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ksm: singly-linked rmap_list
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Free up a pointer in struct rmap_item, by making the mm_slot's rmap_list a
singly-linked list: we always traverse that list sequentially, and we
don't even lose any prefetches (but should consider adding a few later).
Name it rmap_list throughout.

Do we need to free up that pointer?  Not immediately, and in the end, we
could continue to avoid it with a union; but having done the conversion,
let's keep it this way, since there's no downside, and maybe we'll want
more in future (struct rmap_item is a cache-friendly 32 bytes on 32-bit
and 64 bytes on 64-bit, so we shall want to avoid expanding it).

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Hugh Dickins authored and Linus Torvalds committed Dec 15, 2009
1 parent 8dd3557 commit 6514d51
Showing 1 changed file with 26 additions and 30 deletions.
56 changes: 26 additions & 30 deletions mm/ksm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -79,35 +79,36 @@
* struct mm_slot - ksm information per mm that is being scanned
* @link: link to the mm_slots hash list
* @mm_list: link into the mm_slots list, rooted in ksm_mm_head
* @rmap_list: head for this mm_slot's list of rmap_items
* @rmap_list: head for this mm_slot's singly-linked list of rmap_items
* @mm: the mm that this information is valid for
*/
struct mm_slot {
struct hlist_node link;
struct list_head mm_list;
struct list_head rmap_list;
struct rmap_item *rmap_list;
struct mm_struct *mm;
};

/**
* struct ksm_scan - cursor for scanning
* @mm_slot: the current mm_slot we are scanning
* @address: the next address inside that to be scanned
* @rmap_item: the current rmap that we are scanning inside the rmap_list
* @rmap_list: link to the next rmap to be scanned in the rmap_list
* @seqnr: count of completed full scans (needed when removing unstable node)
*
* There is only the one ksm_scan instance of this cursor structure.
*/
struct ksm_scan {
struct mm_slot *mm_slot;
unsigned long address;
struct rmap_item *rmap_item;
struct rmap_item **rmap_list;
unsigned long seqnr;
};

/**
* struct rmap_item - reverse mapping item for virtual addresses
* @link: link into mm_slot's rmap_list (rmap_list is per mm)
* @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
* @filler: unused space we're making available in this patch
* @mm: the memory structure this rmap_item is pointing into
* @address: the virtual address this rmap_item tracks (+ flags in low bits)
* @oldchecksum: previous checksum of the page at that virtual address
Expand All @@ -116,7 +117,8 @@ struct ksm_scan {
* @prev: previous rmap_item hanging off the same node of the stable tree
*/
struct rmap_item {
struct list_head link;
struct rmap_item *rmap_list;
unsigned long filler;
struct mm_struct *mm;
unsigned long address; /* + low bits used for flags below */
union {
Expand Down Expand Up @@ -275,7 +277,6 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
% MM_SLOTS_HASH_HEADS];
mm_slot->mm = mm;
INIT_LIST_HEAD(&mm_slot->rmap_list);
hlist_add_head(&mm_slot->link, bucket);
}

Expand Down Expand Up @@ -479,15 +480,12 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
}

static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
struct list_head *cur)
struct rmap_item **rmap_list)
{
struct rmap_item *rmap_item;

while (cur != &mm_slot->rmap_list) {
rmap_item = list_entry(cur, struct rmap_item, link);
cur = cur->next;
while (*rmap_list) {
struct rmap_item *rmap_item = *rmap_list;
*rmap_list = rmap_item->rmap_list;
remove_rmap_item_from_tree(rmap_item);
list_del(&rmap_item->link);
free_rmap_item(rmap_item);
}
}
Expand Down Expand Up @@ -553,7 +551,7 @@ static int unmerge_and_remove_all_rmap_items(void)
goto error;
}

remove_trailing_rmap_items(mm_slot, mm_slot->rmap_list.next);
remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list);

spin_lock(&ksm_mmlist_lock);
ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
Expand Down Expand Up @@ -1141,20 +1139,19 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
}

static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
struct list_head *cur,
struct rmap_item **rmap_list,
unsigned long addr)
{
struct rmap_item *rmap_item;

while (cur != &mm_slot->rmap_list) {
rmap_item = list_entry(cur, struct rmap_item, link);
while (*rmap_list) {
rmap_item = *rmap_list;
if ((rmap_item->address & PAGE_MASK) == addr)
return rmap_item;
if (rmap_item->address > addr)
break;
cur = cur->next;
*rmap_list = rmap_item->rmap_list;
remove_rmap_item_from_tree(rmap_item);
list_del(&rmap_item->link);
free_rmap_item(rmap_item);
}

Expand All @@ -1163,7 +1160,8 @@ static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
/* It has already been zeroed */
rmap_item->mm = mm_slot->mm;
rmap_item->address = addr;
list_add_tail(&rmap_item->link, cur);
rmap_item->rmap_list = *rmap_list;
*rmap_list = rmap_item;
}
return rmap_item;
}
Expand All @@ -1188,8 +1186,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
spin_unlock(&ksm_mmlist_lock);
next_mm:
ksm_scan.address = 0;
ksm_scan.rmap_item = list_entry(&slot->rmap_list,
struct rmap_item, link);
ksm_scan.rmap_list = &slot->rmap_list;
}

mm = slot->mm;
Expand All @@ -1215,10 +1212,10 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
flush_anon_page(vma, *page, ksm_scan.address);
flush_dcache_page(*page);
rmap_item = get_next_rmap_item(slot,
ksm_scan.rmap_item->link.next,
ksm_scan.address);
ksm_scan.rmap_list, ksm_scan.address);
if (rmap_item) {
ksm_scan.rmap_item = rmap_item;
ksm_scan.rmap_list =
&rmap_item->rmap_list;
ksm_scan.address += PAGE_SIZE;
} else
put_page(*page);
Expand All @@ -1234,14 +1231,13 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)

if (ksm_test_exit(mm)) {
ksm_scan.address = 0;
ksm_scan.rmap_item = list_entry(&slot->rmap_list,
struct rmap_item, link);
ksm_scan.rmap_list = &slot->rmap_list;
}
/*
* Nuke all the rmap_items that are above this current rmap:
* because there were no VM_MERGEABLE vmas with such addresses.
*/
remove_trailing_rmap_items(slot, ksm_scan.rmap_item->link.next);
remove_trailing_rmap_items(slot, ksm_scan.rmap_list);

spin_lock(&ksm_mmlist_lock);
ksm_scan.mm_slot = list_entry(slot->mm_list.next,
Expand Down Expand Up @@ -1423,7 +1419,7 @@ void __ksm_exit(struct mm_struct *mm)
spin_lock(&ksm_mmlist_lock);
mm_slot = get_mm_slot(mm);
if (mm_slot && ksm_scan.mm_slot != mm_slot) {
if (list_empty(&mm_slot->rmap_list)) {
if (!mm_slot->rmap_list) {
hlist_del(&mm_slot->link);
list_del(&mm_slot->mm_list);
easy_to_free = 1;
Expand Down

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