[Xen-devel] Re: [PATCH V4 2/3] xen-blkfront: teach blkfront driver to handle discard requests

[Li Dongyang <lidongyang@xxxxxxxxxx>]

On Thu, Sep 1, 2011 at 11:25 PM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Thu, Sep 01, 2011 at 06:39:09PM +0800, Li Dongyang wrote:
>> The blkfront driver now will read discard related nodes from xenstore,
>> and set up the request queue, then we can forward the
>> discard requests to backend driver.
>
>
> A better description is as follow:
>
> xen-blkfront: Handle discard requests.
>
>    If the backend advertises 'feature-discard', then interrogate
>    the backend for alignment, granularity, and max discard block size.
>    Setup the request queue with the appropiate values and send the
>    discard operation as required.
>
thanks for the description, way better than mine :-)
>
>>
>> Signed-off-by: Li Dongyang <lidongyang@xxxxxxxxxx>
>> ---
>>  drivers/block/xen-blkfront.c |  111 
>> +++++++++++++++++++++++++++++++++---------
>>  1 files changed, 88 insertions(+), 23 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index 9ea8c25..86e2c63 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -98,6 +98,9 @@ struct blkfront_info
>>       unsigned long shadow_free;
>>       unsigned int feature_flush;
>>       unsigned int flush_op;
>> +     unsigned int feature_discard;
>> +     unsigned int discard_granularity;
>> +     unsigned int discard_alignment;
>>       int is_ready;
>>  };
>>
>> @@ -302,29 +305,36 @@ static int blkif_queue_request(struct request *req)
>>               ring_req->operation = info->flush_op;
>>       }
>>
>> -     ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
>> -     BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
>> +     if (unlikely(req->cmd_flags & REQ_DISCARD)) {
>> +             /* id, sector_number and handle are set above. */
>> +             ring_req->operation = BLKIF_OP_DISCARD;
>> +             ring_req->nr_segments = 0;
>> +             ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
>> +     } else {
>> +             ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
>> +             BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
>>
>> -     for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
>> -             buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
>> -             fsect = sg->offset >> 9;
>> -             lsect = fsect + (sg->length >> 9) - 1;
>> -             /* install a grant reference. */
>> -             ref = gnttab_claim_grant_reference(&gref_head);
>> -             BUG_ON(ref == -ENOSPC);
>> +             for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
>> +                     buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
>> +                     fsect = sg->offset >> 9;
>> +                     lsect = fsect + (sg->length >> 9) - 1;
>> +                     /* install a grant reference. */
>> +                     ref = gnttab_claim_grant_reference(&gref_head);
>> +                     BUG_ON(ref == -ENOSPC);
>>
>> -             gnttab_grant_foreign_access_ref(
>> -                             ref,
>> -                             info->xbdev->otherend_id,
>> -                             buffer_mfn,
>> -                             rq_data_dir(req) );
>> -
>> -             info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
>> -             ring_req->u.rw.seg[i] =
>> -                             (struct blkif_request_segment) {
>> -                                     .gref       = ref,
>> -                                     .first_sect = fsect,
>> -                                     .last_sect  = lsect };
>> +                     gnttab_grant_foreign_access_ref(
>> +                                     ref,
>> +                                     info->xbdev->otherend_id,
>> +                                     buffer_mfn,
>> +                                     rq_data_dir(req));
>> +
>> +                     info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
>> +                     ring_req->u.rw.seg[i] =
>> +                                     (struct blkif_request_segment) {
>> +                                             .gref       = ref,
>> +                                             .first_sect = fsect,
>> +                                             .last_sect  = lsect };
>> +             }
>>       }
>>
>>       info->ring.req_prod_pvt++;
>> @@ -399,6 +409,7 @@ wait:
>>  static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
>>  {
>>       struct request_queue *rq;
>> +     struct blkfront_info *info = gd->private_data;
>>
>>       rq = blk_init_queue(do_blkif_request, &blkif_io_lock);
>>       if (rq == NULL)
>> @@ -406,6 +417,13 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 
>> sector_size)
>>
>>       queue_flag_set_unlocked(QUEUE_FLAG_VIRT, rq);
>>
>> +     if (info->feature_discard) {
>> +             queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, rq);
>> +             blk_queue_max_discard_sectors(rq, get_capacity(gd));
>
> This is not correct. I took a look at the SCSI support for this
> ('sd_config_discard') and if you look there carefully you will see that
> the value can be 64KB for example.
well I don't think so, max_discard_sectors are used to split the bio,
make them not to
exceed the max_discard_sectors, see blkdev_issue_discard, and
that's fine if we use the capacity as max_discard_sectors
in guest: for file backend, there's no need for max_discard_sectors,
and for phy backend, we'll make the blkfront sent out a single big
discard request,
and when we deal with that in blkback by calling blkdev_issue_discard,
the real max_discard_sectors
from phy dev will be used, and take care of splitting bios, Thanks

>
> You need to provide a mechanism similar to 'discard-*' to fetch that data
> from the backend.
>
>> +             rq->limits.discard_granularity = info->discard_granularity;
>> +             rq->limits.discard_alignment = info->discard_alignment;
>> +     }
>> +
>>       /* Hard sector size and max sectors impersonate the equiv. hardware. */
>>       blk_queue_logical_block_size(rq, sector_size);
>>       blk_queue_max_hw_sectors(rq, 512);
>> @@ -722,6 +740,19 @@ static irqreturn_t blkif_interrupt(int irq, void 
>> *dev_id)
>>
>>               error = (bret->status == BLKIF_RSP_OKAY) ? 0 : -EIO;
>>               switch (bret->operation) {
>> +             case BLKIF_OP_DISCARD:
>> +                     if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
>> +                             struct request_queue *rq = info->rq;
>> +                             printk(KERN_WARNING "blkfront: %s: discard op 
>> failed\n",
>> +                                        info->gd->disk_name);
>> +                             error = -EOPNOTSUPP;
>> +                             info->feature_discard = 0;
>> +                             spin_lock(rq->queue_lock);
>> +                             queue_flag_clear(QUEUE_FLAG_DISCARD, rq);
>> +                             spin_unlock(rq->queue_lock);
>> +                     }
>> +                     __blk_end_request_all(req, error);
>> +                     break;
>>               case BLKIF_OP_FLUSH_DISKCACHE:
>>               case BLKIF_OP_WRITE_BARRIER:
>>                       if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
>> @@ -1098,6 +1129,33 @@ blkfront_closing(struct blkfront_info *info)
>>       bdput(bdev);
>>  }
>>
>> +static void blkfront_setup_discard(struct blkfront_info *info)
>> +{
>> +     int err;
>> +     char *type;
>> +     unsigned int discard_granularity;
>> +     unsigned int discard_alignment;
>> +
>> +     type = xenbus_read(XBT_NIL, info->xbdev->otherend, "type", NULL);
>> +     if (IS_ERR(type))
>> +             return;
>> +
>> +     if (strncmp(type, "phy", 3) == 0) {
>> +             err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
>> +                     "discard-granularity", "%u", &discard_granularity,
>> +                     "discard-alignment", "%u", &discard_alignment,
>> +                     NULL);
>> +             if (!err) {
>> +                     info->feature_discard = 1;
>> +                     info->discard_granularity = discard_granularity;
>> +                     info->discard_alignment = discard_alignment;
>> +             }
>> +     } else if (strncmp(type, "file", 4) == 0)
>> +             info->feature_discard = 1;
>> +
>> +     kfree(type);
>> +}
>> +
>>  /*
>>   * Invoked when the backend is finally 'ready' (and has told produced
>>   * the details about the physical device - #sectors, size, etc).
>> @@ -1108,7 +1166,7 @@ static void blkfront_connect(struct blkfront_info 
>> *info)
>>       unsigned long sector_size;
>>       unsigned int binfo;
>>       int err;
>> -     int barrier, flush;
>> +     int barrier, flush, discard;
>>
>>       switch (info->connected) {
>>       case BLKIF_STATE_CONNECTED:
>> @@ -1178,7 +1236,14 @@ static void blkfront_connect(struct blkfront_info 
>> *info)
>>               info->feature_flush = REQ_FLUSH;
>>               info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
>>       }
>> -
>> +
>> +     err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
>> +                         "feature-discard", "%d", &discard,
>> +                         NULL);
>> +
>> +     if (!err && discard)
>> +             blkfront_setup_discard(info);
>> +
>>       err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
>>       if (err) {
>>               xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
>> --
>> 1.7.6
>

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