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Date: Tue, 28 Sep 2021 20:41:50 +0800
From: Muchun Song <songmuchun@...edance.com>
To: mike.kravetz@...cle.com, akpm@...ux-foundation.org,
osalvador@...e.de, mhocko@...e.com, song.bao.hua@...ilicon.com,
david@...hat.com, chenhuang5@...wei.com, bodeddub@...zon.com,
corbet@....net, willy@...radead.org, 21cnbao@...il.com
Cc: duanxiongchun@...edance.com, fam.zheng@...edance.com,
smuchun@...il.com, zhengqi.arch@...edance.com,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Muchun Song <songmuchun@...edance.com>
Subject: [PATCH v5 3/5] mm: sparsemem: use page table lock to protect kernel pmd operations
The init_mm.page_table_lock is used to protect kernel page tables, we
can use it to serialize splitting vmemmap PMD mappings instead of mmap
write lock, which can increase the concurrency of vmemmap_remap_free().
Actually, It increase the concurrency between allocations of HugeTLB
pages. But it is not the only benefit. There are a lot of users of
mmap read lock of init_mm. The mmap write lock is holding through
vmemmap_remap_free(), removing mmap write lock usage to make it does
not affect other users of mmap read lock. It is not making anything
worse and always a win to move.
Now the kernel page table walker does not hold the page_table_lock when
walking pmd entries. There may be consistency issue of a pmd entry,
because pmd entry might change from a huge pmd entry to a PTE page table.
There is only one user of kernel page table walker, namely ptdump. The
ptdump already considers the consistency, which use a local variable to
cache the value of pmd entry. But we also need to update ->action to
ACTION_CONTINUE to make sure the walker does not walk every pte entry
again when concurrent thread has split the huge pmd.
Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
mm/ptdump.c | 16 ++++++++++++----
mm/sparse-vmemmap.c | 49 ++++++++++++++++++++++++++++++++++---------------
2 files changed, 46 insertions(+), 19 deletions(-)
diff --git a/mm/ptdump.c b/mm/ptdump.c
index da751448d0e4..eea3d28d173c 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -40,8 +40,10 @@ static int ptdump_pgd_entry(pgd_t *pgd, unsigned long addr,
if (st->effective_prot)
st->effective_prot(st, 0, pgd_val(val));
- if (pgd_leaf(val))
+ if (pgd_leaf(val)) {
st->note_page(st, addr, 0, pgd_val(val));
+ walk->action = ACTION_CONTINUE;
+ }
return 0;
}
@@ -61,8 +63,10 @@ static int ptdump_p4d_entry(p4d_t *p4d, unsigned long addr,
if (st->effective_prot)
st->effective_prot(st, 1, p4d_val(val));
- if (p4d_leaf(val))
+ if (p4d_leaf(val)) {
st->note_page(st, addr, 1, p4d_val(val));
+ walk->action = ACTION_CONTINUE;
+ }
return 0;
}
@@ -82,8 +86,10 @@ static int ptdump_pud_entry(pud_t *pud, unsigned long addr,
if (st->effective_prot)
st->effective_prot(st, 2, pud_val(val));
- if (pud_leaf(val))
+ if (pud_leaf(val)) {
st->note_page(st, addr, 2, pud_val(val));
+ walk->action = ACTION_CONTINUE;
+ }
return 0;
}
@@ -101,8 +107,10 @@ static int ptdump_pmd_entry(pmd_t *pmd, unsigned long addr,
if (st->effective_prot)
st->effective_prot(st, 3, pmd_val(val));
- if (pmd_leaf(val))
+ if (pmd_leaf(val)) {
st->note_page(st, addr, 3, pmd_val(val));
+ walk->action = ACTION_CONTINUE;
+ }
return 0;
}
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index e881f5db7091..ba76d8765211 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -53,8 +53,8 @@ struct vmemmap_remap_walk {
struct list_head *vmemmap_pages;
};
-static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
- struct vmemmap_remap_walk *walk)
+static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
+ struct vmemmap_remap_walk *walk)
{
pmd_t __pmd;
int i;
@@ -76,15 +76,37 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
set_pte_at(&init_mm, addr, pte, entry);
}
- /* Make pte visible before pmd. See comment in pmd_install(). */
- smp_wmb();
- pmd_populate_kernel(&init_mm, pmd, pgtable);
+ spin_lock(&init_mm.page_table_lock);
+ if (likely(pmd_leaf(*pmd))) {
+ /* Make pte visible before pmd. See comment in pmd_install(). */
+ smp_wmb();
+ pmd_populate_kernel(&init_mm, pmd, pgtable);
+ flush_tlb_kernel_range(start, start + PMD_SIZE);
+ spin_unlock(&init_mm.page_table_lock);
- flush_tlb_kernel_range(start, start + PMD_SIZE);
+ return 0;
+ }
+ spin_unlock(&init_mm.page_table_lock);
+ pte_free_kernel(&init_mm, pgtable);
return 0;
}
+static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
+ struct vmemmap_remap_walk *walk)
+{
+ int ret;
+
+ spin_lock(&init_mm.page_table_lock);
+ ret = pmd_leaf(*pmd);
+ spin_unlock(&init_mm.page_table_lock);
+
+ if (ret)
+ ret = __split_vmemmap_huge_pmd(pmd, start, walk);
+
+ return ret;
+}
+
static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
unsigned long end,
struct vmemmap_remap_walk *walk)
@@ -121,13 +143,12 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
pmd = pmd_offset(pud, addr);
do {
- if (pmd_leaf(*pmd)) {
- int ret;
+ int ret;
+
+ ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
+ if (ret)
+ return ret;
- ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
- if (ret)
- return ret;
- }
next = pmd_addr_end(addr, end);
vmemmap_pte_range(pmd, addr, next, walk);
} while (pmd++, addr = next, addr != end);
@@ -321,10 +342,8 @@ int vmemmap_remap_free(unsigned long start, unsigned long end,
*/
BUG_ON(start - reuse != PAGE_SIZE);
- mmap_write_lock(&init_mm);
+ mmap_read_lock(&init_mm);
ret = vmemmap_remap_range(reuse, end, &walk);
- mmap_write_downgrade(&init_mm);
-
if (ret && walk.nr_walked) {
end = reuse + walk.nr_walked * PAGE_SIZE;
/*
--
2.11.0
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