Commit c63e09ec authored by Al Viro's avatar Al Viro
Browse files

Make allocation of anon devices cheaper 



Standard trick - add a new variable (start) such that
for each n < start n is known to be busy.  Allocation can
skip checking everything in [0..start) and if it returns
n, we can set start to n + 1.  Freeing below start sets
start to what we'd just freed.

Of course, it still sucks if we do something like
	free 0
	allocate
	allocate
in a loop - still O(n^2) time.  However, on saner loads it
improves the things a lot and the entire thing is not worth
the trouble of switching to something with better worst-case
behaviour.

Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 7e325d3a

fs/super.c

+6 −1
Original line number Diff line number Diff line
@@ -608,6 +608,7 @@ void emergency_remount(void) 


static DEFINE_IDA(unnamed_dev_ida);

static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */

static int unnamed_dev_start = 0; /* don't bother trying below it */



int set_anon_super(struct super_block *s, void *data)

{
@@ -618,7 +619,8 @@ int set_anon_super(struct super_block *s, void *data) 
	if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)

		return -ENOMEM;

	spin_lock(&unnamed_dev_lock);

	error = ida_get_new(&unnamed_dev_ida, &dev);

	error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);

	unnamed_dev_start = dev + 1;

	spin_unlock(&unnamed_dev_lock);

	if (error == -EAGAIN)

		/* We raced and lost with another CPU. */
@@ -629,6 +631,7 @@ int set_anon_super(struct super_block *s, void *data) 
	if ((dev & MAX_ID_MASK) == (1 << MINORBITS)) {

		spin_lock(&unnamed_dev_lock);

		ida_remove(&unnamed_dev_ida, dev);

		unnamed_dev_start = dev;

		spin_unlock(&unnamed_dev_lock);

		return -EMFILE;

	}
@@ -645,6 +648,8 @@ void kill_anon_super(struct super_block *sb) 
	generic_shutdown_super(sb);

	spin_lock(&unnamed_dev_lock);

	ida_remove(&unnamed_dev_ida, slot);

	if (slot < unnamed_dev_start)

		unnamed_dev_start = slot;

	spin_unlock(&unnamed_dev_lock);

}

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