linux-kernel - [PATCH v8 02/12] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE

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Message-Id: <20201210035526.38938-3-songmuchun@bytedance.com>
Date:   Thu, 10 Dec 2020 11:55:16 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, mhocko@...e.com, song.bao.hua@...ilicon.com,
        david@...hat.com
Cc:     duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: [PATCH v8 02/12] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

The purpose of introducing HUGETLB_PAGE_FREE_VMEMMAP is to configure
whether to enable the feature of freeing unused vmemmap associated with
HugeTLB pages. And this is just for dependency check. Now only support
x86-64.

Because this config depends on HAVE_BOOTMEM_INFO_NODE. And the function
of the register_page_bootmem_info() is aimed to register bootmem info.
So we should register bootmem info when this config is enabled.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 arch/x86/mm/init_64.c |  2 +-
 fs/Kconfig            | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0a45f062826e..0435bee2e172 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;
 
 static void __init register_page_bootmem_info(void)
 {
-#ifdef CONFIG_NUMA
+#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
 	int i;
 
 	for_each_online_node(i)
diff --git a/fs/Kconfig b/fs/Kconfig
index 976e8b9033c4..4c3a9c614983 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -245,6 +245,21 @@ config HUGETLBFS
 config HUGETLB_PAGE
 	def_bool HUGETLBFS
 
+config HUGETLB_PAGE_FREE_VMEMMAP
+	def_bool HUGETLB_PAGE
+	depends on X86_64
+	depends on SPARSEMEM_VMEMMAP
+	depends on HAVE_BOOTMEM_INFO_NODE
+	help
+	  When using HUGETLB_PAGE_FREE_VMEMMAP, the system can save up some
+	  memory from pre-allocated HugeTLB pages when they are not used.
+	  6 pages per HugeTLB page of the pmd level mapping and (PAGE_SIZE - 2)
+	  pages per HugeTLB page of the pud level mapping.
+
+	  When the pages are going to be used or freed up, the vmemmap array
+	  representing that range needs to be remapped again and the pages
+	  we discarded earlier need to be rellocated again.
+
 config MEMFD_CREATE
 	def_bool TMPFS || HUGETLBFS
 
-- 
2.11.0