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Subject[PATCH 8/9] swap_info: note SWAP_MAP_SHMEM
While we're fiddling with the swap_map values, let's assign a particular
value to shmem/tmpfs swap pages: their swap counts are never incremented,
and it helps swapoff's try_to_unuse() a little if it can immediately
distinguish those pages from process pages.

Since we've no use for SWAP_MAP_BAD | COUNT_CONTINUED,
we might as well use that 0xbf value for SWAP_MAP_SHMEM.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---

include/linux/swap.h | 6 +++++
mm/shmem.c | 11 +++++++--
mm/swapfile.c | 47 +++++++++++++++++++++++------------------
3 files changed, 42 insertions(+), 22 deletions(-)

--- si7/include/linux/swap.h 2009-10-14 21:26:57.000000000 +0100
+++ si8/include/linux/swap.h 2009-10-14 21:27:07.000000000 +0100
@@ -157,6 +157,7 @@ enum {
#define SWAP_HAS_CACHE 0x40 /* Flag page is cached, in first swap_map */
#define SWAP_CONT_MAX 0x7f /* Max count, in each swap_map continuation */
#define COUNT_CONTINUED 0x80 /* See swap_map continuation for full count */
+#define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs, in first swap_map */

/*
* The in-memory structure used to track swap areas.
@@ -315,6 +316,7 @@ extern swp_entry_t get_swap_page(void);
extern swp_entry_t get_swap_page_of_type(int);
extern int valid_swaphandles(swp_entry_t, unsigned long *);
extern int add_swap_count_continuation(swp_entry_t, gfp_t);
+extern void swap_shmem_alloc(swp_entry_t);
extern int swap_duplicate(swp_entry_t);
extern int swapcache_prepare(swp_entry_t);
extern void swap_free(swp_entry_t);
@@ -393,6 +395,10 @@ static inline int add_swap_count_continu
return 0;
}

+static inline void swap_shmem_alloc(swp_entry_t swp)
+{
+}
+
static inline int swap_duplicate(swp_entry_t swp)
{
return 0;
--- si7/mm/shmem.c 2009-09-28 00:28:41.000000000 +0100
+++ si8/mm/shmem.c 2009-10-14 21:27:07.000000000 +0100
@@ -1017,7 +1017,14 @@ int shmem_unuse(swp_entry_t entry, struc
goto out;
}
mutex_unlock(&shmem_swaplist_mutex);
-out: return found; /* 0 or 1 or -ENOMEM */
+ /*
+ * Can some race bring us here? We've been holding page lock,
+ * so I think not; but would rather try again later than BUG()
+ */
+ unlock_page(page);
+ page_cache_release(page);
+out:
+ return (found < 0) ? found : 0;
}

/*
@@ -1080,7 +1087,7 @@ static int shmem_writepage(struct page *
else
inode = NULL;
spin_unlock(&info->lock);
- swap_duplicate(swap);
+ swap_shmem_alloc(swap);
BUG_ON(page_mapped(page));
page_cache_release(page); /* pagecache ref */
swap_writepage(page, wbc);
--- si7/mm/swapfile.c 2009-10-14 21:26:57.000000000 +0100
+++ si8/mm/swapfile.c 2009-10-14 21:27:07.000000000 +0100
@@ -548,6 +548,12 @@ static unsigned char swap_entry_free(str
if (usage == SWAP_HAS_CACHE) {
VM_BUG_ON(!has_cache);
has_cache = 0;
+ } else if (count == SWAP_MAP_SHMEM) {
+ /*
+ * Or we could insist on shmem.c using a special
+ * swap_shmem_free() and free_shmem_swap_and_cache()...
+ */
+ count = 0;
} else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
if (count == COUNT_CONTINUED) {
if (swap_count_continued(p, offset, count))
@@ -1031,7 +1037,6 @@ static int try_to_unuse(unsigned int typ
swp_entry_t entry;
unsigned int i = 0;
int retval = 0;
- int shmem;

/*
* When searching mms for an entry, a good strategy is to
@@ -1107,17 +1112,18 @@ static int try_to_unuse(unsigned int typ

/*
* Remove all references to entry.
- * Whenever we reach init_mm, there's no address space
- * to search, but use it as a reminder to search shmem.
*/
- shmem = 0;
swcount = *swap_map;
- if (swap_count(swcount)) {
- if (start_mm == &init_mm)
- shmem = shmem_unuse(entry, page);
- else
- retval = unuse_mm(start_mm, entry, page);
+ if (swap_count(swcount) == SWAP_MAP_SHMEM) {
+ retval = shmem_unuse(entry, page);
+ /* page has already been unlocked and released */
+ if (retval < 0)
+ break;
+ continue;
}
+ if (swap_count(swcount) && start_mm != &init_mm)
+ retval = unuse_mm(start_mm, entry, page);
+
if (swap_count(*swap_map)) {
int set_start_mm = (*swap_map >= swcount);
struct list_head *p = &start_mm->mmlist;
@@ -1128,7 +1134,7 @@ 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_count(*swap_map) && !retval && !shmem &&
+ while (swap_count(*swap_map) && !retval &&
(p = p->next) != &start_mm->mmlist) {
mm = list_entry(p, struct mm_struct, mmlist);
if (!atomic_inc_not_zero(&mm->mm_users))
@@ -1142,10 +1148,9 @@ static int try_to_unuse(unsigned int typ
swcount = *swap_map;
if (!swap_count(swcount)) /* any usage ? */
;
- else if (mm == &init_mm) {
+ else if (mm == &init_mm)
set_start_mm = 1;
- shmem = shmem_unuse(entry, page);
- } else
+ else
retval = unuse_mm(mm, entry, page);

if (set_start_mm &&
@@ -1162,13 +1167,6 @@ static int try_to_unuse(unsigned int typ
mmput(start_mm);
start_mm = new_start_mm;
}
- if (shmem) {
- /* page has already been unlocked and released */
- if (shmem > 0)
- continue;
- retval = shmem;
- break;
- }
if (retval) {
unlock_page(page);
page_cache_release(page);
@@ -2128,6 +2126,15 @@ bad_file:
}

/*
+ * Help swapoff by noting that swap entry belongs to shmem/tmpfs
+ * (in which case its reference count is never incremented).
+ */
+void swap_shmem_alloc(swp_entry_t entry)
+{
+ __swap_duplicate(entry, SWAP_MAP_SHMEM);
+}
+
+/*
* increase reference count of swap entry by 1.
*/
int swap_duplicate(swp_entry_t entry)

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