On Thu, Sep 01, 2022 at 05:36:57PM +0530, Kashyap Desai wrote: > > -----Original Message----- > > From: Jason Gunthorpe [mailto:jgg@xxxxxxxxxx] > > Sent: Friday, August 26, 2022 6:45 PM > > To: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx> > > Cc: linux-rdma@xxxxxxxxxxxxxxx; leonro@xxxxxxxxxx; Selvin Xavier > > <selvin.xavier@xxxxxxxxxxxx>; Andrew Gospodarek > > <andrew.gospodarek@xxxxxxxxxxxx> > > Subject: Re: [PATCH rdma-rc v1] RDMA/core: fix sg_to_page mapping for > > boundary condition > > > > On Mon, Aug 22, 2022 at 07:51:22PM +0530, Kashyap Desai wrote: > > > > > Now, we will enter into below loop with dma_addr = page_addr = > > > 0xffffffffffffe000 and "end_dma_addr = dma_addr + dma_len" is ZERO. > > > eval 0xffffffffffffe000 + 8192 > > > hexadecimal: 0 > > > > This is called overflow. > > Is this not DMAable for 64bit DMA mask device ? It is DMAable. So not sure > why you call it as overflow. ? Beacuse the normal math overflowed. Should it work? Yes. Is it a special edge case that might have bugs? Certainly. So the IOMMU layer shouldn't be stressing this edge case at all. It is crazy, there is no reason to do this. > I agree that such mapping is obviously dangerous, but it is not illegal as > well. > Same sgl mapping works if it is direct attached Storage, so there will be > a logical question why IB stack is not handling this. Oh that is probably very driver dependent. > > You need to write the code so you never create the situation where > > A+B=0 - don't try to fix things up after that happens. > > In proposed patch, A + B = 0 is possible, but it will be considered as end > of the loop. Like I said, don't do that. End of the loop is -1 which requires a different loop logic design, so send a patch like that. But I would still send a patch for iommu to not create this in the first place. Jason
