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mm: sparsemem: use page table lock to protect kernel pmd operations

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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.

Link: https://lkml.kernel.org/r/20211101031651.75851-4-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>
Cc: Bodeddula Balasubramaniam <bodeddub@amazon.com>
Cc: Chen Huang <chenhuang5@huawei.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Fam Zheng <fam.zheng@bytedance.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Muchun Song 2022-03-22 14:45:06 -07:00 committed by Linus Torvalds
parent a6b40850c4
commit d8d55f5616
2 changed files with 43 additions and 20 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

16
mm/ptdump.c
View file

@ -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;
}

33
mm/sparse-vmemmap.c
View file

@ -53,8 +53,7 @@ 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)
{
pmd_t __pmd;
int i;
@ -76,15 +75,34 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
set_pte_at(&init_mm, addr, pte, entry);
}
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);
} else {
pte_free_kernel(&init_mm, pgtable);
}
spin_unlock(&init_mm.page_table_lock);
return 0;
}
static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
{
int leaf;
spin_lock(&init_mm.page_table_lock);
leaf = pmd_leaf(*pmd);
spin_unlock(&init_mm.page_table_lock);
if (!leaf)
return 0;
return __split_vmemmap_huge_pmd(pmd, start);
}
static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
unsigned long end,
struct vmemmap_remap_walk *walk)
@ -121,13 +139,12 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
pmd = pmd_offset(pud, addr);
do {
if (pmd_leaf(*pmd)) {
int ret;
ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
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 +338,8 @@ 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;
/*