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Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect grant with 64KB pages

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  Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect  grant with 64KB pages Roger Pau Monné <roger.pau@citrix.com> - 2015-10-06 11:40 +0200
    Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect  grant with 64KB pages Julien Grall <julien.grall@citrix.com> - 2015-10-06 12:00 +0200
      Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect  grant with 64KB pages Roger Pau Monné <roger.pau@citrix.com> - 2015-10-06 12:10 +0200
        Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect  grant with 64KB pages Julien Grall <julien.grall@citrix.com> - 2015-10-12 20:10 +0200

#1240315 — Re: [Xen-devel] [PATCH 2/2] block/xen-blkfront: Handle non-indirect grant with 64KB pages

El 05/10/15 a les 19.05, Julien Grall ha escrit:
> On 05/10/15 17:01, Roger Pau Monné wrote:
>> El 11/09/15 a les 21.32, Julien Grall ha escrit:
>>> The minimal size of request in the block framework is always PAGE_SIZE.
>>> It means that when 64KB guest is support, the request will at least be
>>> 64KB.
>>>
>>> Although, if the backend doesn't support indirect grant (such as QDISK
>>> in QEMU), a ring request is only able to accomodate 11 segments of 4KB
>>> (i.e 44KB).
>>>
>>> The current frontend is assuming that an I/O request will always fit in
>>> a ring request. This is not true any more when using 64KB page
>>> granularity and will therefore crash during the boot.
>>                                        ^ during boot.
>>>
>>> On ARM64, the ABI is completely neutral to the page granularity used by
>>> the domU. The guest has the choice between different page granularity
>>> supported by the processors (for instance on ARM64: 4KB, 16KB, 64KB).
>>> This can't be enforced by the hypervisor and therefore it's possible to
>>> run guests using different page granularity.
>>>
>>> So we can't mandate the block backend to support non-indirect grant
>>> when the frontend is using 64KB page granularity and have to fix it
>>> properly in the frontend.
>>>
>>> The solution exposed below is based on modifying directly the frontend
>>> guest rather than asking the block framework to support smaller size
>>> (i.e < PAGE_SIZE). This is because the change is the block framework are
>>> not trivial as everything seems to relying on a struct *page (see [1]).
>>> Although, it may be possible that someone succeed to do it in the future
>>> and we would therefore be able to use advantage.
>>                                        ^ it. (no advantage IMHO)
>>>
>>> Given that a block request may not fit in a single ring request, a
>>> second request is introduced for the data that cannot fit in the first
>>> one. This means that the second request should never be used on Linux
>>> configuration using a page granularity < 44KB.
>>   ^ if the page size is smaller than 44KB.
>>>
>>> Note that the parameters blk_queue_max_* helpers haven't been updated.
>>> The block code will set mimimum size supported and we may be able  to
>>                           ^ the minimum                extra space ^
>>> support directly any change in the block framework that lower down the
>>> mimimal size of a request.
>>   ^ minimal
>>
>> I have a concern regarding the splitting done in this patch.
>>
>> What happens with FUA requests when split? For example the frontend
>> receives a FUA requests with 64KB of data, and this is split into two
>> different requests on the ring, is this going to cause trouble in the
>> higher layers if for example the first request is completed but the
>> second is not? Could we leave the disk in a bad state as a consequence
>> of this?
> 
> If a block request is split into two ring requests, we will wait the two
> responses before reporting the completion to the higher layers (see
> blkif_interrupt and blkif_completion).
> 
> Furthermore, the second ring request will always use the same operation
> as the first one. Note that you will flush twice which is not nice but
> could be improved.
> 
>>
>>> [1] http://lists.xen.org/archives/html/xen-devel/2015-08/msg02200.html
>>>
>>> Signed-off-by: Julien Grall <julien.grall@citrix.com>
>>>
>>> ---
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
>>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>> Cc: David Vrabel <david.vrabel@citrix.com>
>>> ---
>>>  drivers/block/xen-blkfront.c | 199 +++++++++++++++++++++++++++++++++++++++----
>>>  1 file changed, 183 insertions(+), 16 deletions(-)
>>>
>>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>>> index f9d55c3..03772c9 100644
>>> --- a/drivers/block/xen-blkfront.c
>>> +++ b/drivers/block/xen-blkfront.c
>>> @@ -60,6 +60,20 @@
>>>  
>>>  #include <asm/xen/hypervisor.h>
>>>  
>>> +/*
>>> + * The block framework is always working on segment of PAGE_SIZE minimum.
>>
>> The above sentence needs to be reworded.
> 
> What about:
> 
> "The mininal size of the segment supported by the block framework is
> PAGE_SIZE."

That sounds fine.

> 
>>
>>> + * When Linux is using a different page size than xen, it may not be possible
>>> + * to put all the data in a single segment.
>>> + * This can happen when the backend doesn't support indirect grant and
>>                                      indirect requests ^
>>> + * therefore the maximum amount of data that a request can carry is
>>> + * BLKIF_MAX_SEGMENTS_PER_REQUEST * XEN_PAGE_SIZE = 44KB
>>> + *
>>> + * Note that we only support one extra request. So the Linux page size
>>> + * should be <= ( 2 * BLKIF_MAX_SEGMENTS_PER_REQUEST * XEN_PAGE_SIZE) =
>>> + * 88KB.
>>> + */
>>> +#define HAS_EXTRA_REQ (BLKIF_MAX_SEGMENTS_PER_REQUEST < XEN_PFN_PER_PAGE)
>>
>> Since you are already introducing a preprocessor define, I would like
>> the code added to be protected by it.
> 
> Most of the code is protected by an if (HAS_REQ_EXTRA && ...) or
> variable setting based on HAS_REQ_EXTRA. Furthermore there is extra
> protection with some BUG_ON.
> 
> I would rather avoid to use the preprocessor to avoid ending up with:
> 
> #ifdef HAS_EXTRA_REQ
> /* Code */
> #else
> /* Code */
> #endif
> 
> and potentially miss update on the ARM64 with 64KB pages because
> currently most of people are developing PV drivers on x86.

Ok, I was also thinking whether a compile time assert would be fine:

BUILD_BUG_ON(PAGE_SIZE > 2 * BLKIF_MAX_SEGMENTS_PER_REQUEST *
XEN_PAGE_SIZE);

But then if indirect descriptors are available the assert is no longer true.

>>>  enum blkif_state {
>>>  	BLKIF_STATE_DISCONNECTED,
>>>  	BLKIF_STATE_CONNECTED,
>>> @@ -79,6 +93,19 @@ struct blk_shadow {
>>>  	struct grant **indirect_grants;
>>>  	struct scatterlist *sg;
>>>  	unsigned int num_sg;
>>
>> #if HAS_EXTRA_REQ
>>
>>> +	enum
>>> +	{
>>> +		REQ_WAITING,
>>> +		REQ_DONE,
>>> +		REQ_FAIL
>>> +	} status;
>>> +
>>> +	#define NO_ASSOCIATED_ID ~0UL
>>> +	/*
>>> +	 * Id of the sibling if we ever need 2 requests when handling a
>>> +	 * block I/O request
>>> +	 */
>>> +	unsigned long associated_id;
>>
>> #endif
> 
> See my remark above.
> 
>>
>>>  };
>>>  
>>>  struct split_bio {
>>> @@ -467,6 +494,8 @@ static unsigned long blkif_ring_get_request(struct blkfront_info *info,
>>>  
>>>  	id = get_id_from_freelist(info);
>>>  	info->shadow[id].request = req;
>>> +	info->shadow[id].status = REQ_WAITING;
>>> +	info->shadow[id].associated_id = NO_ASSOCIATED_ID;
>>>  
>>>  	(*ring_req)->u.rw.id = id;
>>>  
>>> @@ -508,6 +537,9 @@ struct setup_rw_req {
>>>  	bool need_copy;
>>>  	unsigned int bvec_off;
>>>  	char *bvec_data;
>>> +
>>> +	bool require_extra_req;
>>> +	struct blkif_request *ring_req2;
>>
>> extra_ring_req?
> 
> Will do.
> 
> 
>>>  };
>>>  
>>>  static void blkif_setup_rw_req_grant(unsigned long gfn, unsigned int offset,
>>> @@ -521,8 +553,24 @@ static void blkif_setup_rw_req_grant(unsigned long gfn, unsigned int offset,
>>>  	unsigned int grant_idx = setup->grant_idx;
>>>  	struct blkif_request *ring_req = setup->ring_req;
>>>  	struct blkfront_info *info = setup->info;
>>> +	/*
>>> +	 * We always use the shadow of the first request to store the list
>>> +	 * of grant associated to the block I/O request. This made the
>>               ^ grants                                        ^ makes
>>> +	 * completion more easy to handle even if the block I/O request is
>>> +	 * split.
>>> +	 */
>>>  	struct blk_shadow *shadow = &info->shadow[setup->id];
>>>  
>>> +	if (unlikely(setup->require_extra_req &&
>>> +		     grant_idx >= BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
>>> +		/*
>>> +		 * We are using the second request, setup grant_idx
>>> +		 * to be the index of the segment array
>>> +		 */
>>> +		grant_idx -= BLKIF_MAX_SEGMENTS_PER_REQUEST;
>>> +		ring_req = setup->ring_req2;
>>> +	}
>>> +
>>>  	if ((ring_req->operation == BLKIF_OP_INDIRECT) &&
>>>  	    (grant_idx % GRANTS_PER_INDIRECT_FRAME == 0)) {
>>>  		if (setup->segments)
>>> @@ -537,7 +585,11 @@ static void blkif_setup_rw_req_grant(unsigned long gfn, unsigned int offset,
>>>  
>>>  	gnt_list_entry = get_grant(&setup->gref_head, gfn, info);
>>>  	ref = gnt_list_entry->gref;
>>> -	shadow->grants_used[grant_idx] = gnt_list_entry;
>>> +	/*
>>> +	 * All the grants are stored in the shadow of the first
>>> +	 * request. Therefore we have to use the global index
>>> +	 */
>>> +	shadow->grants_used[setup->grant_idx] = gnt_list_entry;
>>>  
>>>  	if (setup->need_copy) {
>>>  		void *shared_data;
>>> @@ -579,11 +631,31 @@ static void blkif_setup_rw_req_grant(unsigned long gfn, unsigned int offset,
>>>  	(setup->grant_idx)++;
>>>  }
>>>  
>>> +static void blkif_setup_extra_req(struct blkif_request *first,
>>> +				  struct blkif_request *second)
>>> +{
>>> +	uint16_t nr_segments = first->u.rw.nr_segments;
>>> +
>>> +	/*
>>> +	 * The second request is only present when the first request uses
>>> +	 * all its segments. It's always the continuity of the first one
>>> +	 */
>>> +	first->u.rw.nr_segments = BLKIF_MAX_SEGMENTS_PER_REQUEST;
>>> +
>>> +	second->u.rw.nr_segments = nr_segments - BLKIF_MAX_SEGMENTS_PER_REQUEST;
>>> +	second->u.rw.sector_number = first->u.rw.sector_number +
>>> +		(BLKIF_MAX_SEGMENTS_PER_REQUEST * XEN_PAGE_SIZE) / 512;
>>
>> Does this need to take into account if sectors have been skipped in the
>> previous requests due to empty data?
> 
> I'm not sure to understand this question.
> 
> AFAIU, the data is always contiguous in a block segment even though it
> can be split accross multiple Linux page. The number of segments is
> calculated from the amount of data see "Calculate the number of grant
> used" the code. The second request will only be created if this number
> is greater than BLKIF_MAX_SEGMENTS_PER_REQUEST.
> 
> So the second request will always be the continuity of the first one.

Ack.

>> Also I'm not sure how correct it is to hardcode 512 here.
> 
> Well, we are hardcoding it in blkfront and the block framework does the
> same (see blk_limits_max_hw_sectors).

Right, seems like the whole block system is always working with 512 as
the basic unit.

>>
>>> +
>>> +	second->u.rw.handle = first->u.rw.handle;
>>> +	second->operation = first->operation;
>>> +}
>>> +
>>>  static int blkif_queue_rw_req(struct request *req)
>>>  {
>>>  	struct blkfront_info *info = req->rq_disk->private_data;
>>> -	struct blkif_request *ring_req;
>>> -	unsigned long id;
>>> +	struct blkif_request *ring_req, *ring_req2 = NULL;
>>> +	unsigned long id, id2 = NO_ASSOCIATED_ID;
>>> +	bool require_extra_req = false;
>>>  	int i;
>>>  	struct setup_rw_req setup = {
>>>  		.grant_idx = 0,
>>> @@ -628,19 +700,19 @@ static int blkif_queue_rw_req(struct request *req)
>>>  	/* Fill out a communications ring structure. */
>>>  	id = blkif_ring_get_request(info, req, &ring_req);
>>>  
>>> -	BUG_ON(info->max_indirect_segments == 0 &&
>>> -	       GREFS(req->nr_phys_segments) > BLKIF_MAX_SEGMENTS_PER_REQUEST);
>>> -	BUG_ON(info->max_indirect_segments &&
>>> -	       GREFS(req->nr_phys_segments) > info->max_indirect_segments);
>>> -
>>>  	num_sg = blk_rq_map_sg(req->q, req, info->shadow[id].sg);
>>>  	num_grant = 0;
>>>  	/* Calculate the number of grant used */
>>>  	for_each_sg(info->shadow[id].sg, sg, num_sg, i)
>>>  	       num_grant += gnttab_count_grant(sg->offset, sg->length);
>>>  
>>> +	require_extra_req = info->max_indirect_segments == 0 &&
>>> +		num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST;
>>> +	BUG_ON(!HAS_EXTRA_REQ && require_extra_req);
>>> +
>>>  	info->shadow[id].num_sg = num_sg;
>>> -	if (num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST) {
>>> +	if (num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST &&
>>> +	    likely(!require_extra_req)) {
>>>  		/*
>>>  		 * The indirect operation can only be a BLKIF_OP_READ or
>>>  		 * BLKIF_OP_WRITE
>>> @@ -680,10 +752,29 @@ static int blkif_queue_rw_req(struct request *req)
>>>  			}
>>>  		}
>>>  		ring_req->u.rw.nr_segments = num_grant;
>>> +		if (unlikely(require_extra_req)) {
>>> +			id2 = blkif_ring_get_request(info, req, &ring_req2);
>>
>> How can you guarantee that there's always going to be another free
>> request? AFAICT blkif_queue_rq checks for RING_FULL, but you don't
>> actually know if there's only one slot or more than one available.
> 
> Because the depth of the queue is divided by 2 when the extra request is
> used (see xlvbd_init_blk_queue).

I see, that's quite restrictive but I guess it's better than introducing
a new ring macro in order to figure out if there are at least two free
slots.

Roger.

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#1240327

Hi Roger,

On 06/10/2015 10:39, Roger Pau Monné wrote:
> El 05/10/15 a les 19.05, Julien Grall ha escrit:
>> On 05/10/15 17:01, Roger Pau Monné wrote:
>>> El 11/09/15 a les 21.32, Julien Grall ha escrit:
>>>>   		ring_req->u.rw.nr_segments = num_grant;
>>>> +		if (unlikely(require_extra_req)) {
>>>> +			id2 = blkif_ring_get_request(info, req, &ring_req2);
>>>
>>> How can you guarantee that there's always going to be another free
>>> request? AFAICT blkif_queue_rq checks for RING_FULL, but you don't
>>> actually know if there's only one slot or more than one available.
>>
>> Because the depth of the queue is divided by 2 when the extra request is
>> used (see xlvbd_init_blk_queue).

I just noticed that I didn't mention this restriction in the commit 
message. I will do it in the next revision.

> I see, that's quite restrictive but I guess it's better than introducing
> a new ring macro in order to figure out if there are at least two free
> slots.

I actually didn't think about your suggestion. I choose to divide by two 
based on the assumption that the block framework will always try to send 
a request with the maximum data possible.

I don't know if this assumption is correct as I'm not fully aware how 
the block framework is working.

If it's valid, in the case of 64KB guest, the maximum size of a request 
would be 64KB when indirect segment is not supported. So we would end up 
with a lot of 64KB request which will require 2 ring request.

Regards,

-- 
Julien Grall
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#1240335

El 06/10/15 a les 11.58, Julien Grall ha escrit:
> Hi Roger,
> 
> On 06/10/2015 10:39, Roger Pau Monné wrote:
>> El 05/10/15 a les 19.05, Julien Grall ha escrit:
>>> On 05/10/15 17:01, Roger Pau Monné wrote:
>>>> El 11/09/15 a les 21.32, Julien Grall ha escrit:
>>>>>           ring_req->u.rw.nr_segments = num_grant;
>>>>> +        if (unlikely(require_extra_req)) {
>>>>> +            id2 = blkif_ring_get_request(info, req, &ring_req2);
>>>>
>>>> How can you guarantee that there's always going to be another free
>>>> request? AFAICT blkif_queue_rq checks for RING_FULL, but you don't
>>>> actually know if there's only one slot or more than one available.
>>>
>>> Because the depth of the queue is divided by 2 when the extra request is
>>> used (see xlvbd_init_blk_queue).
> 
> I just noticed that I didn't mention this restriction in the commit
> message. I will do it in the next revision.
> 
>> I see, that's quite restrictive but I guess it's better than introducing
>> a new ring macro in order to figure out if there are at least two free
>> slots.
> 
> I actually didn't think about your suggestion. I choose to divide by two
> based on the assumption that the block framework will always try to send
> a request with the maximum data possible.

AFAIK that depends on the request itself, the block layer will try to
merge requests if possible, but you can also expect that there are going
to be requests that will just contain a single block.

> I don't know if this assumption is correct as I'm not fully aware how
> the block framework is working.
> 
> If it's valid, in the case of 64KB guest, the maximum size of a request
> would be 64KB when indirect segment is not supported. So we would end up
> with a lot of 64KB request which will require 2 ring request.

I guess you could add a counter in order to see how many requests were
split vs total number of requests.

Roger.

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#1244974

On 06/10/15 11:06, Roger Pau Monné wrote:
> El 06/10/15 a les 11.58, Julien Grall ha escrit:
>> Hi Roger,
>>
>> On 06/10/2015 10:39, Roger Pau Monné wrote:
>>> El 05/10/15 a les 19.05, Julien Grall ha escrit:
>>>> On 05/10/15 17:01, Roger Pau Monné wrote:
>>>>> El 11/09/15 a les 21.32, Julien Grall ha escrit:
>>>>>>           ring_req->u.rw.nr_segments = num_grant;
>>>>>> +        if (unlikely(require_extra_req)) {
>>>>>> +            id2 = blkif_ring_get_request(info, req, &ring_req2);
>>>>>
>>>>> How can you guarantee that there's always going to be another free
>>>>> request? AFAICT blkif_queue_rq checks for RING_FULL, but you don't
>>>>> actually know if there's only one slot or more than one available.
>>>>
>>>> Because the depth of the queue is divided by 2 when the extra request is
>>>> used (see xlvbd_init_blk_queue).
>>
>> I just noticed that I didn't mention this restriction in the commit
>> message. I will do it in the next revision.
>>
>>> I see, that's quite restrictive but I guess it's better than introducing
>>> a new ring macro in order to figure out if there are at least two free
>>> slots.
>>
>> I actually didn't think about your suggestion. I choose to divide by two
>> based on the assumption that the block framework will always try to send
>> a request with the maximum data possible.
> 
> AFAIK that depends on the request itself, the block layer will try to
> merge requests if possible, but you can also expect that there are going
> to be requests that will just contain a single block.
> 
>> I don't know if this assumption is correct as I'm not fully aware how
>> the block framework is working.
>>
>> If it's valid, in the case of 64KB guest, the maximum size of a request
>> would be 64KB when indirect segment is not supported. So we would end up
>> with a lot of 64KB request which will require 2 ring request.
> 
> I guess you could add a counter in order to see how many requests were
> split vs total number of requests.

So the number of 64KB request is fairly small compare to the total
number of request (277 for 4687 request) for general usage (i.e cd, find).

Although as soon as I use dd, the block request will be merged. So I
guess a common usage will not provide enough data to fill a 64KB request.

Although as soon as I use dd with a block size of 64KB, most of the
request fill 64KB and an extra request is required.

Note that I had to implement quickly xen_biovec_phys_mergeable for 64KB
page as I left this aside. Without it, the biovec won't be merge except
with dd if you specific the block size (bs=64KB).

I've also looked to the code to see if it's possible to check if there
is 2 ring requests free and if not waiting until they are available.

Currently, we don't need to check if a request if free because the queue
is sized according to the number of request supported by the ring. This
means that the block layer is handling the check and we will always have
space in the ring.

If we decide to avoid dividing the number of request enqueue by the
block layer, we would have to handle ourself if there is 2 ring requests
free.
AFAICT, when BLK_MQ_RQ_BUSY is returned the block layer will stop the
queue. So we need to have some logic in blkfront to know when the 2 ring
requests become free and restart the queue. I guest it would be similar
to gnttab_request_free_callback.

I'd like your advice to know whether this is worth to implement it in
blockfront given that it will be only used for 64KB guest with backend
not supporting indirect grant.

Regards,

-- 
Julien Grall
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