The patch titled
Subject: mm: remove an avoidable load of page refcount in page_ref_add_unless
has been added to the -mm mm-unstable branch. Its filename is
mm-remove-an-avoidable-load-of-page-refcount-in-page_ref_add_unless.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-remove-an-avoidable-load-of-page-refcount-in-page_ref_add_unless.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: Mateusz Guzik <mjguzik@xxxxxxxxx>
Subject: mm: remove an avoidable load of page refcount in page_ref_add_unless
Date: Sat, 7 Dec 2024 09:29:31 +0100
Explicitly pre-checking the count adds nothing as atomic_add_unless starts
with doing the same thing. iow no functional changes.
disasm of stock filemap_get_read_batch from perf top while running
readseek2_processes -t 24:
0.04 â?? cb: mov 0x34(%rbx),%eax # first load
73.11 â?? test %eax,%eax
â?? â?? je 1bd
0.09 â?? mov 0x34(%rbx),%eax # second load
1.01 â?? d9: test %eax,%eax
â?? â?? je 1bd
0.06 â?? lea 0x1(%rax),%edx
0.00 â?? lea 0x34(%rbx),%r14
0.00 â?? lock cmpxchg %edx,0x34(%rbx)
14.06 â?? â?? jne d9
Link: https://lkml.kernel.org/r/20241207082931.1707465-1-mjguzik@xxxxxxxxx
Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/page_ref.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/include/linux/page_ref.h~mm-remove-an-avoidable-load-of-page-refcount-in-page_ref_add_unless
+++ a/include/linux/page_ref.h
@@ -234,7 +234,7 @@ static inline bool page_ref_add_unless(s
rcu_read_lock();
/* avoid writing to the vmemmap area being remapped */
- if (!page_is_fake_head(page) && page_ref_count(page) != u)
+ if (!page_is_fake_head(page))
ret = atomic_add_unless(&page->_refcount, nr, u);
rcu_read_unlock();
_
Patches currently in -mm which might be from mjguzik@xxxxxxxxx are
mm-remove-an-avoidable-load-of-page-refcount-in-page_ref_add_unless.patch
get_task_exe_file-check-pf_kthread-locklessly.patch
