[PATCH 2/4] modify swap_map and add SWAP_HAS_CACHE flag.

KAMEZAWA Hiroyuki
2009-05-28T05:20:44Z

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

This is a part of patches for fixing memcg's swap account leak. But, IMHO,
not a bad patch even if no memcg.

Now, reference to swap is counted by swap_map[], an array of unsigned short.
There are 2 kinds of references to swap.
- reference from swap entry
- reference from swap cache
Then,
- If there is swap cache && swap's refcnt is 1, there is only swap cache.
(*) swapcount(entry) == 1 && find_get_page(swapper_space, entry) != NULL

This counting logic have worked well for a long time. But considering that
we cannot know there is a _real_ reference or not by swap_map[], current usage
of counter is not very good.

This patch adds a flag SWAP_HAS_CACHE and recored information that a swap entry
has a cache or not. This will remove -1 magic used in swapfile.c and be a help
to avoid unnecessary find_get_page().

Changelog: v1->v2
- fixed swapcache_prepare()'s return code.
- changed swap_duplicate() be void function.
- fixed racy case in swapoff().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
include/linux/swap.h | 14 ++-
mm/swap_state.c | 5 +
mm/swapfile.c | 203 ++++++++++++++++++++++++++++++++++++---------------
3 files changed, 160 insertions(+), 62 deletions(-)

Index: new-trial-swapcount2/include/linux/swap.h
===================================================================
--- new-trial-swapcount2.orig/include/linux/swap.h
+++ new-trial-swapcount2/include/linux/swap.h
@@ -129,9 +129,10 @@ enum {

#define SWAP_CLUSTER_MAX 32

-#define SWAP_MAP_MAX 0x7fff
-#define SWAP_MAP_BAD 0x8000
-
+#define SWAP_MAP_MAX 0x7ffe
+#define SWAP_MAP_BAD 0x7fff
+#define SWAP_HAS_CACHE 0x8000 /* There is a swap cache of entry. */
+#define SWAP_COUNT_MASK (~SWAP_HAS_CACHE)
/*
* The in-memory structure used to track swap areas.
*/
@@ -300,7 +301,7 @@ extern long total_swap_pages;
extern void si_swapinfo(struct sysinfo *);
extern swp_entry_t get_swap_page(void);
extern swp_entry_t get_swap_page_of_type(int);
-extern int swap_duplicate(swp_entry_t);
+extern void swap_duplicate(swp_entry_t);
extern int swapcache_prepare(swp_entry_t);
extern int valid_swaphandles(swp_entry_t, unsigned long *);
extern void swap_free(swp_entry_t);
@@ -372,9 +373,12 @@ static inline void show_swap_cache_info(
}

#define free_swap_and_cache(swp) is_migration_entry(swp)
-#define swap_duplicate(swp) is_migration_entry(swp)
#define swapcache_prepare(swp) is_migration_entry(swp)

+static inline void swap_duplicate(swp_entry_t swp)
+{
+}
+
static inline void swap_free(swp_entry_t swp)
{
}
Index: new-trial-swapcount2/mm/swapfile.c
===================================================================
--- new-trial-swapcount2.orig/mm/swapfile.c
+++ new-trial-swapcount2/mm/swapfile.c
@@ -53,6 +53,26 @@ static struct swap_info_struct swap_info

static DEFINE_MUTEX(swapon_mutex);

+/* For reference count accounting in swap_map */
+static inline int swap_count(unsigned short ent)
+{
+ return ent & SWAP_COUNT_MASK;
+}
+
+static inline int swap_has_cache(unsigned short ent)
+{
+ return ent & SWAP_HAS_CACHE;
+}
+
+static inline unsigned short make_swap_count(int count, int has_cache)
+{
+ unsigned short ret = count;
+
+ if (has_cache)
+ return SWAP_HAS_CACHE | ret;
+ return ret;
+}
+
/*
* We need this because the bdev->unplug_fn can sleep and we cannot
* hold swap_lock while calling the unplug_fn. And swap_lock
@@ -167,7 +187,8 @@ static int wait_for_discard(void *word)
#define SWAPFILE_CLUSTER 256
#define LATENCY_LIMIT 256

-static inline unsigned long scan_swap_map(struct swap_info_struct *si)
+static inline unsigned long scan_swap_map(struct swap_info_struct *si,
+ int cache)
{
unsigned long offset;
unsigned long scan_base;
@@ -285,7 +306,10 @@ checks:
si->lowest_bit = si->max;
si->highest_bit = 0;
}
- si->swap_map[offset] = 1;
+ if (cache) /* at usual swap-out via vmscan.c */
+ si->swap_map[offset] = make_swap_count(0, 1);
+ else /* at suspend */
+ si->swap_map[offset] = make_swap_count(1, 0);
si->cluster_next = offset + 1;
si->flags -= SWP_SCANNING;

@@ -401,7 +425,8 @@ swp_entry_t get_swap_page(void)
continue;

swap_list.next = next;
- offset = scan_swap_map(si);
+ /* This is called for allocating swap entry for cache */
+ offset = scan_swap_map(si, 1);
if (offset) {
spin_unlock(&swap_lock);
return swp_entry(type, offset);
@@ -415,6 +440,7 @@ noswap:
return (swp_entry_t) {0};
}

+/* The only caller of this function is now susupend routine */
swp_entry_t get_swap_page_of_type(int type)
{
struct swap_info_struct *si;
@@ -424,7 +450,8 @@ swp_entry_t get_swap_page_of_type(int ty
si = swap_info + type;
if (si->flags & SWP_WRITEOK) {
nr_swap_pages--;
- offset = scan_swap_map(si);
+ /* This is called for allocating swap entry, not cache */
+ offset = scan_swap_map(si, 0);
if (offset) {
spin_unlock(&swap_lock);
return swp_entry(type, offset);
@@ -471,25 +498,36 @@ out:
return NULL;
}

-static int swap_entry_free(struct swap_info_struct *p, swp_entry_t ent)
+static int swap_entry_free(struct swap_info_struct *p,
+ swp_entry_t ent, int cache)
{
unsigned long offset = swp_offset(ent);
- int count = p->swap_map[offset];
+ int count = swap_count(p->swap_map[offset]);
+ int has_cache = swap_has_cache(p->swap_map[offset]);

- if (count < SWAP_MAP_MAX) {
- count--;
- p->swap_map[offset] = count;
- if (!count) {
- if (offset < p->lowest_bit)
- p->lowest_bit = offset;
- if (offset > p->highest_bit)
- p->highest_bit = offset;
- if (p->prio > swap_info[swap_list.next].prio)
- swap_list.next = p - swap_info;
- nr_swap_pages++;
- p->inuse_pages--;
- mem_cgroup_uncharge_swap(ent);
- }
+ if (!cache) { /* dropping usage count of swap */
+ if (count < SWAP_MAP_MAX) {
+ count--;
+ p->swap_map[offset] = make_swap_count(count, has_cache);
+ }
+ } else { /* dropping swap cache flag */
+ VM_BUG_ON(!has_cache);
+ p->swap_map[offset] = make_swap_count(count, 0);
+
+ }
+ /* return code. */
+ count = p->swap_map[offset];
+ /* free if no reference */
+ if (!count) {
+ if (offset < p->lowest_bit)
+ p->lowest_bit = offset;
+ if (offset > p->highest_bit)
+ p->highest_bit = offset;
+ if (p->prio > swap_info[swap_list.next].prio)
+ swap_list.next = p - swap_info;
+ nr_swap_pages++;
+ p->inuse_pages--;
+ mem_cgroup_uncharge_swap(ent);
}
return count;
}
@@ -504,7 +542,7 @@ void swap_free(swp_entry_t entry)

p = swap_info_get(entry);
if (p) {
- swap_entry_free(p, entry);
+ swap_entry_free(p, entry, 0);
spin_unlock(&swap_lock);
}
}
@@ -514,9 +552,16 @@ void swap_free(swp_entry_t entry)
*/
void swapcache_free(swp_entry_t entry, struct page *page)
{
+ struct swap_info_struct *p;
+
if (page)
mem_cgroup_uncharge_swapcache(page, entry);
- return swap_free(entry);
+ p = swap_info_get(entry);
+ if (p) {
+ swap_entry_free(p, entry, 1);
+ spin_unlock(&swap_lock);
+ }
+ return;
}

/*
@@ -531,8 +576,7 @@ static inline int page_swapcount(struct
entry.val = page_private(page);
p = swap_info_get(entry);
if (p) {
- /* Subtract the 1 for the swap cache itself */
- count = p->swap_map[swp_offset(entry)] - 1;
+ count = swap_count(p->swap_map[swp_offset(entry)]);
spin_unlock(&swap_lock);
}
return count;
@@ -594,7 +638,7 @@ int free_swap_and_cache(swp_entry_t entr

p = swap_info_get(entry);
if (p) {
- if (swap_entry_free(p, entry) == 1) {
+ if (swap_entry_free(p, entry, 0) == SWAP_HAS_CACHE) {
page = find_get_page(&swapper_space, entry.val);
if (page && !trylock_page(page)) {
page_cache_release(page);
@@ -901,7 +945,7 @@ static unsigned int find_next_to_unuse(s
i = 1;
}
count = si->swap_map[i];
- if (count && count != SWAP_MAP_BAD)
+ if (count && swap_count(count) != SWAP_MAP_BAD)
break;
}
return i;
@@ -1005,13 +1049,13 @@ static int try_to_unuse(unsigned int typ
*/
shmem = 0;
swcount = *swap_map;
- if (swcount > 1) {
+ if (swap_count(swcount)) {
if (start_mm == &init_mm)
shmem = shmem_unuse(entry, page);
else
retval = unuse_mm(start_mm, entry, page);
}
- if (*swap_map > 1) {
+ if (swap_count(*swap_map)) {
int set_start_mm = (*swap_map >= swcount);
struct list_head *p = &start_mm->mmlist;
struct mm_struct *new_start_mm = start_mm;
@@ -1021,7 +1065,7 @@ static int try_to_unuse(unsigned int typ
atomic_inc(&new_start_mm->mm_users);
atomic_inc(&prev_mm->mm_users);
spin_lock(&mmlist_lock);
- while (*swap_map > 1 && !retval && !shmem &&
+ while (swap_count(*swap_map) && !retval && !shmem &&
(p = p->next) != &start_mm->mmlist) {
mm = list_entry(p, struct mm_struct, mmlist);
if (!atomic_inc_not_zero(&mm->mm_users))
@@ -1033,14 +1077,16 @@ static int try_to_unuse(unsigned int typ
cond_resched();

swcount = *swap_map;
- if (swcount <= 1)
+ if (!swap_count(swcount)) /* any usage ? */
;
else if (mm == &init_mm) {
set_start_mm = 1;
shmem = shmem_unuse(entry, page);
} else
retval = unuse_mm(mm, entry, page);
- if (set_start_mm && *swap_map < swcount) {
+
+ if (set_start_mm &&
+ swap_count(*swap_map) < swcount) {
mmput(new_start_mm);
atomic_inc(&mm->mm_users);
new_start_mm = mm;
@@ -1067,21 +1113,25 @@ static int try_to_unuse(unsigned int typ
}

/*
- * How could swap count reach 0x7fff when the maximum
- * pid is 0x7fff, and there's no way to repeat a swap
- * page within an mm (except in shmem, where it's the
- * shared object which takes the reference count)?
- * We believe SWAP_MAP_MAX cannot occur in Linux 2.4.
- *
+ * How could swap count reach 0x7ffe ?
+ * There's no way to repeat a swap page within an mm
+ * (except in shmem, where it's the shared object which takes
+ * the reference count)?
+ * We believe SWAP_MAP_MAX cannot occur.(if occur, unsigned
+ * short is too small....)
* If that's wrong, then we should worry more about
* exit_mmap() and do_munmap() cases described above:
* we might be resetting SWAP_MAP_MAX too early here.
* We know "Undead"s can happen, they're okay, so don't
* report them; but do report if we reset SWAP_MAP_MAX.
*/
- if (*swap_map == SWAP_MAP_MAX) {
+ /* We might release the lock_page() in unuse_mm(). */
+ if (!PageSwapCache(page) || page_private(page) != entry.val)
+ goto retry;
+
+ if (swap_count(*swap_map) == SWAP_MAP_MAX) {
spin_lock(&swap_lock);
- *swap_map = 1;
+ *swap_map = make_swap_count(0, 1);
spin_unlock(&swap_lock);
reset_overflow = 1;
}
@@ -1099,7 +1149,8 @@ static int try_to_unuse(unsigned int typ
* pages would be incorrect if swap supported "shared
* private" pages, but they are handled by tmpfs files.
*/
- if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
+ if (swap_count(*swap_map) &&
+ PageDirty(page) && PageSwapCache(page)) {
struct writeback_control wbc = {
.sync_mode = WB_SYNC_NONE,
};
@@ -1126,6 +1177,7 @@ static int try_to_unuse(unsigned int typ
* mark page dirty so shrink_page_list will preserve it.
*/
SetPageDirty(page);
+retry:
unlock_page(page);
page_cache_release(page);

@@ -1952,15 +2004,22 @@ void si_swapinfo(struct sysinfo *val)
*
* Note: if swap_map[] reaches SWAP_MAP_MAX the entries are treated as
* "permanent", but will be reclaimed by the next swapoff.
+ * Returns error code in following case.
+ * - success -> 0
+ * - swp_entry is invalid -> EINVAL
+ * - swp_entry is migration entry -> EINVAL
+ * - swap-cache reference is requested but there is already one. -> EEXIST
+ * - swap-cache reference is requested but the entry is not used. -> ENOENT
*/
-int swap_duplicate(swp_entry_t entry)
+static int __swap_duplicate(swp_entry_t entry, int cache)
{
struct swap_info_struct * p;
unsigned long offset, type;
- int result = 0;
+ int result = -EINVAL;
+ int count, has_cache;

if (is_migration_entry(entry))
- return 1;
+ return -EINVAL;

type = swp_type(entry);
if (type >= nr_swapfiles)
@@ -1969,17 +2028,37 @@ int swap_duplicate(swp_entry_t entry)
offset = swp_offset(entry);

spin_lock(&swap_lock);
- if (offset < p->max && p->swap_map[offset]) {
- if (p->swap_map[offset] < SWAP_MAP_MAX - 1) {
- p->swap_map[offset]++;
- result = 1;
- } else if (p->swap_map[offset] <= SWAP_MAP_MAX) {
+
+ if (unlikely(offset >= p->max))
+ goto unlock_out;
+
+ count = swap_count(p->swap_map[offset]);
+ has_cache = swap_has_cache(p->swap_map[offset]);
+ if (cache) { /* called for swapcache/swapin-readahead */
+ /* set SWAP_HAS_CACHE if there is no cache and entry is used */
+ if (!has_cache && count) {
+ p->swap_map[offset] = make_swap_count(count, 1);
+ result = 0;
+ } else if (has_cache)
+ result = -EEXIST;
+ else if (!count)
+ result = -ENOENT;
+ } else if (count || has_cache) {
+ if (count < SWAP_MAP_MAX - 1) {
+ p->swap_map[offset] = make_swap_count(count + 1,
+ has_cache);
+ result = 0;
+ } else if (count <= SWAP_MAP_MAX) {
if (swap_overflow++ < 5)
- printk(KERN_WARNING "swap_dup: swap entry overflow\n");
- p->swap_map[offset] = SWAP_MAP_MAX;
- result = 1;
- }
- }
+ printk(KERN_WARNING
+ "swap_dup: swap entry overflow\n");
+ p->swap_map[offset] = make_swap_count(SWAP_MAP_MAX,
+ has_cache);
+ result = 0;
+ }
+ } else
+ result = -ENOENT; /* unused swap entry */
+unlock_out:
spin_unlock(&swap_lock);
out:
return result;
@@ -1988,13 +2067,25 @@ bad_file:
printk(KERN_ERR "swap_dup: %s%08lx\n", Bad_file, entry.val);
goto out;
}
+/*
+ * increase reference count of swap entry by 1.
+ */
+void swap_duplicate(swp_entry_t entry)
+{
+ __swap_duplicate(entry, 0);
+}

/*
+ * @entry: swap entry for which we allocate swap cache.
+ *
* Called when allocating swap cache for exising swap entry,
+ * This can return error codes. Returns 0 at success.
+ * -EBUSY means there is a swap cache.
+ * Note: return code is different from swap_duplicate().
*/
int swapcache_prepare(swp_entry_t entry)
{
- return swap_duplicate(entry);
+ return __swap_duplicate(entry, 1);
}

@@ -2035,7 +2126,7 @@ int valid_swaphandles(swp_entry_t entry,
/* Don't read in free or bad pages */
if (!si->swap_map[toff])
break;
- if (si->swap_map[toff] == SWAP_MAP_BAD)
+ if (swap_count(si->swap_map[toff]) == SWAP_MAP_BAD)
break;
}
/* Count contiguous allocated slots below our target */
@@ -2043,7 +2134,7 @@ int valid_swaphandles(swp_entry_t entry,
/* Don't read in free or bad pages */
if (!si->swap_map[toff])
break;
- if (si->swap_map[toff] == SWAP_MAP_BAD)
+ if (swap_count(si->swap_map[toff]) == SWAP_MAP_BAD)
break;
}
spin_unlock(&swap_lock);
Index: new-trial-swapcount2/mm/swap_state.c
===================================================================
--- new-trial-swapcount2.orig/mm/swap_state.c
+++ new-trial-swapcount2/mm/swap_state.c
@@ -292,7 +292,10 @@ struct page *read_swap_cache_async(swp_e
/*
* Swap entry may have been freed since our caller observed it.
*/
- if (!swapcache_prepare(entry))
+ err = swapcache_prepare(entry);
+ if (err == -EEXIST) /* seems racy */
+ continue;
+ if (err) /* swp entry is obsolete ? */
break;

/*