Date:	Tue, 24 Mar 2015 11:30:03 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Jason Low <jason.low2@...com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
	Christoph Lameter <cl@...ux.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] mm: Remove usages of ACCESS_ONCE

On Mon 23-03-15 15:44:40, Jason Low wrote:
> Commit 38c5ce936a08 converted ACCESS_ONCE usage in gup_pmd_range() to
> READ_ONCE, since ACCESS_ONCE doesn't work reliably on non-scalar types.
> 
> This patch removes the rest of the usages of ACCESS_ONCE, and use
> READ_ONCE for the read accesses. This also makes things cleaner,
> instead of using separate/multiple sets of APIs.
> 
> Signed-off-by: Jason Low <jason.low2@...com>

Makes sense to me. I would prefer a patch split into two parts. One which
changes potentially dangerous usage of ACCESS_ONCE and the cleanup. This
will make the life of those who backport patches into older kernels
easier a bit. I won't insist though.

Acked-by: Michal Hocko <mhocko@...e.cz>

> ---
>  mm/gup.c         |    4 ++--
>  mm/huge_memory.c |    4 ++--
>  mm/internal.h    |    4 ++--
>  mm/ksm.c         |   10 +++++-----
>  mm/memcontrol.c  |   18 +++++++++---------
>  mm/memory.c      |    2 +-
>  mm/mmap.c        |    8 ++++----
>  mm/page_alloc.c  |    6 +++---
>  mm/rmap.c        |    6 +++---
>  mm/slub.c        |    4 ++--
>  mm/swap_state.c  |    2 +-
>  mm/swapfile.c    |    2 +-
>  12 files changed, 35 insertions(+), 35 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index ca7b607..6297f6b 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1019,7 +1019,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  		 *
>  		 * for an example see gup_get_pte in arch/x86/mm/gup.c
>  		 */
> -		pte_t pte = ACCESS_ONCE(*ptep);
> +		pte_t pte = READ_ONCE(*ptep);
>  		struct page *page;
>  
>  		/*
> @@ -1309,7 +1309,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	local_irq_save(flags);
>  	pgdp = pgd_offset(mm, addr);
>  	do {
> -		pgd_t pgd = ACCESS_ONCE(*pgdp);
> +		pgd_t pgd = READ_ONCE(*pgdp);
>  
>  		next = pgd_addr_end(addr, end);
>  		if (pgd_none(pgd))
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 3412cc8..3ae874c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -183,7 +183,7 @@ static struct page *get_huge_zero_page(void)
>  	struct page *zero_page;
>  retry:
>  	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
> -		return ACCESS_ONCE(huge_zero_page);
> +		return READ_ONCE(huge_zero_page);
>  
>  	zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
>  			HPAGE_PMD_ORDER);
> @@ -202,7 +202,7 @@ retry:
>  	/* We take additional reference here. It will be put back by shrinker */
>  	atomic_set(&huge_zero_refcount, 2);
>  	preempt_enable();
> -	return ACCESS_ONCE(huge_zero_page);
> +	return READ_ONCE(huge_zero_page);
>  }
>  
>  static void put_huge_zero_page(void)
> diff --git a/mm/internal.h b/mm/internal.h
> index edaab69..a25e359 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -224,13 +224,13 @@ static inline unsigned long page_order(struct page *page)
>   * PageBuddy() should be checked first by the caller to minimize race window,
>   * and invalid values must be handled gracefully.
>   *
> - * ACCESS_ONCE is used so that if the caller assigns the result into a local
> + * READ_ONCE is used so that if the caller assigns the result into a local
>   * variable and e.g. tests it for valid range before using, the compiler cannot
>   * decide to remove the variable and inline the page_private(page) multiple
>   * times, potentially observing different values in the tests and the actual
>   * use of the result.
>   */
> -#define page_order_unsafe(page)		ACCESS_ONCE(page_private(page))
> +#define page_order_unsafe(page)		READ_ONCE(page_private(page))
>  
>  static inline bool is_cow_mapping(vm_flags_t flags)
>  {
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 4162dce..7ee101e 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -542,7 +542,7 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
>  	expected_mapping = (void *)stable_node +
>  				(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
>  again:
> -	kpfn = ACCESS_ONCE(stable_node->kpfn);
> +	kpfn = READ_ONCE(stable_node->kpfn);
>  	page = pfn_to_page(kpfn);
>  
>  	/*
> @@ -551,7 +551,7 @@ again:
>  	 * but on Alpha we need to be more careful.
>  	 */
>  	smp_read_barrier_depends();
> -	if (ACCESS_ONCE(page->mapping) != expected_mapping)
> +	if (READ_ONCE(page->mapping) != expected_mapping)
>  		goto stale;
>  
>  	/*
> @@ -577,14 +577,14 @@ again:
>  		cpu_relax();
>  	}
>  
> -	if (ACCESS_ONCE(page->mapping) != expected_mapping) {
> +	if (READ_ONCE(page->mapping) != expected_mapping) {
>  		put_page(page);
>  		goto stale;
>  	}
>  
>  	if (lock_it) {
>  		lock_page(page);
> -		if (ACCESS_ONCE(page->mapping) != expected_mapping) {
> +		if (READ_ONCE(page->mapping) != expected_mapping) {
>  			unlock_page(page);
>  			put_page(page);
>  			goto stale;
> @@ -600,7 +600,7 @@ stale:
>  	 * before checking whether node->kpfn has been changed.
>  	 */
>  	smp_rmb();
> -	if (ACCESS_ONCE(stable_node->kpfn) != kpfn)
> +	if (READ_ONCE(stable_node->kpfn) != kpfn)
>  		goto again;
>  	remove_node_from_stable_tree(stable_node);
>  	return NULL;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 74a9641..14c2f20 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -674,7 +674,7 @@ static void mem_cgroup_remove_exceeded(struct mem_cgroup_per_zone *mz,
>  static unsigned long soft_limit_excess(struct mem_cgroup *memcg)
>  {
>  	unsigned long nr_pages = page_counter_read(&memcg->memory);
> -	unsigned long soft_limit = ACCESS_ONCE(memcg->soft_limit);
> +	unsigned long soft_limit = READ_ONCE(memcg->soft_limit);
>  	unsigned long excess = 0;
>  
>  	if (nr_pages > soft_limit)
> @@ -1042,7 +1042,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  			goto out_unlock;
>  
>  		do {
> -			pos = ACCESS_ONCE(iter->position);
> +			pos = READ_ONCE(iter->position);
>  			/*
>  			 * A racing update may change the position and
>  			 * put the last reference, hence css_tryget(),
> @@ -1359,13 +1359,13 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
>  	unsigned long limit;
>  
>  	count = page_counter_read(&memcg->memory);
> -	limit = ACCESS_ONCE(memcg->memory.limit);
> +	limit = READ_ONCE(memcg->memory.limit);
>  	if (count < limit)
>  		margin = limit - count;
>  
>  	if (do_swap_account) {
>  		count = page_counter_read(&memcg->memsw);
> -		limit = ACCESS_ONCE(memcg->memsw.limit);
> +		limit = READ_ONCE(memcg->memsw.limit);
>  		if (count <= limit)
>  			margin = min(margin, limit - count);
>  	}
> @@ -2637,7 +2637,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep)
>  		return cachep;
>  
>  	memcg = get_mem_cgroup_from_mm(current->mm);
> -	kmemcg_id = ACCESS_ONCE(memcg->kmemcg_id);
> +	kmemcg_id = READ_ONCE(memcg->kmemcg_id);
>  	if (kmemcg_id < 0)
>  		goto out;
>  
> @@ -5007,7 +5007,7 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
>  	 * tunable will only affect upcoming migrations, not the current one.
>  	 * So we need to save it, and keep it going.
>  	 */
> -	move_flags = ACCESS_ONCE(memcg->move_charge_at_immigrate);
> +	move_flags = READ_ONCE(memcg->move_charge_at_immigrate);
>  	if (move_flags) {
>  		struct mm_struct *mm;
>  		struct mem_cgroup *from = mem_cgroup_from_task(p);
> @@ -5241,7 +5241,7 @@ static u64 memory_current_read(struct cgroup_subsys_state *css,
>  static int memory_low_show(struct seq_file *m, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> -	unsigned long low = ACCESS_ONCE(memcg->low);
> +	unsigned long low = READ_ONCE(memcg->low);
>  
>  	if (low == PAGE_COUNTER_MAX)
>  		seq_puts(m, "max\n");
> @@ -5271,7 +5271,7 @@ static ssize_t memory_low_write(struct kernfs_open_file *of,
>  static int memory_high_show(struct seq_file *m, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> -	unsigned long high = ACCESS_ONCE(memcg->high);
> +	unsigned long high = READ_ONCE(memcg->high);
>  
>  	if (high == PAGE_COUNTER_MAX)
>  		seq_puts(m, "max\n");
> @@ -5301,7 +5301,7 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>  static int memory_max_show(struct seq_file *m, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> -	unsigned long max = ACCESS_ONCE(memcg->memory.limit);
> +	unsigned long max = READ_ONCE(memcg->memory.limit);
>  
>  	if (max == PAGE_COUNTER_MAX)
>  		seq_puts(m, "max\n");
> diff --git a/mm/memory.c b/mm/memory.c
> index 5ec794f..9f6d3c6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2845,7 +2845,7 @@ static void do_fault_around(struct vm_area_struct *vma, unsigned long address,
>  	struct vm_fault vmf;
>  	int off;
>  
> -	nr_pages = ACCESS_ONCE(fault_around_bytes) >> PAGE_SHIFT;
> +	nr_pages = READ_ONCE(fault_around_bytes) >> PAGE_SHIFT;
>  	mask = ~(nr_pages * PAGE_SIZE - 1) & PAGE_MASK;
>  
>  	start_addr = max(address & mask, vma->vm_start);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 06a6076..e65cbe0 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1133,7 +1133,7 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *
>   * by another page fault trying to merge _that_. But that's ok: if it
>   * is being set up, that automatically means that it will be a singleton
>   * acceptable for merging, so we can do all of this optimistically. But
> - * we do that ACCESS_ONCE() to make sure that we never re-load the pointer.
> + * we do that READ_ONCE() to make sure that we never re-load the pointer.
>   *
>   * IOW: that the "list_is_singular()" test on the anon_vma_chain only
>   * matters for the 'stable anon_vma' case (ie the thing we want to avoid
> @@ -1147,7 +1147,7 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *
>  static struct anon_vma *reusable_anon_vma(struct vm_area_struct *old, struct vm_area_struct *a, struct vm_area_struct *b)
>  {
>  	if (anon_vma_compatible(a, b)) {
> -		struct anon_vma *anon_vma = ACCESS_ONCE(old->anon_vma);
> +		struct anon_vma *anon_vma = READ_ONCE(old->anon_vma);
>  
>  		if (anon_vma && list_is_singular(&old->anon_vma_chain))
>  			return anon_vma;
> @@ -2100,7 +2100,7 @@ static int acct_stack_growth(struct vm_area_struct *vma, unsigned long size, uns
>  	actual_size = size;
>  	if (size && (vma->vm_flags & (VM_GROWSUP | VM_GROWSDOWN)))
>  		actual_size -= PAGE_SIZE;
> -	if (actual_size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur))
> +	if (actual_size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur))
>  		return -ENOMEM;
>  
>  	/* mlock limit tests */
> @@ -2108,7 +2108,7 @@ static int acct_stack_growth(struct vm_area_struct *vma, unsigned long size, uns
>  		unsigned long locked;
>  		unsigned long limit;
>  		locked = mm->locked_vm + grow;
> -		limit = ACCESS_ONCE(rlim[RLIMIT_MEMLOCK].rlim_cur);
> +		limit = READ_ONCE(rlim[RLIMIT_MEMLOCK].rlim_cur);
>  		limit >>= PAGE_SHIFT;
>  		if (locked > limit && !capable(CAP_IPC_LOCK))
>  			return -ENOMEM;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1b84950..ebffa0e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1371,7 +1371,7 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
>  	int to_drain, batch;
>  
>  	local_irq_save(flags);
> -	batch = ACCESS_ONCE(pcp->batch);
> +	batch = READ_ONCE(pcp->batch);
>  	to_drain = min(pcp->count, batch);
>  	if (to_drain > 0) {
>  		free_pcppages_bulk(zone, to_drain, pcp);
> @@ -1570,7 +1570,7 @@ void free_hot_cold_page(struct page *page, bool cold)
>  		list_add_tail(&page->lru, &pcp->lists[migratetype]);
>  	pcp->count++;
>  	if (pcp->count >= pcp->high) {
> -		unsigned long batch = ACCESS_ONCE(pcp->batch);
> +		unsigned long batch = READ_ONCE(pcp->batch);
>  		free_pcppages_bulk(zone, batch, pcp);
>  		pcp->count -= batch;
>  	}
> @@ -6207,7 +6207,7 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  	mask <<= (BITS_PER_LONG - bitidx - 1);
>  	flags <<= (BITS_PER_LONG - bitidx - 1);
>  
> -	word = ACCESS_ONCE(bitmap[word_bitidx]);
> +	word = READ_ONCE(bitmap[word_bitidx]);
>  	for (;;) {
>  		old_word = cmpxchg(&bitmap[word_bitidx], word, (word & ~mask) | flags);
>  		if (word == old_word)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 8030382..dad23a4 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -456,7 +456,7 @@ struct anon_vma *page_get_anon_vma(struct page *page)
>  	unsigned long anon_mapping;
>  
>  	rcu_read_lock();
> -	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> +	anon_mapping = (unsigned long)READ_ONCE(page->mapping);
>  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>  		goto out;
>  	if (!page_mapped(page))
> @@ -500,14 +500,14 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
>  	unsigned long anon_mapping;
>  
>  	rcu_read_lock();
> -	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> +	anon_mapping = (unsigned long)READ_ONCE(page->mapping);
>  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>  		goto out;
>  	if (!page_mapped(page))
>  		goto out;
>  
>  	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> -	root_anon_vma = ACCESS_ONCE(anon_vma->root);
> +	root_anon_vma = READ_ONCE(anon_vma->root);
>  	if (down_read_trylock(&root_anon_vma->rwsem)) {
>  		/*
>  		 * If the page is still mapped, then this anon_vma is still
> diff --git a/mm/slub.c b/mm/slub.c
> index d01f912..3b5cb6b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4277,7 +4277,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
>  			int node;
>  			struct page *page;
>  
> -			page = ACCESS_ONCE(c->page);
> +			page = READ_ONCE(c->page);
>  			if (!page)
>  				continue;
>  
> @@ -4292,7 +4292,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
>  			total += x;
>  			nodes[node] += x;
>  
> -			page = ACCESS_ONCE(c->partial);
> +			page = READ_ONCE(c->partial);
>  			if (page) {
>  				node = page_to_nid(page);
>  				if (flags & SO_TOTAL)
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 405923f..8bc8e66 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -390,7 +390,7 @@ static unsigned long swapin_nr_pages(unsigned long offset)
>  	unsigned int pages, max_pages, last_ra;
>  	static atomic_t last_readahead_pages;
>  
> -	max_pages = 1 << ACCESS_ONCE(page_cluster);
> +	max_pages = 1 << READ_ONCE(page_cluster);
>  	if (max_pages <= 1)
>  		return 1;
>  
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 63f55cc..a7e7210 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1312,7 +1312,7 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
>  			else
>  				continue;
>  		}
> -		count = ACCESS_ONCE(si->swap_map[i]);
> +		count = READ_ONCE(si->swap_map[i]);
>  		if (count && swap_count(count) != SWAP_MAP_BAD)
>  			break;
>  	}
> -- 
> 1.7.2.5
> 
> 
> 

-- 
Michal Hocko
SUSE Labs
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