Date: Thu, 30 Jul 2015 07:16:02 -0700 From: John Baldwin <jhb@freebsd.org> To: "K. Macy" <kmacy@freebsd.org> Cc: John-Mark Gurney <jmg@funkthat.com>, Laurie Jennings <laurie_jennings_1977@yahoo.com>, "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: Locking Memory Question Message-ID: <5314312.Bb3l71uHLc@ralph.baldwin.cx> In-Reply-To: <CAHM0Q_OP7pQ4WBNL_vw71t9fDn8-qZ%2BjuoTpJQ-TM7Aq1km=jA@mail.gmail.com> References: <20150729232522.GN78154@funkthat.com> <20150730044603.GQ78154@funkthat.com> <CAHM0Q_OP7pQ4WBNL_vw71t9fDn8-qZ%2BjuoTpJQ-TM7Aq1km=jA@mail.gmail.com>
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On Wednesday, July 29, 2015 11:28:03 PM K. Macy wrote: > >> > >> Im not clear how I'd do that. the data being passed up from the kernel is a variable size. To use copyout I'd have to pass a > >> pointer with a static buffer, right? > > > > Correct, you can pass along the size, and if it's not large enough > > try again... Less than ideal... > > > >> Is there a way to malloc user space memory from within an ioctl call? > > > > Well, it is possible that you could do the equivalent of mmap, and pass > > the address back along w/ a length, and leave it up to the user to > > munmap it... This is probably the nicest method if you the size is > > really largely variable, and it's expensive if the userland process > > allocated too much memory... The down side is that this is more > > complex to code... > > > > > Mach has the ability to send large "out of line messages". For smaller > messages where it doesn't do VM tricks to avoid copying it does > exactly this. In the receive path the kernel calls vm_allocate (which > is essentially just a wrapper for mmap) then copies the buffer in to > the newly allocated address space. The message itself contains the > allocated address and size of the allocation. The receiver is expected > to call vm_deallocate (munmap) when it's done with the data. > > The implementation is mixed in with enough other code that it may not > be a useful reference. Nonetheless, I wanted to point at that this > isn't as strange as it might sound. You can do this in FreeBSD by calling vm_mmap() with a NULL handle pointer to simulate a MAP_ANON mapping. Something like this: vm_mmap(&curproc->p_vmspace->vm_map, &addr, <size>, VM_PROT_READ | VM_PROT_WRITE, VM_PROT_READ | VM_PROT_WRITE, MAP_SHARED, OBJT_DEFAULT, NULL, 0); It's not great for a true shared memory buffer, but is fine for a one-time copy. -- John Baldwin
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