Re: [PATCH V5 3/5] RDMA/iser: Limit sg tablesize and max_sectors to device fastreg max depth

[Or Gerlitz <ogerlitz-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>]

On 7/7/2015 4:59 PM, Steve Wise wrote:
>>>>> > >>>diff --git a/drivers/infiniband/ulp/iser/iscsi_iser.c b/drivers/infiniband/ulp/iser/iscsi_iser.c
>>>>> > >>>index 6a594aa..de8730d 100644
>>>>> > >>>--- a/drivers/infiniband/ulp/iser/iscsi_iser.c
>>>>> > >>>+++ b/drivers/infiniband/ulp/iser/iscsi_iser.c
>>>>> > >>>@@ -640,6 +640,15 @@ iscsi_iser_session_create(struct iscsi_endpoint *ep,
>>>>> > >>>    						   SHOST_DIX_GUARD_CRC);
>>>>> > >>>    		}
>>>>> > >>>
>>>>> > >>>+		/*
>>>>> > >>>+		 * Limit the sg_tablesize and max_sectors based on the device
>>>>> > >>>+		 * max fastreg page list length.
>>>>> > >>>+		 */
>>>>> > >>>+		shost->sg_tablesize = min_t(unsigned short, shost->sg_tablesize,
>>>>> > >>>+			ib_conn->device->dev_attr.max_fast_reg_page_list_len);
>>>>> > >>>+		shost->max_sectors = min_t(unsigned int,
>>>>> > >>>+			1024, (shost->sg_tablesize * PAGE_SIZE) >> 9);
>>>>> > >>>+
>>>> > >>
>>>> > >>The min statement is meaningless for max_sectors - you do a min between
>>>> > >>default sg_tablesize and frpl length - so the maximum sg_tablesize is
>>>> > >>128 which is 1024 max_sectors.
>>> > >
>>> > >I'm not following.  What if ib_conn->device->dev_attr.max_fast_reg_page_list_len is say, 32?
>>> > >Then shost->sg_tablesize is set to 32, and max_sectors is set to (32*4K) >> 9 == 256 512B sectors.
>> >
>> >Correct - but it cannot exceed 1024 (as it is derived from sg_tablesize
>> >which is maximum 128).
> Actually it is initialized to 1024 in iscsi_iser_sht / iscsi_iser.c, so it isn't derived from sg_tables (although it probably should be).  I can remove the min_t() though.
>
> Hey Or, thoughts?

Originally, we've put the double restriction of 128 SG entries AND 1024 
sectors to make sure that whatever SG is up there, it'snot spanning > 
512KB.

Think on SG whose one/some of their element/s is > one page or on 
systems with > 4KB page size or others examples... since your patch 
touched the number of SG entries I was thinking you need to make sure no 
regression was introduced re the max_sectors to be <= 1024

If U2 are @ consensus that this is  the case with the original patch w.o 
further changes, let it be.

Or.
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