Re: [Xen-devel] [PATCH 1/5] xen-blkback: don't store dev_bus_addr

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On 19/03/13 16:10, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 19, 2013 at 02:55:56PM +0000, Jan Beulich wrote:
>>>>> On 19.03.13 at 15:32, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
>>>>> wrote:
>>> On Mon, Mar 18, 2013 at 05:49:32PM +0100, Roger Pau Monne wrote:
>>>> dev_bus_addr returned in the grant ref map operation is the mfn of the
>>>> passed page, there's no need to store it in the persistent grant
>>>> entry, since we can always get it provided that we have the page.
>>>>
>>>> This reduces the memory overhead of persistent grants in blkback.
>>>
>>> I took this patch, but I redid it a bit:
>>>
>>> commit 1d4cb410befdb8b373c6fad604b39e0200e0bee0
>>> Author: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>>> Date:   Mon Mar 18 17:49:32 2013 +0100
>>>
>>>     xen-blkback: don't store dev_bus_addr
>>>     
>>>     dev_bus_addr returned in the grant ref map operation is the mfn of the
>>>     passed page, there's no need to store it in the persistent grant
>>>     entry, since we can always get it provided that we have the page.
>>>     
>>>     This reduces the memory overhead of persistent grants in blkback.
>>>     
>>>     While at it, rename the 'seg[i].buf' to be 'seg[i].offset' as
>>>     it makes much more sense - as we use that value in bio_add_page
>>>     which as the fourth argument expects the offset.
>>>     
>>>     We hadn't used the physical address as part of this at all.
>>>     
>>>     Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>     Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>>     Cc: xen-devel@xxxxxxxxxxxxx 
>>>     [v1: s/buf/offset/]
>>>     Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>>
>>> diff --git a/drivers/block/xen-blkback/blkback.c 
>>> b/drivers/block/xen-blkback/blkback.c
>>> index 2cf8381..061c202 100644
>>> --- a/drivers/block/xen-blkback/blkback.c
>>> +++ b/drivers/block/xen-blkback/blkback.c
>>> @@ -442,7 +442,7 @@ int xen_blkif_schedule(void *arg)
>>>  }
>>>  
>>>  struct seg_buf {
>>> -   unsigned long buf;
>>> +   unsigned long offset;
>>
>> If you touch this anyway, why don't you reduce the type to
>> "unsigned int", halving the overall structure size?
>>
>> Even more, the field seems pointless to me altogether, since ...
>>
>>>     unsigned int nsec;
>>>  };
>>>  /*
>>> @@ -621,30 +621,21 @@ static int xen_blkbk_map(struct blkif_request *req,
>>>                              * If this is a new persistent grant
>>>                              * save the handler
>>>                              */
>>> -                           persistent_gnts[i]->handle = map[j].handle;
>>> -                           persistent_gnts[i]->dev_bus_addr =
>>> -                                   map[j++].dev_bus_addr;
>>> +                           persistent_gnts[i]->handle = map[j++].handle;
>>>                     }
>>>                     pending_handle(pending_req, i) =
>>>                             persistent_gnts[i]->handle;
>>>  
>>>                     if (ret)
>>>                             continue;
>>> -
>>> -                   seg[i].buf = persistent_gnts[i]->dev_bus_addr |
>>> -                           (req->u.rw.seg[i].first_sect << 9);
>>>             } else {
>>> -                   pending_handle(pending_req, i) = map[j].handle;
>>> +                   pending_handle(pending_req, i) = map[j++].handle;
>>>                     bitmap_set(pending_req->unmap_seg, i, 1);
>>>  
>>> -                   if (ret) {
>>> -                           j++;
>>> +                   if (ret)
>>>                             continue;
>>> -                   }
>>> -
>>> -                   seg[i].buf = map[j++].dev_bus_addr |
>>> -                           (req->u.rw.seg[i].first_sect << 9);
>>>             }
>>> +           seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
>>
>> ... this uses "i" as index on both sides, so ...
>>
>>>     }
>>>     return ret;
>>>  }
>>> @@ -971,7 +962,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
>>>                    (bio_add_page(bio,
>>>                                  pages[i],
>>>                                  seg[i].nsec << 9,
>>> -                                seg[i].buf & ~PAGE_MASK) == 0)) {
>>> +                                seg[i].offset & ~PAGE_MASK) == 0)) {
>>
>> ... this one could as well use the original field.
>>
>> And the masking with ~PAGE_MASK is not pointless in any case.
> 
> Good point. In which might as well make the 'struct seg_buf' be an
> simple array of unsigned int.

I didn't do this because later on (when using indirect segments) I need
a place to store the offset, since I don't have the indirect segment
pages mapped during the whole operation, but I guess I could change that.

>>
>> Jan
>>
>>>  
>>>                     bio = bio_alloc(GFP_KERNEL, nseg-i);
>>>                     if (unlikely(bio == NULL))
>>> diff --git a/drivers/block/xen-blkback/common.h 
>>> b/drivers/block/xen-blkback/common.h
>>> index da78346..60103e2 100644
>>> --- a/drivers/block/xen-blkback/common.h
>>> +++ b/drivers/block/xen-blkback/common.h
>>> @@ -187,7 +187,6 @@ struct persistent_gnt {
>>>     struct page *page;
>>>     grant_ref_t gnt;
>>>     grant_handle_t handle;
>>> -   uint64_t dev_bus_addr;
>>>     struct rb_node node;
>>>  };
>>>  
>>>
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@xxxxxxxxxxxxx 
>>> http://lists.xen.org/xen-devel 
>>
>>


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