Christoph Lameter asked: > > What does it mean that the "application layer has to be determine what > pages are registered"? The application does not know which of its pages > are currently in memory. It can only force these pages to stay in > memory if their are mlocked. >
An application that advertises an RDMA accessible buffer to a remote peer *does* have to know that its pages *are* currently in memory. The application does *not* need for the virtual-to-physical mapping of those pages to be frozen for the lifespan of the Memory Region. But it is issuing an invitation to its peer to perform direct writes to the advertised buffer. When the peer decides to exercise that invitation the pages have to be there. An analogy: when you write a check for $100 you do not have to identify the serial numbers of ten $10 bills, but you are expected to have the funds in your account. Issuing a buffer advertisement for memory you do not have is the network equivalent of writing a check that you do not have funds for. Now, just as your bank may offer overdraft protection, an RDMA device could merely report a page fault rather than tearing down the connection itself. But that does not grant permission for applications to advertise buffer space that they do not have committed, it merely helps recovery from a programming fault. A suspend/resume interface between the Virtual Memory Manager and the RDMA layer allows pages to be re-arranged at the convenience of the Virtual Memory Manager without breaking the application layer peer-to-peer contract. The current interfaces that pin exact pages are really the equivalent of having to tell the bank that when Joe cashes this $100 check that you should give him *these* ten $10 bills. It works, but it adds too much overhead and is very inflexible. So there are a lot of good reasons to evolve this interface to better deal with these issues. Other areas of possible evolution include allowing growing or trimming of Memory Regions without invalidating their advertised handles. But the more fundamental issue is recognizing that applications that use direct interfaces need to know that buffers that they enable truly have committed resources. They need a way to ask for twenty *real* pages, not twenty pages of address space. And they need to do it in a way that allows memory to be rearranged or even migrated with them to a new host. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
