\
 	(__bvec_iter_bvec((bvec), (iter))->bv_offset + (iter).bi_bvec_done)
@@ -130,12 +135,15 @@ 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)
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index f9f9931e02d6..0c9642acc670 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -60,8 +60,12 @@ static inline int pidns_memfd_noexec_scope(struct pid_namespace *ns)
 {
 	int scope = MEMFD_NOEXEC_SCOPE_EXEC;

-	for (; ns; ns = ns->parent)
-		scope = max(scope, READ_ONCE(ns->memfd_noexec_scope));
+	for (; ns; ns = ns->parent) {
+		int ns_scope = READ_ONCE(ns->memfd_noexec_scope);
+
+		if (ns_scope > scope)
+			scope = ns_scope;
+	}

 	return scope;
 }
--
2.45.2