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Date: Tue, 29 Mar 2016 11:27:47 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc: Hugh Dickins <hughd@...gle.com>,
Johannes Berg <johannes@...solutions.net>,
"David S. Miller" <davem@...emloft.net>,
Sunil Goutham <sgoutham@...ium.com>,
Chris Metcalf <cmetcalf@...lanox.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 2/2] mm: rename _count, field of the struct page, to
_refcount
On 03/28/2016 08:30 AM, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> Many developer already know that field for reference count of
> the struct page is _count and atomic type. They would try to handle it
> directly and this could break the purpose of page reference count
> tracepoint. To prevent direct _count modification, this patch rename it
> to _refcount and add warning message on the code. After that, developer
> who need to handle reference count will find that field should not be
> accessed directly.
>
> v2: change more _count usages to _refcount
There's also
Documentation/vm/transhuge.txt talking about ->_count
include/linux/mm.h: * requires to already have an elevated page->_count.
include/linux/mm_types.h: * Keep _count separate from slub cmpxchg_double data.
include/linux/mm_types.h: * slab_lock but _count is not.
include/linux/pagemap.h: * If the page is free (_count == 0), then _count is untouched, and 0
include/linux/pagemap.h: * is returned. Otherwise, _count is incremented by 1 and 1 is returned.
include/linux/pagemap.h: * this allows allocators to use a synchronize_rcu() to stabilize _count.
include/linux/pagemap.h: * Remove-side that cares about stability of _count (eg. reclaim) has the
mm/huge_memory.c: * tail_page->_count is zero and not changing from under us. But
mm/huge_memory.c: /* Prevent deferred_split_scan() touching ->_count */
mm/internal.h: * Turn a non-refcounted page (->_count == 0) into refcounted with
mm/page_alloc.c: bad_reason = "nonzero _count";
mm/page_alloc.c: bad_reason = "nonzero _count";
mm/page_alloc.c: * because their page->_count is zero at all time.
mm/slub.c: * as page->_count. If we assign to ->counters directly
mm/slub.c: * we run the risk of losing updates to page->_count, so
mm/vmscan.c: * load is not satisfied before that of page->_count.
mm/vmscan.c: * The downside is that we have to touch page->_count against each page.
I've arrived at the following command to find this:
git grep "[^a-zA-Z0-9_]_count[^_]"
Not that many false positives in the output :)
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
> arch/tile/mm/init.c | 2 +-
> include/linux/mm_types.h | 8 ++++++--
> include/linux/page_ref.h | 26 +++++++++++++-------------
> kernel/kexec_core.c | 2 +-
> 4 files changed, 21 insertions(+), 17 deletions(-)
>
> diff --git a/arch/tile/mm/init.c b/arch/tile/mm/init.c
> index a0582b7..adce254 100644
> --- a/arch/tile/mm/init.c
> +++ b/arch/tile/mm/init.c
> @@ -679,7 +679,7 @@ static void __init init_free_pfn_range(unsigned long start, unsigned long end)
> * Hacky direct set to avoid unnecessary
> * lock take/release for EVERY page here.
> */
> - p->_count.counter = 0;
> + p->_refcount.counter = 0;
> p->_mapcount.counter = -1;
> }
> init_page_count(page);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 944b2b3..9e8eb5a 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -97,7 +97,11 @@ struct page {
> };
> int units; /* SLOB */
> };
> - atomic_t _count; /* Usage count, see below. */
> + /*
> + * Usage count, *USE WRAPPER FUNCTION*
> + * when manual accounting. See page_ref.h
> + */
> + atomic_t _refcount;
> };
> unsigned int active; /* SLAB */
> };
> @@ -248,7 +252,7 @@ struct page_frag_cache {
> __u32 offset;
> #endif
> /* we maintain a pagecount bias, so that we dont dirty cache line
> - * containing page->_count every time we allocate a fragment.
> + * containing page->_refcount every time we allocate a fragment.
> */
> unsigned int pagecnt_bias;
> bool pfmemalloc;
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index e596d5d9..8b5e0a9 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -63,17 +63,17 @@ static inline void __page_ref_unfreeze(struct page *page, int v)
>
> static inline int page_ref_count(struct page *page)
> {
> - return atomic_read(&page->_count);
> + return atomic_read(&page->_refcount);
> }
>
> static inline int page_count(struct page *page)
> {
> - return atomic_read(&compound_head(page)->_count);
> + return atomic_read(&compound_head(page)->_refcount);
> }
>
> static inline void set_page_count(struct page *page, int v)
> {
> - atomic_set(&page->_count, v);
> + atomic_set(&page->_refcount, v);
> if (page_ref_tracepoint_active(__tracepoint_page_ref_set))
> __page_ref_set(page, v);
> }
> @@ -89,35 +89,35 @@ static inline void init_page_count(struct page *page)
>
> static inline void page_ref_add(struct page *page, int nr)
> {
> - atomic_add(nr, &page->_count);
> + atomic_add(nr, &page->_refcount);
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> __page_ref_mod(page, nr);
> }
>
> static inline void page_ref_sub(struct page *page, int nr)
> {
> - atomic_sub(nr, &page->_count);
> + atomic_sub(nr, &page->_refcount);
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> __page_ref_mod(page, -nr);
> }
>
> static inline void page_ref_inc(struct page *page)
> {
> - atomic_inc(&page->_count);
> + atomic_inc(&page->_refcount);
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> __page_ref_mod(page, 1);
> }
>
> static inline void page_ref_dec(struct page *page)
> {
> - atomic_dec(&page->_count);
> + atomic_dec(&page->_refcount);
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
> __page_ref_mod(page, -1);
> }
>
> static inline int page_ref_sub_and_test(struct page *page, int nr)
> {
> - int ret = atomic_sub_and_test(nr, &page->_count);
> + int ret = atomic_sub_and_test(nr, &page->_refcount);
>
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_and_test))
> __page_ref_mod_and_test(page, -nr, ret);
> @@ -126,7 +126,7 @@ static inline int page_ref_sub_and_test(struct page *page, int nr)
>
> static inline int page_ref_dec_and_test(struct page *page)
> {
> - int ret = atomic_dec_and_test(&page->_count);
> + int ret = atomic_dec_and_test(&page->_refcount);
>
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_and_test))
> __page_ref_mod_and_test(page, -1, ret);
> @@ -135,7 +135,7 @@ static inline int page_ref_dec_and_test(struct page *page)
>
> static inline int page_ref_dec_return(struct page *page)
> {
> - int ret = atomic_dec_return(&page->_count);
> + int ret = atomic_dec_return(&page->_refcount);
>
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_and_return))
> __page_ref_mod_and_return(page, -1, ret);
> @@ -144,7 +144,7 @@ static inline int page_ref_dec_return(struct page *page)
>
> static inline int page_ref_add_unless(struct page *page, int nr, int u)
> {
> - int ret = atomic_add_unless(&page->_count, nr, u);
> + int ret = atomic_add_unless(&page->_refcount, nr, u);
>
> if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_unless))
> __page_ref_mod_unless(page, nr, ret);
> @@ -153,7 +153,7 @@ static inline int page_ref_add_unless(struct page *page, int nr, int u)
>
> static inline int page_ref_freeze(struct page *page, int count)
> {
> - int ret = likely(atomic_cmpxchg(&page->_count, count, 0) == count);
> + int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);
>
> if (page_ref_tracepoint_active(__tracepoint_page_ref_freeze))
> __page_ref_freeze(page, count, ret);
> @@ -165,7 +165,7 @@ static inline void page_ref_unfreeze(struct page *page, int count)
> VM_BUG_ON_PAGE(page_count(page) != 0, page);
> VM_BUG_ON(count == 0);
>
> - atomic_set(&page->_count, count);
> + atomic_set(&page->_refcount, count);
> if (page_ref_tracepoint_active(__tracepoint_page_ref_unfreeze))
> __page_ref_unfreeze(page, count);
> }
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index f826e11..e0e95b0 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -1410,7 +1410,7 @@ static int __init crash_save_vmcoreinfo_init(void)
> VMCOREINFO_STRUCT_SIZE(list_head);
> VMCOREINFO_SIZE(nodemask_t);
> VMCOREINFO_OFFSET(page, flags);
> - VMCOREINFO_OFFSET(page, _count);
> + VMCOREINFO_OFFSET(page, _refcount);
> VMCOREINFO_OFFSET(page, mapping);
> VMCOREINFO_OFFSET(page, lru);
> VMCOREINFO_OFFSET(page, _mapcount);
>
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