Patchwork [KNOWN,BUGGY,RFC,4/3] block: skip elevator initialization for flush requests

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Submitter Mike Snitzer
Date Jan. 25, 2011, 8:41 p.m.
Message ID <20110125204158.GA3013@redhat.com>
Download mbox | patch
Permalink /patch/80403/
State New
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Comments

Mike Snitzer - Jan. 25, 2011, 8:41 p.m.
Hi Tejun,

On Fri, Jan 21 2011 at 10:59am -0500,
Tejun Heo <tj@kernel.org> wrote:
> 
> * As flush requests are never put on the IO scheduler, request fields
>   used for flush share space with rq->rb_node.  rq->completion_data is
>   moved out of the union.  This increases the request size by one
>   pointer.
> 
>   As rq->elevator_private* are used only by the iosched too, it is
>   possible to reduce the request size further.  However, to do that,
>   we need to modify request allocation path such that iosched data is
>   not allocated for flush requests.

I decided to take a crack at using rq->elevator_private* and came up
with the following patch.

Unfortunately, in testing I found that flush requests that have data do
in fact eventually get added to the queue as normal requests, via:
1) "data but flush is not necessary" case in blk_insert_flush
2) REQ_FSEQ_DATA case in blk_flush_complete_seq

I know this because in my following get_request() change to _not_ call
elv_set_request() for flush requests hit cfq_put_request()'s
BUG_ON(!cfqq->allocated[rw]).  cfqq->allocated[rw] gets set via
elv_set_request()'s call to cfq_set_request().

So this seems to call in to question the running theory that flush
requests can share 'struct request' space with elevator-specific members
(via union) -- be it rq->rb_node or rq->elevator_private*.

Please advise, thanks!
Mike



From: Mike Snitzer <snitzer@redhat.com>

Skip elevator initialization for flush requests by passing priv=0 to
blk_alloc_request() in get_request().  As such elv_set_request() is
never called for flush requests.

Move elevator_private* into 'struct elevator' and have the flush fields
share a union with it.

Reclaim the space lost in 'struct request' by moving 'completion_data'
back in to the union with 'rb_node'.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-core.c       |   13 +++++++++----
 block/cfq-iosched.c    |   18 +++++++++---------
 block/elevator.c       |    2 +-
 include/linux/blkdev.h |   26 +++++++++++++++-----------
 4 files changed, 34 insertions(+), 25 deletions(-)

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Mike Snitzer - Jan. 25, 2011, 9:55 p.m.
On Tue, Jan 25 2011 at  3:41pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> Hi Tejun,
> 
> On Fri, Jan 21 2011 at 10:59am -0500,
> Tejun Heo <tj@kernel.org> wrote:
> > 
> > * As flush requests are never put on the IO scheduler, request fields
> >   used for flush share space with rq->rb_node.  rq->completion_data is
> >   moved out of the union.  This increases the request size by one
> >   pointer.
> > 
> >   As rq->elevator_private* are used only by the iosched too, it is
> >   possible to reduce the request size further.  However, to do that,
> >   we need to modify request allocation path such that iosched data is
> >   not allocated for flush requests.
> 
> I decided to take a crack at using rq->elevator_private* and came up
> with the following patch.
> 
> Unfortunately, in testing I found that flush requests that have data do
> in fact eventually get added to the queue as normal requests, via:
> 1) "data but flush is not necessary" case in blk_insert_flush
> 2) REQ_FSEQ_DATA case in blk_flush_complete_seq

Vivek helped me understand that adding the request to the queue doesn't
mean it goes to the elevator.  It is inserting the request directly to
the underlying queue.

That embarassing oversight aside, the flush request is still getting to
the elevator somehow -- even though elv_set_request() was clearly not
called.

It is an interesting duality that:
1) REQ_ELVPRIV is never set because priv=0 is passed to blk_alloc_request
2) yet when blk_free_request() checks rq->cmd_flags REQ_ELVPRIV is set;
   resulting in the call to elv_put_request()

> I know this because in my following get_request() change to _not_ call
> elv_set_request() for flush requests hit cfq_put_request()'s
> BUG_ON(!cfqq->allocated[rw]).

FYI, here is the backtrace:

PID: 0      TASK: ffff88007ccd6b30  CPU: 1   COMMAND: "swapper"
 #0 [ffff880002103930] show_trace_log_lvl at ffffffff8100f3ec
 #1 [ffff880002103980] delay_tsc at ffffffff8125e62a
 #2 [ffff8800021039b0] __const_udelay at ffffffff8125e5d6
 #3 [ffff8800021039c0] panic at ffffffff814c3604
 #4 [ffff880002103a40] oops_end at ffffffff814c7622
 #5 [ffff880002103a70] die at ffffffff8100f33b
 #6 [ffff880002103aa0] do_trap at ffffffff814c6ec4
 #7 [ffff880002103b00] do_invalid_op at ffffffff8100cee5
 #8 [ffff880002103ba0] invalid_op at ffffffff8100bf5b
    [exception RIP: cfq_put_request+128]
    RIP: ffffffff8124f000  RSP: ffff880002103c58  RFLAGS: 00010046
    RAX: 0000000000000019  RBX: ffff88007b5668a0  RCX: 0000000000002c7b
    RDX: 0000000000000000  RSI: ffff88007b5668a0  RDI: ffff88007b5668a0
    RBP: ffff880002103c68   R8: 0000000000000003   R9: 0000000000000001
    R10: 0000000000000003  R11: 0000000000000003  R12: ffff88007b566940
    R13: 00000000018c2441  R14: 0000000000000001  R15: 00000000000000a5
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #9 [ffff880002103c70] elv_put_request at ffffffff8123208e
#10 [ffff880002103c80] __blk_put_request at ffffffff8123ae23
#11 [ffff880002103cb0] blk_finish_request at ffffffff8123b049
#12 [ffff880002103d00] __blk_end_request_all at ffffffff8123b0fb
#13 [ffff880002103d20] blk_flush_complete_seq at ffffffff8123de4c
#14 [ffff880002103d50] flush_end_io at ffffffff8123e095
#15 [ffff880002103da0] blk_finish_request at ffffffff8123aedb
#16 [ffff880002103df0] __blk_end_request_all at ffffffff8123b0fb
#17 [ffff880002103e10] blk_done at ffffffffa002e085
#18 [ffff880002103e50] vring_interrupt at ffffffffa001f19c
#19 [ffff880002103e70] vp_vring_interrupt at ffffffffa00264bb
#20 [ffff880002103ec0] vp_interrupt at ffffffffa0026544
#21 [ffff880002103ee0] handle_IRQ_event at ffffffff810d10b0
#22 [ffff880002103f30] handle_fasteoi_irq at ffffffff810d38a9
#23 [ffff880002103f60] handle_irq at ffffffff8100dfb9
#24 [ffff880002103f80] do_IRQ at ffffffff814cb32c
--- <IRQ stack> ---
#25 [ffff88007ccf9e28] ret_from_intr at ffffffff8100bad3
    [exception RIP: native_safe_halt+11]
    RIP: ffffffff81033f0b  RSP: ffff88007ccf9ed8  RFLAGS: 00000246
    RAX: 0000000000000000  RBX: ffff88007ccf9ed8  RCX: 0000000000000000
    RDX: 0000000000000000  RSI: 0000000000000001  RDI: ffffffff81d0f1e8
    RBP: ffffffff8100bace   R8: 0000000000000000   R9: 0000000000000001
    R10: 0000000000000000  R11: 00000000fffba7c1  R12: 0000000000000001
    R13: ffff88007ccf9e68  R14: 0000000281075d93  R15: ffff88007ccf9e98
    ORIG_RAX: ffffffffffffffc4  CS: 0010  SS: 0018
#26 [ffff88007ccf9ee0] default_idle at ffffffff81013e0d
#27 [ffff88007ccf9f00] cpu_idle at ffffffff81009e96
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Mike Snitzer - Jan. 26, 2011, 5:27 a.m.
On Tue, Jan 25 2011 at  4:55pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Tue, Jan 25 2011 at  3:41pm -0500,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
> > Hi Tejun,
> > 
> > On Fri, Jan 21 2011 at 10:59am -0500,
> > Tejun Heo <tj@kernel.org> wrote:
> > > 
> > > * As flush requests are never put on the IO scheduler, request fields
> > >   used for flush share space with rq->rb_node.  rq->completion_data is
> > >   moved out of the union.  This increases the request size by one
> > >   pointer.
> > > 
> > >   As rq->elevator_private* are used only by the iosched too, it is
> > >   possible to reduce the request size further.  However, to do that,
> > >   we need to modify request allocation path such that iosched data is
> > >   not allocated for flush requests.
> > 
> > I decided to take a crack at using rq->elevator_private* and came up
> > with the following patch.

...

> It is an interesting duality that:
> 1) REQ_ELVPRIV is never set because priv=0 is passed to blk_alloc_request
> 2) yet when blk_free_request() checks rq->cmd_flags REQ_ELVPRIV is set;
>    resulting in the call to elv_put_request()

Turns out priv=0 was never actually being established in get_request()
on the kernel I was testing (see below).

> > I know this because in my following get_request() change to _not_ call
> > elv_set_request() for flush requests hit cfq_put_request()'s
> > BUG_ON(!cfqq->allocated[rw]).
> 
> FYI, here is the backtrace:

That backtrace was from a RHEL6 port of your recent flush merge work.
One RHELism associated with the RHEL6 flush+fua port in general (and now
this flush merge port) is that bio and request flags are _not_ shared.

As such I introduced BIO_FLUSH and BIO_FUA flags in RHEL6.  Long story
short, my 4/3 patch works just fine ontop of your 3 patches that Jens
just staged for 2.6.39.

So sorry for the noise!  I'm kicking myself for having tainted my patch
(and your work indirectly) by making an issue out of nothing.

Taking a step back, what do you think of my proposed patch?

Mike
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Tejun Heo - Jan. 26, 2011, 10:03 a.m.
Hello,

On Tue, Jan 25, 2011 at 03:41:58PM -0500, Mike Snitzer wrote:
> Unfortunately, in testing I found that flush requests that have data do
> in fact eventually get added to the queue as normal requests, via:
> 1) "data but flush is not necessary" case in blk_insert_flush
> 2) REQ_FSEQ_DATA case in blk_flush_complete_seq
> 
> I know this because in my following get_request() change to _not_ call
> elv_set_request() for flush requests hit cfq_put_request()'s
> BUG_ON(!cfqq->allocated[rw]).  cfqq->allocated[rw] gets set via
> elv_set_request()'s call to cfq_set_request().
> 
> So this seems to call in to question the running theory that flush
> requests can share 'struct request' space with elevator-specific members
> (via union) -- be it rq->rb_node or rq->elevator_private*.

As this part seems to have already been solved, I'm skipping this
part.

> diff --git a/block/blk-core.c b/block/blk-core.c
> index 72dd23b..f507888 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -764,7 +764,7 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
>  	struct request_list *rl = &q->rq;
>  	struct io_context *ioc = NULL;
>  	const bool is_sync = rw_is_sync(rw_flags) != 0;
> -	int may_queue, priv;
> +	int may_queue, priv = 0;
>  
>  	may_queue = elv_may_queue(q, rw_flags);
>  	if (may_queue == ELV_MQUEUE_NO)
> @@ -808,9 +808,14 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
>  	rl->count[is_sync]++;
>  	rl->starved[is_sync] = 0;
>  
> -	priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> -	if (priv)
> -		rl->elvpriv++;
> +	/*
> +	 * Skip elevator initialization for flush requests
> +	 */
> +	if (!(bio && (bio->bi_rw & (REQ_FLUSH | REQ_FUA)))) {
> +		priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> +		if (priv)
> +			rl->elvpriv++;
> +	}

I thought about doing it this way but I think we're burying the
REQ_FLUSH|REQ_FUA test logic too deep.  get_request() shouldn't
"magically" know not to allocate elevator data.  The decision should
be made higher in the stack and passed down to get_request().  e.g. if
REQ_SORTED is set in @rw, elevator data is allocated; otherwise, not.

> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 8a082a5..0c569ec 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -99,25 +99,29 @@ struct request {
>  	/*
>  	 * The rb_node is only used inside the io scheduler, requests
>  	 * are pruned when moved to the dispatch queue. So let the
> -	 * flush fields share space with the rb_node.
> +	 * completion_data share space with the rb_node.
>  	 */
>  	union {
>  		struct rb_node rb_node;	/* sort/lookup */
> -		struct {
> -			unsigned int			seq;
> -			struct list_head		list;
> -		} flush;
> +		void *completion_data;
>  	};
>  
> -	void *completion_data;
> -
>  	/*
>  	 * Three pointers are available for the IO schedulers, if they need
> -	 * more they have to dynamically allocate it.
> +	 * more they have to dynamically allocate it.  Let the flush fields
> +	 * share space with these three pointers.
>  	 */
> -	void *elevator_private;
> -	void *elevator_private2;
> -	void *elevator_private3;
> +	union {
> +		struct {
> +			void *private;
> +			void *private2;
> +			void *private3;
> +		} elevator;
> +		struct {
> +			unsigned int			seq;
> +			struct list_head		list;
> +		} flush;
> +	};

Another thing is, can we please make private* an array?  The number
postfixes are irksome.  It's even one based instead of zero!

Also, it would be great to better describe the lifetime difference
between the first and the second unions and why it has be organized
this way (rb_node and completion_data can live together but rb_node
and flush can't).

Thank you.
Tejun Heo - Jan. 26, 2011, 10:05 a.m.
On Wed, Jan 26, 2011 at 11:03:22AM +0100, Tejun Heo wrote:
> Also, it would be great to better describe the lifetime difference
> between the first and the second unions and why it has be organized
> this way (rb_node and completion_data can live together but rb_node
> and flush can't).

Oops, what can't live together are elevator_private* and
completion_data.

Thanks.
Mike Snitzer - Feb. 1, 2011, 5:38 p.m.
On Wed, Jan 26 2011 at  5:03am -0500,
Tejun Heo <tj@kernel.org> wrote:

> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index 72dd23b..f507888 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -764,7 +764,7 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
> >  	struct request_list *rl = &q->rq;
> >  	struct io_context *ioc = NULL;
> >  	const bool is_sync = rw_is_sync(rw_flags) != 0;
> > -	int may_queue, priv;
> > +	int may_queue, priv = 0;
> >  
> >  	may_queue = elv_may_queue(q, rw_flags);
> >  	if (may_queue == ELV_MQUEUE_NO)
> > @@ -808,9 +808,14 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
> >  	rl->count[is_sync]++;
> >  	rl->starved[is_sync] = 0;
> >  
> > -	priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> > -	if (priv)
> > -		rl->elvpriv++;
> > +	/*
> > +	 * Skip elevator initialization for flush requests
> > +	 */
> > +	if (!(bio && (bio->bi_rw & (REQ_FLUSH | REQ_FUA)))) {
> > +		priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> > +		if (priv)
> > +			rl->elvpriv++;
> > +	}
> 
> I thought about doing it this way but I think we're burying the
> REQ_FLUSH|REQ_FUA test logic too deep.  get_request() shouldn't
> "magically" know not to allocate elevator data.

There is already a considerable amount of REQ_FLUSH|REQ_FUA special
casing magic sprinkled though-out the block layer.  Why is this
get_request() change the case that goes too far?

> The decision should
> be made higher in the stack and passed down to get_request().  e.g. if
> REQ_SORTED is set in @rw, elevator data is allocated; otherwise, not.

Considering REQ_SORTED is set in elv_insert(), well after get_request() 
is called, I'm not seeing what you're suggesting.

Anyway, I agree that ideally we'd have a mechanism to explicitly
short-circuit elevator initialization.  But doing so in a meaningful way
would likely require a fair amount of refactoring of get_request* and
its callers.  I'll come back to this and have another look but my gut is
this interface churn wouldn't _really_ help -- all things considered.

> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index 8a082a5..0c569ec 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -99,25 +99,29 @@ struct request {
> >  	/*
> >  	 * The rb_node is only used inside the io scheduler, requests
> >  	 * are pruned when moved to the dispatch queue. So let the
> > -	 * flush fields share space with the rb_node.
> > +	 * completion_data share space with the rb_node.
> >  	 */
> >  	union {
> >  		struct rb_node rb_node;	/* sort/lookup */
> > -		struct {
> > -			unsigned int			seq;
> > -			struct list_head		list;
> > -		} flush;
> > +		void *completion_data;
> >  	};
> >  
> > -	void *completion_data;
> > -
> >  	/*
> >  	 * Three pointers are available for the IO schedulers, if they need
> > -	 * more they have to dynamically allocate it.
> > +	 * more they have to dynamically allocate it.  Let the flush fields
> > +	 * share space with these three pointers.
> >  	 */
> > -	void *elevator_private;
> > -	void *elevator_private2;
> > -	void *elevator_private3;
> > +	union {
> > +		struct {
> > +			void *private;
> > +			void *private2;
> > +			void *private3;
> > +		} elevator;
> > +		struct {
> > +			unsigned int			seq;
> > +			struct list_head		list;
> > +		} flush;
> > +	};
> 
> Another thing is, can we please make private* an array?  The number
> postfixes are irksome.  It's even one based instead of zero!

Sure, I can sort that out.

> > Also, it would be great to better describe the lifetime difference
> > between the first and the second unions and why it has be organized
> > this way (rb_node and completion_data can live together but rb_node
> > and flush can't).
>
> Oops, what can't live together are elevator_private* and
> completion_data.

I'll better describe the 2nd union's sharing in the next revision.

Mike
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Tejun Heo - Feb. 1, 2011, 6:52 p.m.
Hello,

On Tue, Feb 01, 2011 at 12:38:46PM -0500, Mike Snitzer wrote:
> > I thought about doing it this way but I think we're burying the
> > REQ_FLUSH|REQ_FUA test logic too deep.  get_request() shouldn't
> > "magically" know not to allocate elevator data.
> 
> There is already a considerable amount of REQ_FLUSH|REQ_FUA special
> casing magic sprinkled though-out the block layer.  Why is this
> get_request() change the case that goes too far?

After the reimplementation, FLUSH implementation seems to be pretty
well isolated.  Also, having REQ_FLUSH logic in the issue and
completion paths is logical and preventing them from leaking to other
places sounds like a good idea.

> > The decision should
> > be made higher in the stack and passed down to get_request().  e.g. if
> > REQ_SORTED is set in @rw, elevator data is allocated; otherwise, not.
> 
> Considering REQ_SORTED is set in elv_insert(), well after get_request() 
> is called, I'm not seeing what you're suggesting.

I was suggesting using REQ_SORTED in @rw parameter to indicate "this
request may be sorted and thus needs elevator data allocation".

> Anyway, I agree that ideally we'd have a mechanism to explicitly
> short-circuit elevator initialization.  But doing so in a meaningful way
> would likely require a fair amount of refactoring of get_request* and
> its callers.  I'll come back to this and have another look but my gut is
> this interface churn wouldn't _really_ help -- all things considered.

I don't know.  I agree that it's not a critical issue but, to me,
subjectively of course, it feels a bit too subtle.  The sharing of
fields using unions is already subtle enough.  I with that at least
the allocation switching would be obvious and explicit.  The combined
subtleties scare me.

Thank you.

Patch

diff --git a/block/blk-core.c b/block/blk-core.c
index 72dd23b..f507888 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -764,7 +764,7 @@  static struct request *get_request(struct request_queue *q, int rw_flags,
 	struct request_list *rl = &q->rq;
 	struct io_context *ioc = NULL;
 	const bool is_sync = rw_is_sync(rw_flags) != 0;
-	int may_queue, priv;
+	int may_queue, priv = 0;
 
 	may_queue = elv_may_queue(q, rw_flags);
 	if (may_queue == ELV_MQUEUE_NO)
@@ -808,9 +808,14 @@  static struct request *get_request(struct request_queue *q, int rw_flags,
 	rl->count[is_sync]++;
 	rl->starved[is_sync] = 0;
 
-	priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
-	if (priv)
-		rl->elvpriv++;
+	/*
+	 * Skip elevator initialization for flush requests
+	 */
+	if (!(bio && (bio->bi_rw & (REQ_FLUSH | REQ_FUA)))) {
+		priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
+		if (priv)
+			rl->elvpriv++;
+	}
 
 	if (blk_queue_io_stat(q))
 		rw_flags |= REQ_IO_STAT;
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 501ffdf..580ae0a 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -54,9 +54,9 @@  static const int cfq_hist_divisor = 4;
 #define CFQQ_SEEKY(cfqq)	(hweight32(cfqq->seek_history) > 32/8)
 
 #define RQ_CIC(rq)		\
-	((struct cfq_io_context *) (rq)->elevator_private)
-#define RQ_CFQQ(rq)		(struct cfq_queue *) ((rq)->elevator_private2)
-#define RQ_CFQG(rq)		(struct cfq_group *) ((rq)->elevator_private3)
+	((struct cfq_io_context *) (rq)->elevator.private)
+#define RQ_CFQQ(rq)		(struct cfq_queue *) ((rq)->elevator.private2)
+#define RQ_CFQG(rq)		(struct cfq_group *) ((rq)->elevator.private3)
 
 static struct kmem_cache *cfq_pool;
 static struct kmem_cache *cfq_ioc_pool;
@@ -3609,12 +3609,12 @@  static void cfq_put_request(struct request *rq)
 
 		put_io_context(RQ_CIC(rq)->ioc);
 
-		rq->elevator_private = NULL;
-		rq->elevator_private2 = NULL;
+		rq->elevator.private = NULL;
+		rq->elevator.private2 = NULL;
 
 		/* Put down rq reference on cfqg */
 		cfq_put_cfqg(RQ_CFQG(rq));
-		rq->elevator_private3 = NULL;
+		rq->elevator.private3 = NULL;
 
 		cfq_put_queue(cfqq);
 	}
@@ -3702,9 +3702,9 @@  new_queue:
 
 	cfqq->allocated[rw]++;
 	cfqq->ref++;
-	rq->elevator_private = cic;
-	rq->elevator_private2 = cfqq;
-	rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);
+	rq->elevator.private = cic;
+	rq->elevator.private2 = cfqq;
+	rq->elevator.private3 = cfq_ref_get_cfqg(cfqq->cfqg);
 
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
diff --git a/block/elevator.c b/block/elevator.c
index 270e097..02b66be 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -764,7 +764,7 @@  int elv_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
 	if (e->ops->elevator_set_req_fn)
 		return e->ops->elevator_set_req_fn(q, rq, gfp_mask);
 
-	rq->elevator_private = NULL;
+	rq->elevator.private = NULL;
 	return 0;
 }
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 8a082a5..0c569ec 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -99,25 +99,29 @@  struct request {
 	/*
 	 * The rb_node is only used inside the io scheduler, requests
 	 * are pruned when moved to the dispatch queue. So let the
-	 * flush fields share space with the rb_node.
+	 * completion_data share space with the rb_node.
 	 */
 	union {
 		struct rb_node rb_node;	/* sort/lookup */
-		struct {
-			unsigned int			seq;
-			struct list_head		list;
-		} flush;
+		void *completion_data;
 	};
 
-	void *completion_data;
-
 	/*
 	 * Three pointers are available for the IO schedulers, if they need
-	 * more they have to dynamically allocate it.
+	 * more they have to dynamically allocate it.  Let the flush fields
+	 * share space with these three pointers.
 	 */
-	void *elevator_private;
-	void *elevator_private2;
-	void *elevator_private3;
+	union {
+		struct {
+			void *private;
+			void *private2;
+			void *private3;
+		} elevator;
+		struct {
+			unsigned int			seq;
+			struct list_head		list;
+		} flush;
+	};
 
 	struct gendisk *rq_disk;
 	struct hd_struct *part;