linux-kernel - [PATCH v2 5/7] mm/vmalloc: skip the uninitilized vmalloc areas

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Date:   Sat, 17 Dec 2022 09:54:33 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, urezki@...il.com, stephen.s.brennan@...cle.com,
        willy@...radead.org, akpm@...ux-foundation.org, hch@...radead.org,
        Baoquan He <bhe@...hat.com>
Subject: [PATCH v2 5/7] mm/vmalloc: skip the uninitilized vmalloc areas

For areas allocated via vmalloc_xxx() APIs, it searches for unmapped area
to reserve and allocates new pages to map into, please see function
__vmalloc_node_range(). During the process, flag VM_UNINITIALIZED is set
in vm->flags to indicate that the pages allocation and mapping haven't
been done, until clear_vm_uninitialized_flag() is called to clear it.

For this kind of area, if VM_UNINITIALIZED is still set, let's ignore
it in vread() because pages newly allocated and being mapped in that
area only contains zero data. reading them out by aligned_vread() is
wasting time.

Signed-off-by: Baoquan He <bhe@...hat.com>
---
 mm/vmalloc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 3bfa872a4513..bdaceda1b878 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3617,6 +3617,11 @@ long vread(char *buf, char *addr, unsigned long count)
 		if (!vm && !flags)
 			continue;

+		if (vm->flags & VM_UNINITIALIZED)
+			continue;
+		/* Pair with smp_wmb() in clear_vm_uninitialized_flag() */
+		smp_rmb();
+
 		vaddr = (char *) va->va_start;
 		size = flags ? va_size(va) : get_vm_area_size(vm);

-- 
2.34.1