On Thu 12-10-17 10:19:16, Mike Kravetz wrote: > On 10/12/2017 07:37 AM, Michal Hocko wrote: > > On Wed 11-10-17 18:46:11, Mike Kravetz wrote: > >> Add new MAP_CONTIG flag to mmap system call. Check for flag in normal > >> mmap flag processing. If present, pre-allocate a contiguous set of > >> pages to back the mapping. These pages will be used a fault time, and > >> the MAP_CONTIG flag implies populating the mapping at the mmap time. > > > > I have only briefly read through the previous discussion and it is still > > not clear to me _why_ we want such a interface. I didn't give it much > > time yet but I do not think this is a good idea at all. > > Thanks for looking Michal. The primary use case comes from devices that can > realize performance benefits if operating on physically contiguous memory. > What sparked this effort was Christoph and Guy's plumbers presentation > where they showed RDMA performance benefits that could be realized with > contiguous memory. I also remember sitting in a presentation about > Intel's QuackAssist technology at Vault last year. The presenter mentioned > that their compression engine needed to be passed a physically contiguous > buffer. I asked how a user could obtain such a buffer. They said they > had a special driver/ioctl for that. Yuck! I'm guessing there are other > specific use cases. That is why I wanted to start the discussion as to > whether there should be an interface to provide this functionality. I would, quite contrary, suggest a device specific mmap implementation which would guarantee both the best memory wrt. physical contiguous aspect as well as the placement - what if the device have a restriction on that as well? > > any user to simply consume larger order memory blocks? What would > > prevent from that? > > We certainly would want to put restrictions in place for contiguous > memory allocations. Since it makes sense to pre-populate and lock > contiguous allocations, using the same restrictions as mlock is a start. > However, I can see the possible need for more restrictions. Absolutely. mlock limit is per process (resp. mm) so a single user could simply deplete large blocks. No good... > > Does the memory always stays contiguous? How much contiguous it will be? > > Yes, it remains contiguous. It is locked in memory. Hmm, so hugetlb on steroids... > > Who is going to use such an interface? And probably many other > > questions... > > Thanks for asking. I am just throwing out the idea of providing an interface > for doing contiguous memory allocations from user space. There are at least > two (and possibly more) devices that could benefit from such an interface. I am not really convinced this is a good interface. You are basically trying to bypass virtual memory abstraction and that is quite contradicting the mmap API to me. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
