Commit d8d55f56 authored by Muchun Song's avatar Muchun Song Committed by Linus Torvalds
Browse files

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.

Link: https://lkml.kernel.org/r/20211101031651.75851-4-songmuchun@bytedance.com


Signed-off-by: default avatarMuchun 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent a6b40850

mm/ptdump.c

+12 −4
Original line number Diff line number Diff line
@@ -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;

}

mm/sparse-vmemmap.c

+31 −16
Original line number Diff line number Diff line
@@ -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;
@@ -76,13 +75,32 @@ 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,
@@ -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;

		/*