Messages in this thread | | | Subject | Re: [PATCH] net: add per device sg_max_frags for skb | From | Hans Westgaard Ry <> | Date | Fri, 8 Jan 2016 10:55:56 +0100 |
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On 01/06/2016 02:59 PM, David Laight wrote: > From: Hans Westgaard Ry >> Sent: 06 January 2016 13:16 >> Devices may have limits on the number of fragments in an skb they >> support. Current codebase uses a constant as maximum for number of >> fragments (MAX_SKB_FRAGS) one skb can hold and use. >> >> When enabling scatter/gather and running traffic with many small >> messages the codebase uses the maximum number of fragments and thereby >> violates the max for certain devices. >> >> An example of such a violation is when running IPoIB on a HCA >> supporting 16 SGE on an architecture with 4K pagesize. The >> MAX_SKB_FRAGS will be 17 (64K/4K+1) and because IPoIB adds yet another >> segment we end up with send_requests with 18 SGE resulting in >> kernel-panic. >> >> The patch allows the device to limit the maximum number fragments used >> in one skb. > This doesn't seem to me to be the correct way to fix this. > Anything that adds an extra fragment (in this case IPoIB) should allow > for the skb already having the maximum number of fragments. > Fully linearising the skb is overkill, but I think the first fragment > can be added to the linear part of the skb. > > David > > When IpoIB handles a skb-request it converts fragments to SGEs to be handled by a HCA. The problem arises when the HCA have a limited number of SGEs less than MAX_SKB_FRAGS. (it gets a little worse since IPoIB need to yet another segment) I have not found any easy way of fixing this with currenct codebase.
Hans
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