On 09/01/2015 14:13, Russell King - ARM Linux wrote: > On Fri, Jan 09, 2015 at 01:59:10PM +0100, Mason wrote: > >> 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.
I don't understand why. mmap allocates virtual addresses in the user-space process, yes? So if I had VMSPLIT_2G, user-space processes would be limited to 2G virtual addresses, and could not create a single 2G map on top of its stack and text space. Or am I missing something? >> - 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. This means remap_pfn_range doesn't "put" anything in the kernel's virtual address space. >> If I requested the same mapping *within the kernel* using ioremap, >> would that fail because of limited VMALLOC space? > > Correct. OK. >> 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. I see. Thanks for spelling it out. Regards. -- 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/
