On Fri, Jan 09, 2015 at 01:59:10PM +0100, Mason wrote: > Hello everyone, > > Yesterday, I used /dev/mem to mmap 2 GB and (to my surprise) it worked. > Specifically, I opened /dev/mem O_RDWR | O_SYNC > then called > mmap(NULL, 1U<<31, PROT_WRITE, MAP_SHARED, fd, 0x80000000);
So you asked to map 2GB starting at 2GB physical. > And mmap returned a valid pointer. And that mapping would have been created to map physical addresses 0x80000000-0xffffffff inclusive. > I was expecting it to fail. > > - the kernel is configured with VMSPLIT_3G (3G/1G user/kernel) This has no bearing on the above. > - the kernel manages 256 MB RAM > - there is roughly 750 MB of VMALLOC space, no highmem vmalloc has no bearing on the above, mmap() doesn't allocate anything into vmalloc space. > If I requested the same mapping *within the kernel* using ioremap, > would that fail because of limited VMALLOC space? Correct. > Moving to arch-specific questions (namely ARM Cortex-A9). > If I understand correctly (which is very possibly NOT the case) > the CPU has two registers pointing to page tables, one for > the current process, one for the kernel. And the CPU automatically > picks the correct one, based on the active context? > It would seem possible to have a full 4G for process, and a full 4G > for the kernel, using that method, no? (Like Ingo's old 4G/4G split). > Without the performance overhead of fiddling with the page tables. > What am I missing? It's possible to use both, but the CPU selects the page table register according to the virtual address. So it's not possible to have 4G for both. There's only a restricted set of options: 2G / 2G, where the bottom 2G of virtual space uses TTBR0 and the upper 2G uses TTBR1. 1G / 3G (1G for TTBR0, 3G for TTBR1). We don't use it because most people run with 3G for userspace, which isn't supported in hardware. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- 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/
