[PATCH 2/2] [mmotm] ramzswap: use slot free callback to eliminate stale data

[Nitin Gupta <ngupta@vflare.org>]

ramzswap driver creates RAM backed block devices which are
used as swap disks. Pages swapped to these disks are compressed
and stored in memory itself. However, when a swap page becomes
stale i.e. it is no longer referenced by any process (say, when
owning process exits), the driver does not get any notification
about this. So, it has to keep such pages in memory until kernel
swaps to the same swap slot again thereby overwriting previous
(stale) page.

Often, a large number of such stale pages accumulate which defeats
the whole purpose of in-memory compressed swapping and it begins
to have a negative impact on system performance.

To overcome this problem, we now register a callback function
which is called as soon as a swap slot is freed which allows
us to free corresponding memory, eliminating any stale data
in (compressed) memory.

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---
 drivers/staging/ramzswap/ramzswap_drv.c   |   15 ++++++++++++++-
 drivers/staging/ramzswap/ramzswap_drv.h   |    1 +
 drivers/staging/ramzswap/ramzswap_ioctl.h |    1 +
 3 files changed, 16 insertions(+), 1 deletions(-)

diff --git a/drivers/staging/ramzswap/ramzswap_drv.c b/drivers/staging/ramzswap/ramzswap_drv.c
index b839f05..bada4ae 100644
--- a/drivers/staging/ramzswap/ramzswap_drv.c
+++ b/drivers/staging/ramzswap/ramzswap_drv.c
@@ -276,6 +276,7 @@ void ramzswap_ioctl_get_stats(struct ramzswap *rzs,
 	s->failed_reads = rs->failed_reads;
 	s->failed_writes = rs->failed_writes;
 	s->invalid_io = rs->invalid_io;
+	s->notify_free = rs->notify_free;
 	s->pages_zero = rs->pages_zero;
 
 	s->good_compress_pct = good_compress_perc;
@@ -1310,9 +1311,21 @@ out:
 	return ret;
 }
 
+void ramzswap_slot_free_notify(struct block_device *bdev, unsigned long index)
+{
+	struct ramzswap *rzs;
+
+	rzs = bdev->bd_disk->private_data;
+	ramzswap_free_page(rzs, index);
+	stat_inc(rzs->stats.notify_free);
+
+	return;
+}
+
 static struct block_device_operations ramzswap_devops = {
 	.ioctl = ramzswap_ioctl,
-	.owner = THIS_MODULE,
+	.swap_slot_free_notify = ramzswap_slot_free_notify,
+	.owner = THIS_MODULE
 };
 
 static void create_device(struct ramzswap *rzs, int device_id)
diff --git a/drivers/staging/ramzswap/ramzswap_drv.h b/drivers/staging/ramzswap/ramzswap_drv.h
index a6ea240..adc841a 100644
--- a/drivers/staging/ramzswap/ramzswap_drv.h
+++ b/drivers/staging/ramzswap/ramzswap_drv.h
@@ -124,6 +124,7 @@ struct ramzswap_stats {
 	u64 failed_reads;	/* can happen when memory is too low */
 	u64 failed_writes;	/* should NEVER! happen */
 	u64 invalid_io;		/* non-swap I/O requests */
+	u64 notify_free;	/* no. of swap slot free notifications */
 	u32 pages_zero;		/* no. of zero filled pages */
 	u32 pages_stored;	/* no. of pages currently stored */
 	u32 good_compress;	/* % of pages with compression ratio<=50% */
diff --git a/drivers/staging/ramzswap/ramzswap_ioctl.h b/drivers/staging/ramzswap/ramzswap_ioctl.h
index c713a09..ec50416 100644
--- a/drivers/staging/ramzswap/ramzswap_ioctl.h
+++ b/drivers/staging/ramzswap/ramzswap_ioctl.h
@@ -27,6 +27,7 @@ struct ramzswap_ioctl_stats {
 	u64 failed_reads;	/* can happen when memory is too low */
 	u64 failed_writes;	/* should NEVER! happen */
 	u64 invalid_io;		/* non-swap I/O requests */
+	u64 notify_free;	/* no. of swap slot free notifications */
 	u32 pages_zero;		/* no. of zero filled pages */
 	u32 good_compress_pct;	/* no. of pages with compression ratio<=50% */
 	u32 pages_expand_pct;	/* no. of incompressible pages */
-- 
1.6.2.5