linux-kernel - [PATCH] vmalloc: Use atomic_long_add_return

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Message-Id: <20250415112646.113091-1-urezki@gmail.com>
Date: Tue, 15 Apr 2025 13:26:46 +0200
From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Baoquan He <bhe@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: [PATCH] vmalloc: Use atomic_long_add_return_relaxed()

Switch from the atomic_long_add_return() to its relaxed version.

We do not need a full memory barrier or any memory ordering during
increasing the "vmap_lazy_nr" variable. What we only need is to do it
atomically. This is what atomic_long_add_return_relaxed() guarantees.

AARCH64:

<snip>
Default:
    40ec:       d34cfe94        lsr     x20, x20, #12
    40f0:       14000044        b       4200 <free_vmap_area_noflush+0x19c>
    40f4:       94000000        bl      0 <__sanitizer_cov_trace_pc>
    40f8:       90000000        adrp    x0, 0 <__traceiter_alloc_vmap_area>
    40fc:       91000000        add     x0, x0, #0x0
    4100:       f8f40016        ldaddal x20, x22, [x0]
    4104:       8b160296        add     x22, x20, x22

Relaxed:
    40ec:       d34cfe94        lsr     x20, x20, #12
    40f0:       14000044        b       4200 <free_vmap_area_noflush+0x19c>
    40f4:       94000000        bl      0 <__sanitizer_cov_trace_pc>
    40f8:       90000000        adrp    x0, 0 <__traceiter_alloc_vmap_area>
    40fc:       91000000        add     x0, x0, #0x0
    4100:       f8340016        ldadd   x20, x22, [x0]
    4104:       8b160296        add     x22, x20, x22
<snip>

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7bb32f498d39..9d4027041a3f 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2370,7 +2370,7 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 	if (WARN_ON_ONCE(!list_empty(&va->list)))
 		return;
 
-	nr_lazy = atomic_long_add_return(va_size(va) >> PAGE_SHIFT,
+	nr_lazy = atomic_long_add_return_relaxed(va_size(va) >> PAGE_SHIFT,
 					 &vmap_lazy_nr);
 
 	/*
-- 
2.39.5