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Subject[PATCH] rmaplock 2/6 kill page_map_lock
The pte_chains rmap used pte_chain_lock (bit_spin_lock on PG_chainlock)
to lock its pte_chains. We kept this (as page_map_lock: bit_spin_lock
on PG_maplock) when we moved to objrmap. But the file objrmap locks its
vma tree with mapping->i_mmap_lock, and the anon objrmap locks its vma
list with anon_vma->lock: so isn't the page_map_lock superfluous?

Pretty much, yes. The mapcount was protected by it, and needs to become
an atomic: starting at -1 like page _count, so nr_mapped can be tracked
precisely up and down. The last page_remove_rmap can't clear anon page
mapping any more, because of races with page_add_rmap; from which some
BUG_ONs must go for the same reason, but they've served their purpose.

vmscan decisions are naturally racy, little change there beyond removing
page_map_lock/unlock. But to stabilize the file-backed page->mapping
against truncation while acquiring i_mmap_lock, page_referenced_file now
needs page lock to be held even for refill_inactive_zone. There's a
similar issue in acquiring anon_vma->lock, where page lock doesn't help:
which this patch pretends to handle, but actually it needs the next.

Roughly 10% cut off lmbench fork numbers on my 2*HT*P4. Must confess
my testing failed to show the races even while they were knowingly
exposed: would benefit from testing on racier equipment.

Signed-off-by: Hugh Dickins <hugh@veritas.com>

include/linux/mm.h | 22 ++++++-
include/linux/page-flags.h | 3 -
include/linux/rmap.h | 13 +---
mm/page_alloc.c | 10 +--
mm/rmap.c | 135 +++++++++++++++++++++++++--------------------
mm/vmscan.c | 37 ++----------
6 files changed, 110 insertions(+), 110 deletions(-)
--- rmaplock1/include/linux/mm.h 2004-07-12 18:20:07.949982720 +0100
+++ rmaplock2/include/linux/mm.h 2004-07-12 18:20:22.331796352 +0100
@@ -195,10 +195,9 @@ struct page {
page_flags_t flags; /* Atomic flags, some possibly
* updated asynchronously */
atomic_t _count; /* Usage count, see below. */
- unsigned int mapcount; /* Count of ptes mapped in mms,
+ atomic_t _mapcount; /* Count of ptes mapped in mms,
* to show when page is mapped
- * & limit reverse map searches,
- * protected by PG_maplock.
+ * & limit reverse map searches.
*/
unsigned long private; /* Mapping-private opaque data:
* usually used for buffer_heads
@@ -472,11 +471,26 @@ static inline pgoff_t page_index(struct
}

/*
+ * The atomic page->_mapcount, like _count, starts from -1:
+ * so that transitions both from it and to it can be tracked,
+ * using atomic_inc_and_test and atomic_add_negative(-1).
+ */
+static inline void reset_page_mapcount(struct page *page)
+{
+ atomic_set(&(page)->_mapcount, -1);
+}
+
+static inline int page_mapcount(struct page *page)
+{
+ return atomic_read(&(page)->_mapcount) + 1;
+}
+
+/*
* Return true if this page is mapped into pagetables.
*/
static inline int page_mapped(struct page *page)
{
- return page->mapcount != 0;
+ return atomic_read(&(page)->_mapcount) >= 0;
}

/*
--- rmaplock1/include/linux/page-flags.h 2004-07-12 18:20:07.950982568 +0100
+++ rmaplock2/include/linux/page-flags.h 2004-07-12 18:20:22.332796200 +0100
@@ -69,12 +69,11 @@
#define PG_private 12 /* Has something at ->private */
#define PG_writeback 13 /* Page is under writeback */
#define PG_nosave 14 /* Used for system suspend/resume */
-#define PG_maplock 15 /* Lock bit for rmap to ptes */
+#define PG_compound 15 /* Part of a compound page */

#define PG_swapcache 16 /* Swap page: swp_entry_t in private */
#define PG_mappedtodisk 17 /* Has blocks allocated on-disk */
#define PG_reclaim 18 /* To be reclaimed asap */
-#define PG_compound 19 /* Part of a compound page */


/*
--- rmaplock1/include/linux/rmap.h 2004-06-16 06:20:36.000000000 +0100
+++ rmaplock2/include/linux/rmap.h 2004-07-12 18:20:22.333796048 +0100
@@ -9,11 +9,6 @@
#include <linux/slab.h>
#include <linux/spinlock.h>

-#define page_map_lock(page) \
- bit_spin_lock(PG_maplock, (unsigned long *)&(page)->flags)
-#define page_map_unlock(page) \
- bit_spin_unlock(PG_maplock, (unsigned long *)&(page)->flags)
-
/*
* The anon_vma heads a list of private "related" vmas, to scan if
* an anonymous page pointing to this anon_vma needs to be unmapped:
@@ -87,15 +82,13 @@ void page_remove_rmap(struct page *);
*/
static inline void page_dup_rmap(struct page *page)
{
- page_map_lock(page);
- page->mapcount++;
- page_map_unlock(page);
+ atomic_inc(&page->_mapcount);
}

/*
* Called from mm/vmscan.c to handle paging out
*/
-int page_referenced(struct page *);
+int page_referenced(struct page *, int is_locked);
int try_to_unmap(struct page *);

#else /* !CONFIG_MMU */
@@ -104,7 +97,7 @@ int try_to_unmap(struct page *);
#define anon_vma_prepare(vma) (0)
#define anon_vma_link(vma) do {} while (0)

-#define page_referenced(page) TestClearPageReferenced(page)
+#define page_referenced(page,l) TestClearPageReferenced(page)
#define try_to_unmap(page) SWAP_FAIL

#endif /* CONFIG_MMU */
--- rmaplock1/mm/page_alloc.c 2004-07-12 18:20:07.952982264 +0100
+++ rmaplock2/mm/page_alloc.c 2004-07-12 18:20:22.335795744 +0100
@@ -78,7 +78,7 @@ static void bad_page(const char *functio
function, current->comm, page);
printk(KERN_EMERG "flags:0x%08lx mapping:%p mapcount:%d count:%d\n",
(unsigned long)page->flags, page->mapping,
- (int)page->mapcount, page_count(page));
+ page_mapcount(page), page_count(page));
printk(KERN_EMERG "Backtrace:\n");
dump_stack();
printk(KERN_EMERG "Trying to fix it up, but a reboot is needed\n");
@@ -87,12 +87,11 @@ static void bad_page(const char *functio
1 << PG_lru |
1 << PG_active |
1 << PG_dirty |
- 1 << PG_maplock |
1 << PG_swapcache |
1 << PG_writeback);
set_page_count(page, 0);
+ reset_page_mapcount(page);
page->mapping = NULL;
- page->mapcount = 0;
}

#ifndef CONFIG_HUGETLB_PAGE
@@ -228,7 +227,6 @@ static inline void free_pages_check(cons
1 << PG_active |
1 << PG_reclaim |
1 << PG_slab |
- 1 << PG_maplock |
1 << PG_swapcache |
1 << PG_writeback )))
bad_page(function, page);
@@ -350,7 +348,6 @@ static void prep_new_page(struct page *p
1 << PG_active |
1 << PG_dirty |
1 << PG_reclaim |
- 1 << PG_maplock |
1 << PG_swapcache |
1 << PG_writeback )))
bad_page(__FUNCTION__, page);
@@ -512,6 +509,8 @@ static void fastcall free_hot_cold_page(

kernel_map_pages(page, 1, 0);
inc_page_state(pgfree);
+ if (PageAnon(page))
+ page->mapping = NULL;
free_pages_check(__FUNCTION__, page);
pcp = &zone->pageset[get_cpu()].pcp[cold];
local_irq_save(flags);
@@ -1399,6 +1398,7 @@ void __init memmap_init_zone(struct page
for (page = start; page < (start + size); page++) {
set_page_zone(page, NODEZONE(nid, zone));
set_page_count(page, 0);
+ reset_page_mapcount(page);
SetPageReserved(page);
INIT_LIST_HEAD(&page->lru);
#ifdef WANT_PAGE_VIRTUAL
--- rmaplock1/mm/rmap.c 2004-07-12 18:20:07.954981960 +0100
+++ rmaplock2/mm/rmap.c 2004-07-12 18:20:22.338795288 +0100
@@ -163,13 +163,6 @@ void __init anon_vma_init(void)
sizeof(struct anon_vma), 0, SLAB_PANIC, anon_vma_ctor, NULL);
}

-/* this needs the page->flags PG_maplock held */
-static inline void clear_page_anon(struct page *page)
-{
- BUG_ON(!page->mapping);
- page->mapping = NULL;
-}
-
/*
* At what user virtual address is page expected in vma?
*/
@@ -239,15 +232,22 @@ out:
return referenced;
}

-static inline int page_referenced_anon(struct page *page)
+static int page_referenced_anon(struct page *page)
{
- unsigned int mapcount = page->mapcount;
+ unsigned int mapcount;
struct anon_vma *anon_vma = (void *) page->mapping - PAGE_MAPPING_ANON;
struct vm_area_struct *vma;
int referenced = 0;

+ /*
+ * Recheck mapcount: it is not safe to take anon_vma->lock after
+ * last page_remove_rmap, since struct anon_vma might be reused.
+ */
+ mapcount = page_mapcount(page);
+ if (!mapcount)
+ return referenced;
+
spin_lock(&anon_vma->lock);
- BUG_ON(list_empty(&anon_vma->head));
list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
referenced += page_referenced_one(page, vma, &mapcount);
if (!mapcount)
@@ -271,18 +271,39 @@ static inline int page_referenced_anon(s
* The spinlock address_space->i_mmap_lock is tried. If it can't be gotten,
* assume a reference count of 0, so try_to_unmap will then have a go.
*/
-static inline int page_referenced_file(struct page *page)
+static int page_referenced_file(struct page *page)
{
- unsigned int mapcount = page->mapcount;
+ unsigned int mapcount;
struct address_space *mapping = page->mapping;
pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
struct vm_area_struct *vma = NULL;
struct prio_tree_iter iter;
int referenced = 0;

+ /*
+ * The caller's checks on page->mapping and !PageAnon have made
+ * sure that this is a file page: the check for page->mapping
+ * excludes the case just before it gets set on an anon page.
+ */
+ BUG_ON(PageAnon(page));
+
+ /*
+ * The page lock not only makes sure that page->mapping cannot
+ * suddenly be NULLified by truncation, it makes sure that the
+ * structure at mapping cannot be freed and reused yet,
+ * so we can safely take mapping->i_mmap_lock.
+ */
+ BUG_ON(!PageLocked(page));
+
if (!spin_trylock(&mapping->i_mmap_lock))
return 0;

+ /*
+ * i_mmap_lock does not stabilize mapcount at all, but mapcount
+ * is more likely to be accurate if we note it after spinning.
+ */
+ mapcount = page_mapcount(page);
+
while ((vma = vma_prio_tree_next(vma, &mapping->i_mmap,
&iter, pgoff, pgoff)) != NULL) {
if ((vma->vm_flags & (VM_LOCKED|VM_MAYSHARE))
@@ -302,12 +323,12 @@ static inline int page_referenced_file(s
/**
* page_referenced - test if the page was referenced
* @page: the page to test
+ * @is_locked: caller holds lock on the page
*
* Quick test_and_clear_referenced for all mappings to a page,
* returns the number of ptes which referenced the page.
- * Caller needs to hold the rmap lock.
*/
-int page_referenced(struct page *page)
+int page_referenced(struct page *page, int is_locked)
{
int referenced = 0;

@@ -317,11 +338,17 @@ int page_referenced(struct page *page)
if (TestClearPageReferenced(page))
referenced++;

- if (page->mapcount && page->mapping) {
+ if (page_mapped(page) && page->mapping) {
if (PageAnon(page))
referenced += page_referenced_anon(page);
- else
+ else if (is_locked)
+ referenced += page_referenced_file(page);
+ else if (TestSetPageLocked(page))
+ referenced++;
+ else if (page->mapping) {
referenced += page_referenced_file(page);
+ unlock_page(page);
+ }
}
return referenced;
}
@@ -348,18 +375,12 @@ void page_add_anon_rmap(struct page *pag
index += vma->vm_pgoff;
index >>= PAGE_CACHE_SHIFT - PAGE_SHIFT;

- page_map_lock(page);
- if (!page->mapcount) {
- BUG_ON(page->mapping);
+ if (atomic_inc_and_test(&page->_mapcount)) {
page->index = index;
page->mapping = (struct address_space *) anon_vma;
inc_page_state(nr_mapped);
- } else {
- BUG_ON(page->index != index);
- BUG_ON(page->mapping != (struct address_space *) anon_vma);
}
- page->mapcount++;
- page_map_unlock(page);
+ /* else checking page index and mapping is racy */
}

/**
@@ -374,11 +395,8 @@ void page_add_file_rmap(struct page *pag
if (!pfn_valid(page_to_pfn(page)) || PageReserved(page))
return;

- page_map_lock(page);
- if (!page->mapcount)
+ if (atomic_inc_and_test(&page->_mapcount))
inc_page_state(nr_mapped);
- page->mapcount++;
- page_map_unlock(page);
}

/**
@@ -390,18 +408,20 @@ void page_add_file_rmap(struct page *pag
void page_remove_rmap(struct page *page)
{
BUG_ON(PageReserved(page));
- BUG_ON(!page->mapcount);

- page_map_lock(page);
- page->mapcount--;
- if (!page->mapcount) {
+ if (atomic_add_negative(-1, &page->_mapcount)) {
+ BUG_ON(page_mapcount(page) < 0);
+ /*
+ * It would be tidy to reset the PageAnon mapping here,
+ * but that might overwrite a racing page_add_anon_rmap
+ * which increments mapcount after us but sets mapping
+ * before us: so leave the reset to free_hot_cold_page,
+ * and remember that it's only reliable while mapped.
+ */
if (page_test_and_clear_dirty(page))
set_page_dirty(page);
- if (PageAnon(page))
- clear_page_anon(page);
dec_page_state(nr_mapped);
}
- page_map_unlock(page);
}

/*
@@ -469,7 +489,7 @@ static int try_to_unmap_one(struct page
* the original page for.
*/
if (PageSwapCache(page) &&
- page_count(page) != page->mapcount + 2) {
+ page_count(page) != page_mapcount(page) + 2) {
ret = SWAP_FAIL;
goto out_unmap;
}
@@ -495,8 +515,7 @@ static int try_to_unmap_one(struct page
}

mm->rss--;
- BUG_ON(!page->mapcount);
- page->mapcount--;
+ page_remove_rmap(page);
page_cache_release(page);

out_unmap:
@@ -607,17 +626,23 @@ out_unlock:
return SWAP_AGAIN;
}

-static inline int try_to_unmap_anon(struct page *page)
+static int try_to_unmap_anon(struct page *page)
{
struct anon_vma *anon_vma = (void *) page->mapping - PAGE_MAPPING_ANON;
struct vm_area_struct *vma;
int ret = SWAP_AGAIN;

+ /*
+ * Recheck mapped: it is not safe to take anon_vma->lock after
+ * last page_remove_rmap, since struct anon_vma might be reused.
+ */
+ if (!page_mapped(page))
+ return ret;
+
spin_lock(&anon_vma->lock);
- BUG_ON(list_empty(&anon_vma->head));
list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
ret = try_to_unmap_one(page, vma);
- if (ret == SWAP_FAIL || !page->mapcount)
+ if (ret == SWAP_FAIL || !page_mapped(page))
break;
}
spin_unlock(&anon_vma->lock);
@@ -636,7 +661,7 @@ static inline int try_to_unmap_anon(stru
* The spinlock address_space->i_mmap_lock is tried. If it can't be gotten,
* return a temporary error.
*/
-static inline int try_to_unmap_file(struct page *page)
+static int try_to_unmap_file(struct page *page)
{
struct address_space *mapping = page->mapping;
pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
@@ -654,7 +679,7 @@ static inline int try_to_unmap_file(stru
while ((vma = vma_prio_tree_next(vma, &mapping->i_mmap,
&iter, pgoff, pgoff)) != NULL) {
ret = try_to_unmap_one(page, vma);
- if (ret == SWAP_FAIL || !page->mapcount)
+ if (ret == SWAP_FAIL || !page_mapped(page))
goto out;
}

@@ -683,8 +708,9 @@ static inline int try_to_unmap_file(stru
* The mapcount of the page we came in with is irrelevant,
* but even so use it as a guide to how hard we should try?
*/
- mapcount = page->mapcount;
- page_map_unlock(page);
+ mapcount = page_mapcount(page);
+ if (!mapcount)
+ goto out;
cond_resched_lock(&mapping->i_mmap_lock);

max_nl_size = (max_nl_size + CLUSTER_SIZE - 1) & CLUSTER_MASK;
@@ -707,7 +733,7 @@ static inline int try_to_unmap_file(stru
cursor += CLUSTER_SIZE;
vma->vm_private_data = (void *) cursor;
if ((int)mapcount <= 0)
- goto relock;
+ goto out;
}
if (ret != SWAP_FAIL)
vma->vm_private_data =
@@ -728,8 +754,6 @@ static inline int try_to_unmap_file(stru
if (!(vma->vm_flags & VM_RESERVED))
vma->vm_private_data = NULL;
}
-relock:
- page_map_lock(page);
out:
spin_unlock(&mapping->i_mmap_lock);
return ret;
@@ -740,11 +764,11 @@ out:
* @page: the page to get unmapped
*
* Tries to remove all the page table entries which are mapping this
- * page, used in the pageout path. Caller must hold the page lock
- * and its rmap lock. Return values are:
+ * page, used in the pageout path. Caller must hold the page lock.
+ * Return values are:
*
* SWAP_SUCCESS - we succeeded in removing all mappings
- * SWAP_AGAIN - we missed a trylock, try again later
+ * SWAP_AGAIN - we missed a mapping, try again later
* SWAP_FAIL - the page is unswappable
*/
int try_to_unmap(struct page *page)
@@ -753,20 +777,13 @@ int try_to_unmap(struct page *page)

BUG_ON(PageReserved(page));
BUG_ON(!PageLocked(page));
- BUG_ON(!page->mapcount);

if (PageAnon(page))
ret = try_to_unmap_anon(page);
else
ret = try_to_unmap_file(page);

- if (!page->mapcount) {
- if (page_test_and_clear_dirty(page))
- set_page_dirty(page);
- if (PageAnon(page))
- clear_page_anon(page);
- dec_page_state(nr_mapped);
+ if (!page_mapped(page))
ret = SWAP_SUCCESS;
- }
return ret;
}
--- rmaplock1/mm/vmscan.c 2004-07-09 10:53:47.000000000 +0100
+++ rmaplock2/mm/vmscan.c 2004-07-12 18:20:22.340794984 +0100
@@ -209,7 +209,7 @@ static int shrink_slab(unsigned long sca
return 0;
}

-/* Must be called with page's rmap lock held. */
+/* Called without lock on whether page is mapped, so answer is unstable */
static inline int page_mapping_inuse(struct page *page)
{
struct address_space *mapping;
@@ -367,26 +367,19 @@ static int shrink_list(struct list_head
if (page_mapped(page) || PageSwapCache(page))
sc->nr_scanned++;

- page_map_lock(page);
- referenced = page_referenced(page);
- if (referenced && page_mapping_inuse(page)) {
- /* In active use or really unfreeable. Activate it. */
- page_map_unlock(page);
+ referenced = page_referenced(page, 1);
+ /* In active use or really unfreeable? Activate it. */
+ if (referenced && page_mapping_inuse(page))
goto activate_locked;
- }

#ifdef CONFIG_SWAP
/*
* Anonymous process memory has backing store?
* Try to allocate it some swap space here.
- *
- * XXX: implement swap clustering ?
*/
if (PageAnon(page) && !PageSwapCache(page)) {
- page_map_unlock(page);
if (!add_to_swap(page))
goto activate_locked;
- page_map_lock(page);
}
#endif /* CONFIG_SWAP */

@@ -401,16 +394,13 @@ static int shrink_list(struct list_head
if (page_mapped(page) && mapping) {
switch (try_to_unmap(page)) {
case SWAP_FAIL:
- page_map_unlock(page);
goto activate_locked;
case SWAP_AGAIN:
- page_map_unlock(page);
goto keep_locked;
case SWAP_SUCCESS:
; /* try to free the page below */
}
}
- page_map_unlock(page);

if (PageDirty(page)) {
if (referenced)
@@ -713,25 +703,12 @@ refill_inactive_zone(struct zone *zone,
page = lru_to_page(&l_hold);
list_del(&page->lru);
if (page_mapped(page)) {
- if (!reclaim_mapped) {
- list_add(&page->lru, &l_active);
- continue;
- }
- page_map_lock(page);
- if (page_referenced(page)) {
- page_map_unlock(page);
+ if (!reclaim_mapped ||
+ (total_swap_pages == 0 && PageAnon(page)) ||
+ page_referenced(page, 0)) {
list_add(&page->lru, &l_active);
continue;
}
- page_map_unlock(page);
- }
- /*
- * FIXME: need to consider page_count(page) here if/when we
- * reap orphaned pages via the LRU (Daniel's locking stuff)
- */
- if (total_swap_pages == 0 && PageAnon(page)) {
- list_add(&page->lru, &l_active);
- continue;
}
list_add(&page->lru, &l_inactive);
}
-
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