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Subject[PATCH 07/11] dm: suspend: add noflush pushback
From: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>

In device-mapper I/O is sometimes queued within targets for later
processing. For example the multipath target can be configured to store
I/O when no paths are available instead of returning it -EIO.

This patch allows the device-mapper core to instruct a target to transfer
the contents of any such in-target queue back into the core. This frees
up the resources used by the target so the core can replace that target
with an alternative one and then resend the I/O to it. Without this patch
the only way to change the target in such circumstances involves returning
the I/O with an error back to the filesystem/application. In the
multipath case, this patch will let us add new paths for existing I/O to
try after all the existing paths have failed.

======================================================================
DMF_NOFLUSH_SUSPENDING
----------------------
If the DM_NOFLUSH_FLAG ioctl option is specified at suspend time, the
DMF_NOFLUSH_SUSPENDING flag is set in md->flags during dm_suspend(). It
is always cleared before dm_suspend() returns.

The flag must be visible while the target is flushing pending I/Os so it
is set before presuspend where the flush starts and unset after the wait
for md->pending where the flush ends.

Target drivers can check this flag by calling dm_noflush_suspending().


DM_MAPIO_REQUEUE / DM_ENDIO_REQUEUE
-----------------------------------
A target's map() function can now return DM_MAPIO_REQUEUE to request the
device mapper core queue the bio.

Similarly, a target's end_io() function can return DM_ENDIO_REQUEUE to request
the same. This has been labelled 'pushback'.

The __map_bio() and clone_endio() functions in the core treat these return
values as errors and call dec_pending() to end the I/O.


dec_pending
-----------
dec_pending() saves the pushback request in struct dm_io->error. Once all
the split clones have ended, dec_pending() will put the original bio on
the md->pushback list. Note that this supercedes any I/O errors.

It is possible for the suspend with DM_NOFLUSH_FLAG to be aborted while
in progress (e.g. by user interrupt). dec_pending() checks for this and
returns -EIO if it happened.

pushdback list and pushback_lock
--------------------------------
The bio is queued on md->pushback temporarily in dec_pending(), and after
all pending I/Os return, md->pushback is merged into md->deferred in
dm_suspend() for re-issuing at resume time.

md->pushback_lock protects md->pushback.
The lock should be held with irq disabled because dec_pending() can be
called from interrupt context.

Queueing bios to md->pushback in dec_pending() must be done atomically
with the check for DMF_NOFLUSH_SUSPENDING flag. So md->pushback_lock is
held when checking the flag. Otherwise dec_pending() may queue a bio to
md->pushback after the interrupted dm_suspend() flushes md->pushback.
Then the bio would be left in md->pushback.

Flag setting in dm_suspend() can be done without md->pushback_lock because
the flag is checked only after presuspend and the set value is already
made visible via the target's presuspend function.

The flag can be checked without md->pushback_lock (e.g. the first part of
the dec_pending() or target drivers), because the flag is checked again
with md->pushback_lock held when the bio is really queued to md->pushback
as described above. So even if the flag is cleared after the lockless
checkings, the bio isn't left in md->pushback but returned to applications
with -EIO.


Other notes on the current patch
--------------------------------
- md->pushback is added to the struct mapped_device instead of using
md->deferred directly because md->io_lock which protects md->deferred is
rw_semaphore and can't be used in interrupt context like dec_pending(),
and md->io_lock protects the DMF_BLOCK_IO flag of md->flags too.

- Don't issue lock_fs() in dm_suspend() if the DM_NOFLUSH_FLAG
ioctl option is specified, because I/Os generated by lock_fs() would be
pushed back and never return if there were no valid devices.

- If an error occurs in dm_suspend() after the DMF_NOFLUSH_SUSPENDING
flag is set, md->pushback must be flushed because I/Os may be queued to
the list already. (flush_and_out label in dm_suspend())


======================================================================
Test results
=-=-=-=-=-=-=

I have tested using multipath target with the next patch.

The following tests are for regression/compatibility:
- I/Os succeed when valid paths exist;
- I/Os fail when there are no valid paths and queue_if_no_path is not
set;
- I/Os are queued in the multipath target when there are no valid paths and
queue_if_no_path is set;
- The queued I/Os above fail when suspend is issued without the
DM_NOFLUSH_FLAG ioctl option. I/Os spanning 2 multipath targets also
fail.

The following tests are for the normal code path of new pushback feature:
- Queued I/Os in the multipath target are flushed from the target
but don't return when suspend is issued with the DM_NOFLUSH_FLAG
ioctl option;
- The I/Os above are queued in the multipath target again when
resume is issued without path recovery;
- The I/Os above succeed when resume is issued after path recovery
or table load;
- Queued I/Os in the multipath target succeed when resume is issued
with the DM_NOFLUSH_FLAG ioctl option after table load. I/Os
spanning 2 multipath targets also succeed.

The following tests are for the error paths of the new pushback feature:
- When the bdget_disk() fails in dm_suspend(), the
DMF_NOFLUSH_SUSPENDING flag is cleared and I/Os already queued to the
pushback list are flushed properly.
- When suspend with the DM_NOFLUSH_FLAG ioctl option is interrupted,
o I/Os which had already been queued to the pushback list
at the time don't return, and are re-issued at resume time;
o I/Os which hadn't been returned at the time return with EIO.


Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: dm-devel@redhat.com

Index: linux-2.6.19-rc6/drivers/md/dm-bio-list.h
===================================================================
--- linux-2.6.19-rc6.orig/drivers/md/dm-bio-list.h 2006-11-22 17:26:45.000000000 +0000
+++ linux-2.6.19-rc6/drivers/md/dm-bio-list.h 2006-11-22 17:27:00.000000000 +0000
@@ -44,6 +44,20 @@ static inline void bio_list_merge(struct
bl->tail = bl2->tail;
}

+static inline void bio_list_merge_head(struct bio_list *bl,
+ struct bio_list *bl2)
+{
+ if (!bl2->head)
+ return;
+
+ if (bl->head)
+ bl2->tail->bi_next = bl->head;
+ else
+ bl->tail = bl2->tail;
+
+ bl->head = bl2->head;
+}
+
static inline struct bio *bio_list_pop(struct bio_list *bl)
{
struct bio *bio = bl->head;
Index: linux-2.6.19-rc6/drivers/md/dm.c
===================================================================
--- linux-2.6.19-rc6.orig/drivers/md/dm.c 2006-11-22 17:26:58.000000000 +0000
+++ linux-2.6.19-rc6/drivers/md/dm.c 2006-11-22 17:27:00.000000000 +0000
@@ -68,10 +68,12 @@ union map_info *dm_get_mapinfo(struct bi
#define DMF_FROZEN 2
#define DMF_FREEING 3
#define DMF_DELETING 4
+#define DMF_NOFLUSH_SUSPENDING 5

struct mapped_device {
struct rw_semaphore io_lock;
struct semaphore suspend_lock;
+ spinlock_t pushback_lock;
rwlock_t map_lock;
atomic_t holders;
atomic_t open_count;
@@ -90,6 +92,7 @@ struct mapped_device {
atomic_t pending;
wait_queue_head_t wait;
struct bio_list deferred;
+ struct bio_list pushback;

/*
* The current mapping.
@@ -444,23 +447,50 @@ int dm_set_geometry(struct mapped_device
* you this clearly demarcated crap.
*---------------------------------------------------------------*/

+static int __noflush_suspending(struct mapped_device *md)
+{
+ return test_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+}
+
/*
* Decrements the number of outstanding ios that a bio has been
* cloned into, completing the original io if necc.
*/
static void dec_pending(struct dm_io *io, int error)
{
- if (error)
+ unsigned long flags;
+
+ /* Push-back supersedes any I/O errors */
+ if (error && !(io->error > 0 && __noflush_suspending(io->md)))
io->error = error;

if (atomic_dec_and_test(&io->io_count)) {
+ if (io->error == DM_ENDIO_REQUEUE) {
+ /*
+ * Target requested pushing back the I/O.
+ * This must be handled before the sleeper on
+ * suspend queue merges the pushback list.
+ */
+ spin_lock_irqsave(&io->md->pushback_lock, flags);
+ if (__noflush_suspending(io->md))
+ bio_list_add(&io->md->pushback, io->bio);
+ else
+ /* noflush suspend was interrupted. */
+ io->error = -EIO;
+ spin_unlock_irqrestore(&io->md->pushback_lock, flags);
+ }
+
if (end_io_acct(io))
/* nudge anyone waiting on suspend queue */
wake_up(&io->md->wait);

- blk_add_trace_bio(io->md->queue, io->bio, BLK_TA_COMPLETE);
+ if (io->error != DM_ENDIO_REQUEUE) {
+ blk_add_trace_bio(io->md->queue, io->bio,
+ BLK_TA_COMPLETE);
+
+ bio_endio(io->bio, io->bio->bi_size, io->error);
+ }

- bio_endio(io->bio, io->bio->bi_size, io->error);
free_io(io->md, io);
}
}
@@ -480,7 +510,11 @@ static int clone_endio(struct bio *bio,

if (endio) {
r = endio(tio->ti, bio, error, &tio->info);
- if (r < 0)
+ if (r < 0 || r == DM_ENDIO_REQUEUE)
+ /*
+ * error and requeue request are handled
+ * in dec_pending().
+ */
error = r;
else if (r == DM_ENDIO_INCOMPLETE)
/* The target will handle the io */
@@ -554,8 +588,8 @@ static void __map_bio(struct dm_target *
clone->bi_sector);

generic_make_request(clone);
- } else if (r < 0) {
- /* error the io and bail out */
+ } else if (r < 0 || r == DM_MAPIO_REQUEUE) {
+ /* error the io and bail out, or requeue it if needed */
md = tio->io->md;
dec_pending(tio->io, r);
/*
@@ -952,6 +986,7 @@ static struct mapped_device *alloc_dev(i
memset(md, 0, sizeof(*md));
init_rwsem(&md->io_lock);
init_MUTEX(&md->suspend_lock);
+ spin_lock_init(&md->pushback_lock);
rwlock_init(&md->map_lock);
atomic_set(&md->holders, 1);
atomic_set(&md->open_count, 0);
@@ -1282,10 +1317,12 @@ static void unlock_fs(struct mapped_devi
int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
{
struct dm_table *map = NULL;
+ unsigned long flags;
DECLARE_WAITQUEUE(wait, current);
struct bio *def;
int r = -EINVAL;
int do_lockfs = suspend_flags & DM_SUSPEND_LOCKFS_FLAG ? 1 : 0;
+ int noflush = suspend_flags & DM_SUSPEND_NOFLUSH_FLAG ? 1 : 0;

down(&md->suspend_lock);

@@ -1294,6 +1331,13 @@ int dm_suspend(struct mapped_device *md,

map = dm_get_table(md);

+ /*
+ * DMF_NOFLUSH_SUSPENDING must be set before presuspend.
+ * This flag is cleared before dm_suspend returns.
+ */
+ if (noflush)
+ set_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+
/* This does not get reverted if there's an error later. */
dm_table_presuspend_targets(map);

@@ -1301,11 +1345,14 @@ int dm_suspend(struct mapped_device *md,
if (!md->suspended_bdev) {
DMWARN("bdget failed in dm_suspend");
r = -ENOMEM;
- goto out;
+ goto flush_and_out;
}

- /* Flush I/O to the device. */
- if (do_lockfs) {
+ /*
+ * Flush I/O to the device.
+ * noflush supersedes do_lockfs, because lock_fs() needs to flush I/Os.
+ */
+ if (do_lockfs && !noflush) {
r = lock_fs(md);
if (r)
goto out;
@@ -1341,6 +1388,14 @@ int dm_suspend(struct mapped_device *md,
down_write(&md->io_lock);
remove_wait_queue(&md->wait, &wait);

+ if (noflush) {
+ spin_lock_irqsave(&md->pushback_lock, flags);
+ clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+ bio_list_merge_head(&md->deferred, &md->pushback);
+ bio_list_init(&md->pushback);
+ spin_unlock_irqrestore(&md->pushback_lock, flags);
+ }
+
/* were we interrupted ? */
r = -EINTR;
if (atomic_read(&md->pending)) {
@@ -1349,7 +1404,7 @@ int dm_suspend(struct mapped_device *md,
__flush_deferred_io(md, def);
up_write(&md->io_lock);
unlock_fs(md);
- goto out;
+ goto out; /* pushback list is already flushed, so skip flush */
}
up_write(&md->io_lock);

@@ -1359,6 +1414,25 @@ int dm_suspend(struct mapped_device *md,

r = 0;

+flush_and_out:
+ if (r && noflush) {
+ /*
+ * Because there may be already I/Os in the pushback list,
+ * flush them before return.
+ */
+ down_write(&md->io_lock);
+
+ spin_lock_irqsave(&md->pushback_lock, flags);
+ clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
+ bio_list_merge_head(&md->deferred, &md->pushback);
+ bio_list_init(&md->pushback);
+ spin_unlock_irqrestore(&md->pushback_lock, flags);
+
+ def = bio_list_get(&md->deferred);
+ __flush_deferred_io(md, def);
+ up_write(&md->io_lock);
+ }
+
out:
if (r && md->suspended_bdev) {
bdput(md->suspended_bdev);
@@ -1445,6 +1519,17 @@ int dm_suspended(struct mapped_device *m
return test_bit(DMF_SUSPENDED, &md->flags);
}

+int dm_noflush_suspending(struct dm_target *ti)
+{
+ struct mapped_device *md = dm_table_get_md(ti->table);
+ int r = __noflush_suspending(md);
+
+ dm_put(md);
+
+ return r;
+}
+EXPORT_SYMBOL_GPL(dm_noflush_suspending);
+
static struct block_device_operations dm_blk_dops = {
.open = dm_blk_open,
.release = dm_blk_close,
Index: linux-2.6.19-rc6/drivers/md/dm.h
===================================================================
--- linux-2.6.19-rc6.orig/drivers/md/dm.h 2006-11-22 17:26:59.000000000 +0000
+++ linux-2.6.19-rc6/drivers/md/dm.h 2006-11-22 17:27:00.000000000 +0000
@@ -36,12 +36,14 @@
* Definitions of return values from target end_io function.
*/
#define DM_ENDIO_INCOMPLETE 1
+#define DM_ENDIO_REQUEUE 2

/*
* Definitions of return values from target map function.
*/
#define DM_MAPIO_SUBMITTED 0
#define DM_MAPIO_REMAPPED 1
+#define DM_MAPIO_REQUEUE DM_ENDIO_REQUEUE

/*
* Suspend feature flags
Index: linux-2.6.19-rc6/include/linux/device-mapper.h
===================================================================
--- linux-2.6.19-rc6.orig/include/linux/device-mapper.h 2006-11-22 17:26:58.000000000 +0000
+++ linux-2.6.19-rc6/include/linux/device-mapper.h 2006-11-22 17:27:00.000000000 +0000
@@ -40,6 +40,7 @@ typedef void (*dm_dtr_fn) (struct dm_tar
* < 0: error
* = 0: The target will handle the io by resubmitting it later
* = 1: simple remap complete
+ * = 2: The target wants to push back the io
*/
typedef int (*dm_map_fn) (struct dm_target *ti, struct bio *bio,
union map_info *map_context);
@@ -50,6 +51,7 @@ typedef int (*dm_map_fn) (struct dm_targ
* 0 : ended successfully
* 1 : for some reason the io has still not completed (eg,
* multipath target might want to requeue a failed io).
+ * 2 : The target wants to push back the io
*/
typedef int (*dm_endio_fn) (struct dm_target *ti,
struct bio *bio, int error,
@@ -188,6 +190,7 @@ int dm_wait_event(struct mapped_device *
const char *dm_device_name(struct mapped_device *md);
struct gendisk *dm_disk(struct mapped_device *md);
int dm_suspended(struct mapped_device *md);
+int dm_noflush_suspending(struct dm_target *ti);

/*
* Geometry functions.
-
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