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Commit 5d337b91 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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[PATCH] swap: swap_lock replace list+device 



The idea of a swap_device_lock per device, and a swap_list_lock over them all,
is appealing; but in practice almost every holder of swap_device_lock must
already hold swap_list_lock, which defeats the purpose of the split.

The only exceptions have been swap_duplicate, valid_swaphandles and an
untrodden path in try_to_unuse (plus a few places added in this series).
valid_swaphandles doesn't show up high in profiles, but swap_duplicate does
demand attention.  However, with the hold time in get_swap_pages so much
reduced, I've not yet found a load and set of swap device priorities to show
even swap_duplicate benefitting from the split.  Certainly the split is mere
overhead in the common case of a single swap device.

So, replace swap_list_lock and swap_device_lock by spinlock_t swap_lock
(generally we seem to prefer an _ in the name, and not hide in a macro).

If someone can show a regression in swap_duplicate, then probably we should
add a hashlock for the swap_map entries alone (shorts being anatomic), so as
to help the case of the single swap device too.

Signed-off-by: default avatarHugh Dickins <hugh@veritas.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 048c27fd

Documentation/vm/locking

+7 −8
Original line number Diff line number Diff line
@@ -83,19 +83,18 @@ single address space optimization, so that the zap_page_range (from 
vmtruncate) does not lose sending ipi's to cloned threads that might 

be spawned underneath it and go to user mode to drag in pte's into tlbs.



swap_list_lock/swap_device_lock

-------------------------------

swap_lock

--------------

The swap devices are chained in priority order from the "swap_list" header. 

The "swap_list" is used for the round-robin swaphandle allocation strategy.

The #free swaphandles is maintained in "nr_swap_pages". These two together

are protected by the swap_list_lock. 

are protected by the swap_lock.



The swap_device_lock, which is per swap device, protects the reference 

counts on the corresponding swaphandles, maintained in the "swap_map"

array, and the "highest_bit" and "lowest_bit" fields.

The swap_lock also protects all the device reference counts on the

corresponding swaphandles, maintained in the "swap_map" array, and the

"highest_bit" and "lowest_bit" fields.



Both of these are spinlocks, and are never acquired from intr level. The

locking hierarchy is swap_list_lock -> swap_device_lock.

The swap_lock is a spinlock, and is never acquired from intr level.



To prevent races between swap space deletion or async readahead swapins

deciding whether a swap handle is being used, ie worthy of being read in

include/linux/swap.h

+2 −9
Original line number Diff line number Diff line
@@ -121,7 +121,7 @@ enum { 
 */

struct swap_info_struct {

	unsigned int flags;

	spinlock_t sdev_lock;

	int prio;			/* swap priority */

	struct file *swap_file;

	struct block_device *bdev;

	struct list_head extent_list;
@@ -135,7 +135,6 @@ struct swap_info_struct { 
	unsigned int pages;

	unsigned int max;

	unsigned int inuse_pages;

	int prio;			/* swap priority */

	int next;			/* next entry on swap list */

};
@@ -221,13 +220,7 @@ extern int can_share_swap_page(struct page *); 
extern int remove_exclusive_swap_page(struct page *);

struct backing_dev_info;



extern struct swap_list_t swap_list;

extern spinlock_t swaplock;



#define swap_list_lock()	spin_lock(&swaplock)

#define swap_list_unlock()	spin_unlock(&swaplock)

#define swap_device_lock(p)	spin_lock(&p->sdev_lock)

#define swap_device_unlock(p)	spin_unlock(&p->sdev_lock)

extern spinlock_t swap_lock;



/* linux/mm/thrash.c */

extern struct mm_struct * swap_token_mm;

mm/filemap.c

+3 −4
Original line number Diff line number Diff line
@@ -54,8 +54,7 @@ 
 *

 *  ->i_mmap_lock		(vmtruncate)

 *    ->private_lock		(__free_pte->__set_page_dirty_buffers)

 *      ->swap_list_lock

 *        ->swap_device_lock	(exclusive_swap_page, others)

 *      ->swap_lock		(exclusive_swap_page, others)

 *        ->mapping->tree_lock

 *

 *  ->i_sem
@@ -86,7 +85,7 @@ 
 *    ->page_table_lock		(anon_vma_prepare and various)

 *

 *  ->page_table_lock

 *    ->swap_device_lock	(try_to_unmap_one)

 *    ->swap_lock		(try_to_unmap_one)

 *    ->private_lock		(try_to_unmap_one)

 *    ->tree_lock		(try_to_unmap_one)

 *    ->zone.lru_lock		(follow_page->mark_page_accessed)

mm/rmap.c

+1 −2
Original line number Diff line number Diff line
@@ -34,9 +34,8 @@ 
 *       anon_vma->lock

 *         mm->page_table_lock

 *           zone->lru_lock (in mark_page_accessed)

 *           swap_list_lock (in swap_free etc's swap_info_get)

 *           swap_lock (in swap_duplicate, swap_info_get)

 *             mmlist_lock (in mmput, drain_mmlist and others)

 *             swap_device_lock (in swap_duplicate, swap_info_get)

 *             mapping->private_lock (in __set_page_dirty_buffers)

 *             inode_lock (in set_page_dirty's __mark_inode_dirty)

 *               sb_lock (within inode_lock in fs/fs-writeback.c)

mm/swapfile.c

+53 −72
Original line number Diff line number Diff line
@@ -31,7 +31,7 @@ 
#include <asm/tlbflush.h>

#include <linux/swapops.h>



DEFINE_SPINLOCK(swaplock);

DEFINE_SPINLOCK(swap_lock);

unsigned int nr_swapfiles;

long total_swap_pages;

static int swap_overflow;
@@ -51,7 +51,7 @@ static DECLARE_MUTEX(swapon_sem); 


/*

 * We need this because the bdev->unplug_fn can sleep and we cannot

 * hold swap_list_lock while calling the unplug_fn. And swap_list_lock

 * hold swap_lock while calling the unplug_fn. And swap_lock

 * cannot be turned into a semaphore.

 */

static DECLARE_RWSEM(swap_unplug_sem);
@@ -105,7 +105,7 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si) 
		si->cluster_nr = SWAPFILE_CLUSTER - 1;

		if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER)

			goto lowest;

		swap_device_unlock(si);

		spin_unlock(&swap_lock);



		offset = si->lowest_bit;

		last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
@@ -115,7 +115,7 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si) 
			if (si->swap_map[offset])

				last_in_cluster = offset + SWAPFILE_CLUSTER;

			else if (offset == last_in_cluster) {

				swap_device_lock(si);

				spin_lock(&swap_lock);

				si->cluster_next = offset-SWAPFILE_CLUSTER-1;

				goto cluster;

			}
@@ -124,7 +124,7 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si) 
				latency_ration = LATENCY_LIMIT;

			}

		}

		swap_device_lock(si);

		spin_lock(&swap_lock);

		goto lowest;

	}
@@ -153,10 +153,10 @@ checks: if (!(si->flags & SWP_WRITEOK)) 
		return offset;

	}



	swap_device_unlock(si);

	spin_unlock(&swap_lock);

	while (++offset <= si->highest_bit) {

		if (!si->swap_map[offset]) {

			swap_device_lock(si);

			spin_lock(&swap_lock);

			goto checks;

		}

		if (unlikely(--latency_ration < 0)) {
@@ -164,7 +164,7 @@ checks: if (!(si->flags & SWP_WRITEOK)) 
			latency_ration = LATENCY_LIMIT;

		}

	}

	swap_device_lock(si);

	spin_lock(&swap_lock);

	goto lowest;



no_page:
@@ -179,7 +179,7 @@ swp_entry_t get_swap_page(void) 
	int type, next;

	int wrapped = 0;



	swap_list_lock();

	spin_lock(&swap_lock);

	if (nr_swap_pages <= 0)

		goto noswap;

	nr_swap_pages--;
@@ -199,19 +199,17 @@ swp_entry_t get_swap_page(void) 
			continue;



		swap_list.next = next;

		swap_device_lock(si);

		swap_list_unlock();

		offset = scan_swap_map(si);

		swap_device_unlock(si);

		if (offset)

		if (offset) {

			spin_unlock(&swap_lock);

			return swp_entry(type, offset);

		swap_list_lock();

		}

		next = swap_list.next;

	}



	nr_swap_pages++;

noswap:

	swap_list_unlock();

	spin_unlock(&swap_lock);

	return (swp_entry_t) {0};

}
@@ -233,8 +231,7 @@ static struct swap_info_struct * swap_info_get(swp_entry_t entry) 
		goto bad_offset;

	if (!p->swap_map[offset])

		goto bad_free;

	swap_list_lock();

	swap_device_lock(p);

	spin_lock(&swap_lock);

	return p;



bad_free:
@@ -252,12 +249,6 @@ static struct swap_info_struct * swap_info_get(swp_entry_t entry) 
	return NULL;

}	



static void swap_info_put(struct swap_info_struct * p)

{

	swap_device_unlock(p);

	swap_list_unlock();

}



static int swap_entry_free(struct swap_info_struct *p, unsigned long offset)

{

	int count = p->swap_map[offset];
@@ -290,7 +281,7 @@ void swap_free(swp_entry_t entry) 
	p = swap_info_get(entry);

	if (p) {

		swap_entry_free(p, swp_offset(entry));

		swap_info_put(p);

		spin_unlock(&swap_lock);

	}

}
@@ -308,7 +299,7 @@ static inline int page_swapcount(struct page *page) 
	if (p) {

		/* Subtract the 1 for the swap cache itself */

		count = p->swap_map[swp_offset(entry)] - 1;

		swap_info_put(p);

		spin_unlock(&swap_lock);

	}

	return count;

}
@@ -365,7 +356,7 @@ int remove_exclusive_swap_page(struct page *page) 
		}

		write_unlock_irq(&swapper_space.tree_lock);

	}

	swap_info_put(p);

	spin_unlock(&swap_lock);



	if (retval) {

		swap_free(entry);
@@ -388,7 +379,7 @@ void free_swap_and_cache(swp_entry_t entry) 
	if (p) {

		if (swap_entry_free(p, swp_offset(entry)) == 1)

			page = find_trylock_page(&swapper_space, entry.val);

		swap_info_put(p);

		spin_unlock(&swap_lock);

	}

	if (page) {

		int one_user;
@@ -558,10 +549,10 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si, 
	int count;



	/*

	 * No need for swap_device_lock(si) here: we're just looking

	 * No need for swap_lock here: we're just looking

	 * for whether an entry is in use, not modifying it; false

	 * hits are okay, and sys_swapoff() has already prevented new

	 * allocations from this area (while holding swap_list_lock()).

	 * allocations from this area (while holding swap_lock).

	 */

	for (;;) {

		if (++i >= max) {
@@ -751,9 +742,9 @@ static int try_to_unuse(unsigned int type) 
		 * report them; but do report if we reset SWAP_MAP_MAX.

		 */

		if (*swap_map == SWAP_MAP_MAX) {

			swap_device_lock(si);

			spin_lock(&swap_lock);

			*swap_map = 1;

			swap_device_unlock(si);

			spin_unlock(&swap_lock);

			reset_overflow = 1;

		}
@@ -817,9 +808,9 @@ static int try_to_unuse(unsigned int type) 
}



/*

 * After a successful try_to_unuse, if no swap is now in use, we know we

 * can empty the mmlist.  swap_list_lock must be held on entry and exit.

 * Note that mmlist_lock nests inside swap_list_lock, and an mm must be

 * After a successful try_to_unuse, if no swap is now in use, we know

 * we can empty the mmlist.  swap_lock must be held on entry and exit.

 * Note that mmlist_lock nests inside swap_lock, and an mm must be

 * added to the mmlist just after page_duplicate - before would be racy.

 */

static void drain_mmlist(void)
@@ -1092,7 +1083,7 @@ asmlinkage long sys_swapoff(const char __user * specialfile) 


	mapping = victim->f_mapping;

	prev = -1;

	swap_list_lock();

	spin_lock(&swap_lock);

	for (type = swap_list.head; type >= 0; type = swap_info[type].next) {

		p = swap_info + type;

		if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
@@ -1103,14 +1094,14 @@ asmlinkage long sys_swapoff(const char __user * specialfile) 
	}

	if (type < 0) {

		err = -EINVAL;

		swap_list_unlock();

		spin_unlock(&swap_lock);

		goto out_dput;

	}

	if (!security_vm_enough_memory(p->pages))

		vm_unacct_memory(p->pages);

	else {

		err = -ENOMEM;

		swap_list_unlock();

		spin_unlock(&swap_lock);

		goto out_dput;

	}

	if (prev < 0) {
@@ -1124,10 +1115,8 @@ asmlinkage long sys_swapoff(const char __user * specialfile) 
	}

	nr_swap_pages -= p->pages;

	total_swap_pages -= p->pages;

	swap_device_lock(p);

	p->flags &= ~SWP_WRITEOK;

	swap_device_unlock(p);

	swap_list_unlock();

	spin_unlock(&swap_lock);



	current->flags |= PF_SWAPOFF;

	err = try_to_unuse(type);
@@ -1135,7 +1124,7 @@ asmlinkage long sys_swapoff(const char __user * specialfile) 


	if (err) {

		/* re-insert swap space back into swap_list */

		swap_list_lock();

		spin_lock(&swap_lock);

		for (prev = -1, i = swap_list.head; i >= 0; prev = i, i = swap_info[i].next)

			if (p->prio >= swap_info[i].prio)

				break;
@@ -1146,10 +1135,8 @@ asmlinkage long sys_swapoff(const char __user * specialfile) 
			swap_info[prev].next = p - swap_info;

		nr_swap_pages += p->pages;

		total_swap_pages += p->pages;

		swap_device_lock(p);

		p->flags |= SWP_WRITEOK;

		swap_device_unlock(p);

		swap_list_unlock();

		spin_unlock(&swap_lock);

		goto out_dput;

	}
@@ -1157,30 +1144,27 @@ asmlinkage long sys_swapoff(const char __user * specialfile) 
	down_write(&swap_unplug_sem);

	up_write(&swap_unplug_sem);



	destroy_swap_extents(p);

	down(&swapon_sem);

	spin_lock(&swap_lock);

	drain_mmlist();



	/* wait for anyone still in scan_swap_map */

	swap_device_lock(p);

	p->highest_bit = 0;		/* cuts scans short */

	while (p->flags >= SWP_SCANNING) {

		swap_device_unlock(p);

		spin_unlock(&swap_lock);

		set_current_state(TASK_UNINTERRUPTIBLE);

		schedule_timeout(1);

		swap_device_lock(p);

		spin_lock(&swap_lock);

	}

	swap_device_unlock(p);



	destroy_swap_extents(p);

	down(&swapon_sem);

	swap_list_lock();

	drain_mmlist();

	swap_device_lock(p);

	swap_file = p->swap_file;

	p->swap_file = NULL;

	p->max = 0;

	swap_map = p->swap_map;

	p->swap_map = NULL;

	p->flags = 0;

	swap_device_unlock(p);

	swap_list_unlock();

	spin_unlock(&swap_lock);

	up(&swapon_sem);

	vfree(swap_map);

	inode = mapping->host;
@@ -1324,7 +1308,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 


	if (!capable(CAP_SYS_ADMIN))

		return -EPERM;

	swap_list_lock();

	spin_lock(&swap_lock);

	p = swap_info;

	for (type = 0 ; type < nr_swapfiles ; type++,p++)

		if (!(p->flags & SWP_USED))
@@ -1343,7 +1327,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 
	 * swp_entry_t or the architecture definition of a swap pte.

	 */

	if (type > swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))) {

		swap_list_unlock();

		spin_unlock(&swap_lock);

		goto out;

	}

	if (type >= nr_swapfiles)
@@ -1357,7 +1341,6 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 
	p->highest_bit = 0;

	p->cluster_nr = 0;

	p->inuse_pages = 0;

	spin_lock_init(&p->sdev_lock);

	p->next = -1;

	if (swap_flags & SWAP_FLAG_PREFER) {

		p->prio =
@@ -1365,7 +1348,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 
	} else {

		p->prio = --least_priority;

	}

	swap_list_unlock();

	spin_unlock(&swap_lock);

	name = getname(specialfile);

	error = PTR_ERR(name);

	if (IS_ERR(name)) {
@@ -1542,8 +1525,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 
	}



	down(&swapon_sem);

	swap_list_lock();

	swap_device_lock(p);

	spin_lock(&swap_lock);

	p->flags = SWP_ACTIVE;

	nr_swap_pages += nr_good_pages;

	total_swap_pages += nr_good_pages;
@@ -1567,8 +1549,7 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 
	} else {

		swap_info[prev].next = p - swap_info;

	}

	swap_device_unlock(p);

	swap_list_unlock();

	spin_unlock(&swap_lock);

	up(&swapon_sem);

	error = 0;

	goto out;
@@ -1579,14 +1560,14 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags) 
	}

	destroy_swap_extents(p);

bad_swap_2:

	swap_list_lock();

	spin_lock(&swap_lock);

	swap_map = p->swap_map;

	p->swap_file = NULL;

	p->swap_map = NULL;

	p->flags = 0;

	if (!(swap_flags & SWAP_FLAG_PREFER))

		++least_priority;

	swap_list_unlock();

	spin_unlock(&swap_lock);

	vfree(swap_map);

	if (swap_file)

		filp_close(swap_file, NULL);
@@ -1610,7 +1591,7 @@ void si_swapinfo(struct sysinfo *val) 
	unsigned int i;

	unsigned long nr_to_be_unused = 0;



	swap_list_lock();

	spin_lock(&swap_lock);

	for (i = 0; i < nr_swapfiles; i++) {

		if (!(swap_info[i].flags & SWP_USED) ||

		     (swap_info[i].flags & SWP_WRITEOK))
@@ -1619,7 +1600,7 @@ void si_swapinfo(struct sysinfo *val) 
	}

	val->freeswap = nr_swap_pages + nr_to_be_unused;

	val->totalswap = total_swap_pages + nr_to_be_unused;

	swap_list_unlock();

	spin_unlock(&swap_lock);

}



/*
@@ -1640,7 +1621,7 @@ int swap_duplicate(swp_entry_t entry) 
	p = type + swap_info;

	offset = swp_offset(entry);



	swap_device_lock(p);

	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]++;
@@ -1652,7 +1633,7 @@ int swap_duplicate(swp_entry_t entry) 
			result = 1;

		}

	}

	swap_device_unlock(p);

	spin_unlock(&swap_lock);

out:

	return result;
@@ -1668,7 +1649,7 @@ get_swap_info_struct(unsigned type) 
}



/*

 * swap_device_lock prevents swap_map being freed. Don't grab an extra

 * swap_lock prevents swap_map being freed. Don't grab an extra

 * reference on the swaphandle, it doesn't matter if it becomes unused.

 */

int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
@@ -1684,7 +1665,7 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset) 
		toff++, i--;

	*offset = toff;



	swap_device_lock(swapdev);

	spin_lock(&swap_lock);

	do {

		/* Don't read-ahead past the end of the swap area */

		if (toff >= swapdev->max)
@@ -1697,6 +1678,6 @@ int valid_swaphandles(swp_entry_t entry, unsigned long *offset) 
		toff++;

		ret++;

	} while (--i);

	swap_device_unlock(swapdev);

	spin_unlock(&swap_lock);

	return ret;

}