[PATCH] Revert "mm: rename _count, field of the struct page, to _refcount"

From: Vitaly Kuznetsov
Date: Thu Jun 16 2016 - 05:23:02 EST

_count -> _refcount rename in commit 0139aa7b7fa12 ("mm: rename _count,
field of the struct page, to _refcount") broke kdump. makedumpfile(8) does
stuff like READ_MEMBER_OFFSET("page._count", page._count) and fails. While
it is definitely possible to fix this particular tool I'm not sure about
other tools which might be doing the same.

I suggest we remember the "we don't break userspace" rule and revert for
4.7 while it's not too late.

This is a partial revert, useful hunks in drivers which do
page_ref_{sub,add,inc} instead of open coded atomic_* operations stay.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
Documentation/vm/transhuge.txt | 10 +++++-----
arch/tile/mm/init.c | 2 +-
drivers/block/aoe/aoecmd.c | 2 +-
drivers/hwtracing/intel_th/msu.c | 2 +-
fs/proc/page.c | 2 +-
include/linux/mm.h | 2 +-
include/linux/mm_types.h | 14 +++++---------
include/linux/page_ref.h | 26 +++++++++++++-------------
include/linux/pagemap.h | 8 ++++----
kernel/kexec_core.c | 2 +-
mm/huge_memory.c | 4 ++--
mm/internal.h | 2 +-
mm/page_alloc.c | 4 ++--
mm/slub.c | 4 ++--
mm/vmscan.c | 4 ++--
15 files changed, 42 insertions(+), 46 deletions(-)

diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
index 7c871d6..75c774c 100644
--- a/Documentation/vm/transhuge.txt
+++ b/Documentation/vm/transhuge.txt
@@ -394,9 +394,9 @@ hugepage natively. Once finished you can drop the page table lock.
Refcounting on THP is mostly consistent with refcounting on other compound
pages:

- - get_page()/put_page() and GUP operate in head page's ->_refcount.
+ - get_page()/put_page() and GUP operate in head page's ->_count.

- - ->_refcount in tail pages is always zero: get_page_unless_zero() never
+ - ->_count in tail pages is always zero: get_page_unless_zero() never
succeed on tail pages.

- map/unmap of the pages with PTE entry increment/decrement ->_mapcount
@@ -426,15 +426,15 @@ requests to split pinned huge page: it expects page count to be equal to
sum of mapcount of all sub-pages plus one (split_huge_page caller must
have reference for head page).

-split_huge_page uses migration entries to stabilize page->_refcount and
+split_huge_page uses migration entries to stabilize page->_count and
page->_mapcount.

We safe against physical memory scanners too: the only legitimate way
scanner can get reference to a page is get_page_unless_zero().

-All tail pages have zero ->_refcount until atomic_add(). This prevents the
+All tail pages have zero ->_count until atomic_add(). This prevents the
scanner from getting a reference to the tail page up to that point. After the
-atomic_add() we don't care about the ->_refcount value. We already known how
+atomic_add() we don't care about the ->_count value. We already known how
many references should be uncharged from the head page.

For head page get_page_unless_zero() will succeed and we don't mind. It's
diff --git a/arch/tile/mm/init.c b/arch/tile/mm/init.c
index adce254..a0582b7 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->_refcount.counter = 0;
+ p->_count.counter = 0;
p->_mapcount.counter = -1;
}
init_page_count(page);
diff --git a/drivers/block/aoe/aoecmd.c b/drivers/block/aoe/aoecmd.c
index d597e43..437b3a8 100644
--- a/drivers/block/aoe/aoecmd.c
+++ b/drivers/block/aoe/aoecmd.c
@@ -861,7 +861,7 @@ rqbiocnt(struct request *r)
* discussion.
*
* We cannot use get_page in the workaround, because it insists on a
- * positive page count as a precondition. So we use _refcount directly.
+ * positive page count as a precondition. So we use _count directly.
*/
static void
bio_pageinc(struct bio *bio)
diff --git a/drivers/hwtracing/intel_th/msu.c b/drivers/hwtracing/intel_th/msu.c
index e8d55a1..0974090 100644
--- a/drivers/hwtracing/intel_th/msu.c
+++ b/drivers/hwtracing/intel_th/msu.c
@@ -1172,7 +1172,7 @@ static void msc_mmap_close(struct vm_area_struct *vma)
if (!atomic_dec_and_mutex_lock(&msc->mmap_count, &msc->buf_mutex))
return;

- /* drop page _refcounts */
+ /* drop page _counts */
for (pg = 0; pg < msc->nr_pages; pg++) {
struct page *page = msc_buffer_get_page(msc, pg);

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 3ecd445..712f1b9 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -142,7 +142,7 @@ u64 stable_page_flags(struct page *page)


/*
- * Caveats on high order pages: page->_refcount will only be set
+ * Caveats on high order pages: page->_count will only be set
* -1 on the head page; SLUB/SLQB do the same for PG_slab;
* SLOB won't set PG_slab at all on compound pages.
*/
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5df5feb..574e055 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -739,7 +739,7 @@ static inline void get_page(struct page *page)
page = compound_head(page);
/*
* Getting a normal page or the head of a compound page
- * requires to already have an elevated page->_refcount.
+ * requires to already have an elevated page->_count.
*/
VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
page_ref_inc(page);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index ca3e517..dd3e765 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -74,9 +74,9 @@ struct page {
unsigned long counters;
#else
/*
- * Keep _refcount separate from slub cmpxchg_double
- * data. As the rest of the double word is protected by
- * slab_lock but _refcount is not.
+ * Keep _count separate from slub cmpxchg_double data.
+ * As the rest of the double word is protected by
+ * slab_lock but _count is not.
*/
unsigned counters;
#endif
@@ -98,11 +98,7 @@ struct page {
};
int units; /* SLOB */
};
- /*
- * Usage count, *USE WRAPPER FUNCTION*
- * when manual accounting. See page_ref.h
- */
- atomic_t _refcount;
+ atomic_t _count; /* Usage count, see below. */
};
unsigned int active; /* SLAB */
};
@@ -253,7 +249,7 @@ struct page_frag_cache {
__u32 offset;
#endif
/* we maintain a pagecount bias, so that we dont dirty cache line
- * containing page->_refcount every time we allocate a fragment.
+ * containing page->_count 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 8b5e0a9..e596d5d9 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->_refcount);
+ return atomic_read(&page->_count);
}

static inline int page_count(struct page *page)
{
- return atomic_read(&compound_head(page)->_refcount);
+ return atomic_read(&compound_head(page)->_count);
}

static inline void set_page_count(struct page *page, int v)
{
- atomic_set(&page->_refcount, v);
+ atomic_set(&page->_count, 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->_refcount);
+ atomic_add(nr, &page->_count);
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->_refcount);
+ atomic_sub(nr, &page->_count);
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->_refcount);
+ atomic_inc(&page->_count);
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->_refcount);
+ atomic_dec(&page->_count);
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->_refcount);
+ int ret = atomic_sub_and_test(nr, &page->_count);

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->_refcount);
+ int ret = atomic_dec_and_test(&page->_count);

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->_refcount);
+ int ret = atomic_dec_return(&page->_count);

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->_refcount, nr, u);
+ int ret = atomic_add_unless(&page->_count, 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->_refcount, count, 0) == count);
+ int ret = likely(atomic_cmpxchg(&page->_count, 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->_refcount, count);
+ atomic_set(&page->_count, count);
if (page_ref_tracepoint_active(__tracepoint_page_ref_unfreeze))
__page_ref_unfreeze(page, count);
}
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 9735410..ce53eff 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -90,12 +90,12 @@ void release_pages(struct page **pages, int nr, bool cold);

/*
* speculatively take a reference to a page.
- * If the page is free (_refcount == 0), then _refcount is untouched, and 0
- * is returned. Otherwise, _refcount is incremented by 1 and 1 is returned.
+ * If the page is free (_count == 0), then _count is untouched, and 0
+ * is returned. Otherwise, _count is incremented by 1 and 1 is returned.
*
* This function must be called inside the same rcu_read_lock() section as has
* been used to lookup the page in the pagecache radix-tree (or page table):
- * this allows allocators to use a synchronize_rcu() to stabilize _refcount.
+ * this allows allocators to use a synchronize_rcu() to stabilize _count.
*
* Unless an RCU grace period has passed, the count of all pages coming out
* of the allocator must be considered unstable. page_count may return higher
@@ -111,7 +111,7 @@ void release_pages(struct page **pages, int nr, bool cold);
* 2. conditionally increment refcount
* 3. check the page is still in pagecache (if no, goto 1)
*
- * Remove-side that cares about stability of _refcount (eg. reclaim) has the
+ * Remove-side that cares about stability of _count (eg. reclaim) has the
* following (with tree_lock held for write):
* A. atomically check refcount is correct and set it to 0 (atomic_cmpxchg)
* B. remove page from pagecache
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 56b3ed0..9b3744f 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1409,7 +1409,7 @@ static int __init crash_save_vmcoreinfo_init(void)
VMCOREINFO_STRUCT_SIZE(list_head);
VMCOREINFO_SIZE(nodemask_t);
VMCOREINFO_OFFSET(page, flags);
- VMCOREINFO_OFFSET(page, _refcount);
+ VMCOREINFO_OFFSET(page, _count);
VMCOREINFO_OFFSET(page, mapping);
VMCOREINFO_OFFSET(page, lru);
VMCOREINFO_OFFSET(page, _mapcount);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9ed5853..a73fa7a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3123,7 +3123,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
VM_BUG_ON_PAGE(page_ref_count(page_tail) != 0, page_tail);

/*
- * tail_page->_refcount is zero and not changing from under us. But
+ * tail_page->_count is zero and not changing from under us. But
* get_page_unless_zero() may be running from under us on the
* tail_page. If we used atomic_set() below instead of atomic_inc(), we
* would then run atomic_set() concurrently with
@@ -3350,7 +3350,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
if (mlocked)
lru_add_drain();

- /* Prevent deferred_split_scan() touching ->_refcount */
+ /* Prevent deferred_split_scan() touching ->_count */
spin_lock_irqsave(&pgdata->split_queue_lock, flags);
count = page_count(head);
mapcount = total_mapcount(head);
diff --git a/mm/internal.h b/mm/internal.h
index a37e5b6..3711310d 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -58,7 +58,7 @@ static inline unsigned long ra_submit(struct file_ra_state *ra,
}

/*
- * Turn a non-refcounted page (->_refcount == 0) into refcounted with
+ * Turn a non-refcounted page (->_count == 0) into refcounted with
* a count of one.
*/
static inline void set_page_refcounted(struct page *page)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6903b69..82481d0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -912,7 +912,7 @@ static void free_pages_check_bad(struct page *page)
if (unlikely(page->mapping != NULL))
bad_reason = "non-NULL mapping";
if (unlikely(page_ref_count(page) != 0))
- bad_reason = "nonzero _refcount";
+ bad_reason = "nonzero _count";
if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_FREE)) {
bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
bad_flags = PAGE_FLAGS_CHECK_AT_FREE;
@@ -7213,7 +7213,7 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
* We can't use page_count without pin a page
* because another CPU can free compound page.
* This check already skips compound tails of THP
- * because their page->_refcount is zero at all time.
+ * because their page->_count is zero at all time.
*/
if (!page_ref_count(page)) {
if (PageBuddy(page))
diff --git a/mm/slub.c b/mm/slub.c
index 825ff45..72d0f96 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -329,8 +329,8 @@ static inline void set_page_slub_counters(struct page *page, unsigned long count
tmp.counters = counters_new;
/*
* page->counters can cover frozen/inuse/objects as well
- * as page->_refcount. If we assign to ->counters directly
- * we run the risk of losing updates to page->_refcount, so
+ * as page->_count. If we assign to ->counters directly
+ * we run the risk of losing updates to page->_count, so
* be careful and only assign to the fields we need.
*/
page->frozen = tmp.frozen;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c4a2f45..074fe2e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -633,7 +633,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
*
* Reversing the order of the tests ensures such a situation cannot
* escape unnoticed. The smp_rmb is needed to ensure the page->flags
- * load is not satisfied before that of page->_refcount.
+ * load is not satisfied before that of page->_count.
*
* Note that if SetPageDirty is always performed via set_page_dirty,
* and thus under tree_lock, then this ordering is not required.
@@ -1716,7 +1716,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
* It is safe to rely on PG_active against the non-LRU pages in here because
* nobody will play with that bit on a non-LRU page.
*
- * The downside is that we have to touch page->_refcount against each page.
+ * The downside is that we have to touch page->_count against each page.
* But we had to alter page->flags anyway.
*/

--
2.5.5