Messages in this thread |  | | Subject | Re: [Patch v3 2/6] cifs: Allocate validate negotiation request through kmalloc | From | Tom Talpey <> | Date | Wed, 18 Apr 2018 13:40:16 -0400 |
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On 4/18/2018 1:11 PM, Long Li wrote: >> Subject: Re: [Patch v3 2/6] cifs: Allocate validate negotiation request through >> kmalloc >> >> On 4/18/2018 9:08 AM, David Laight wrote: >>> From: Tom Talpey >>>> Sent: 18 April 2018 12:32 >>> ... >>>> On 4/17/2018 8:33 PM, Long Li wrote: >>>>> From: Long Li <longli@microsoft.com> >>>>> >>>>> The data buffer allocated on the stack can't be DMA'ed, and hence >>>>> can't send through RDMA via SMB Direct. >>>> >>>> This comment is confusing. Any registered memory can be DMA'd, need >>>> to state the reason for the choice here more clearly. >>> >>> The stack could be allocated with vmalloc(). >>> In which case the pages might not be physically contiguous and there >>> is no >>> (sensible) call to get the physical address required by the dma >>> controller (or other bus master). >> >> Memory registration does not requires pages to be physically contiguous. >> RDMA Regions can and do support very large physical page scatter/gather, >> and the adapter DMA's them readily. Is this the only reason? > > ib_dma_map_page will return an invalid DMA address for a buffer on stack. Even worse, this incorrect address can't be detected by ib_dma_mapping_error. Sending data from this address to hardware will not fail, but the remote peer will get junk data. > > I think it makes sense as stack is dynamic and can shrink as I/O proceeds, so the buffer is gone. Other kernel code use only data on the heap for DMA, e.g. BLK/SCSI layer never use buffer on the stack to send data.
I totally agree that registering the stack is a bad idea. I mainly suggest that you capture these fundamental ib_dma* reasons in the commit. There's no other practical reason why the original approach would not work.
Tom.
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