+ mm-page_alloc-use-try_cmpxchg-in-set_pfnblock_flags_mask.patch added to mm-unstable branch

+ mm-page_alloc-use-try_cmpxchg-in-set_pfnblock_flags_mask.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/page_alloc: use try_cmpxchg in set_pfnblock_flags_mask
has been added to the -mm mm-unstable branch.  Its filename is
     mm-page_alloc-use-try_cmpxchg-in-set_pfnblock_flags_mask.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-use-try_cmpxchg-in-set_pfnblock_flags_mask.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Uros Bizjak <ubizjak@gmail.com>
Subject: mm/page_alloc: use try_cmpxchg in set_pfnblock_flags_mask
Date: Fri, 8 Jul 2022 16:07:36 +0200

Use try_cmpxchg instead of cmpxchg in set_pfnblock_flags_mask.  x86
CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after cmpxchg (and related move instruction in front of cmpxchg). 
The main loop improves from:

    1c5d:	48 89 c2             	mov    %rax,%rdx
    1c60:	48 89 c1             	mov    %rax,%rcx
    1c63:	48 21 fa             	and    %rdi,%rdx
    1c66:	4c 09 c2             	or     %r8,%rdx
    1c69:	f0 48 0f b1 16       	lock cmpxchg %rdx,(%rsi)
    1c6e:	48 39 c1             	cmp    %rax,%rcx
    1c71:	75 ea                	jne    1c5d <...>

to:

    1c60:	48 89 ca             	mov    %rcx,%rdx
    1c63:	48 21 c2             	and    %rax,%rdx
    1c66:	4c 09 c2             	or     %r8,%rdx
    1c69:	f0 48 0f b1 16       	lock cmpxchg %rdx,(%rsi)
    1c6e:	75 f0                	jne    1c60 <...>

Link: https://lkml.kernel.org/r/20220708140736.8737-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

--- a/mm/page_alloc.c~mm-page_alloc-use-try_cmpxchg-in-set_pfnblock_flags_mask
+++ a/mm/page_alloc.c
@@ -602,7 +602,7 @@ void set_pfnblock_flags_mask(struct page
 {
 	unsigned long *bitmap;
 	unsigned long bitidx, word_bitidx;
-	unsigned long old_word, word;
+	unsigned long word;
 
 	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
 	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
@@ -618,12 +618,8 @@ void set_pfnblock_flags_mask(struct page
 	flags <<= bitidx;
 
 	word = READ_ONCE(bitmap[word_bitidx]);
-	for (;;) {
-		old_word = cmpxchg(&bitmap[word_bitidx], word, (word & ~mask) | flags);
-		if (word == old_word)
-			break;
-		word = old_word;
-	}
+	do {
+	} while (!try_cmpxchg(&bitmap[word_bitidx], &word, (word & ~mask) | flags));
 }
 
 void set_pageblock_migratetype(struct page *page, int migratetype)
_

Patches currently in -mm which might be from ubizjak@gmail.com are

mm-page_alloc-use-try_cmpxchg-in-set_pfnblock_flags_mask.patch
compiler-gcch-remove-ancient-workaround-for-gcc-pr-58670.patch