Date: Thu, 30 Jul 2015 12:37:56 -0700 From: Laurie Jennings <laurie_jennings_1977@yahoo.com> To: Hooman Fazaeli <hoomanfazaeli@gmail.com> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: Locking Memory Question Message-ID: <1438285076.38601.YahooMailBasic@web141505.mail.bf1.yahoo.com> In-Reply-To: <55BA6E6C.4080304@gmail.com>
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-------------------------------------------- On Thu, 7/30/15, Hooman Fazaeli <hoomanfazaeli@gmail.com> wrote: Subject: Re: Locking Memory Question To: "Laurie Jennings" <laurie_jennings_1977@yahoo.com> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Date: Thursday, July 30, 2015, 2:35 PM On 7/30/2015 5:22 AM, Laurie Jennings via freebsd-net wrote: > -------------------------------------------- > On Wed, 7/29/15, John-Mark Gurney <jmg@funkthat.com> wrote: > > Subject: Re: Locking Memory Question > To: "Laurie Jennings" <laurie_jennings_1977@yahoo.com> > Cc: "John Baldwin" <jhb@freebsd.org>, freebsd-net@freebsd.org > Date: Wednesday, July 29, 2015, 7:25 PM > > Laurie Jennings via > freebsd-net wrote this message on Wed, Jul 29, 2015 at 15:26 > -0700: > > > > I have a problem and > I can't quite figure out where to look. This is what Im > doing: > > > > I have an > IOCTL to read a block of data, but the data is too large to > return via ioctl. So to get the data, > > I > allocate a block in a kernel module: > > > > > foo = > malloc(1024000,M_DEVBUF,M_WAITOK); > > > > I pass up a pointer and in user space > map it using /dev/kmem: > > An easier solution would be for your ioctl to > pass in a userland > pointer and then use > copyout(9) to push the data to userland... This > means the userland process doesn't have to > have /dev/kmem access... > > Is > there a reason you need to use kmem? The only reason you > list above > is that it's too large via > ioctl, but a copyout is fine, and would > handle all page faults for you.. > > __________________________________ > I'm using kmem because the only options I could think of was to > > 1) use shared memory > 2) use kmem > 3) use a huge ioctl structure. > > 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? Is there a way to malloc user space memory from within an ioctl call? Or > would I just have to pass down a pointer to a huge buffer large enough for the largest possible answer? > > thanks > > Laurie You can use two IOCTLs. Get the block size from kernel module with the first ioctl, and malloc(3) a buffer in userland with that size. Then use a second ioctl to pass the address of allocated buffer to kernel module. The module may use copyout(9) to copy in-kernel data to user space buffer. __________________ I sort of did that. I pass a buffer large enough for 99% of the cases, and if its too small I return the required size and pass a bigger buffer. Its a low volume operation so I'm not too concerned about performance, but doing 2 IOCTLs every time would be particular inefficient. Its cleaner than kmem for sure. Lauriehelp
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