Messages in this thread | | | Date | Sat, 5 May 2001 16:18:19 -0400 | From | Pete Wyckoff <> | Subject | Re: Question on mmap(2) with kernel alocated memory |
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terry@beam.demon.co.uk said: > I am trying to mmap() into user space a kernel buffer and am having > problems. > I have a simple test example, can someone please tell me what I have got > wrong ? > > In a driver I do: > uint* kva; > > kva = (uint*)kmalloc(4096, GFP_KERNEL); > *kva = 0x11223344; > printk("Address: %p %lx %x\n", kva, virt_to_phys(kva), *kva); > > Now in some simple user program I do: > > #include <stdio.h> > #include <string.h> > #include <stdlib.h> > #include <sys/mman.h> > #include <fcntl.h> > > int main(int argc, char** argv){ > int fm; > char* p; > uint* pi; > uint v; > uint add = 0x74b000; > > if((fm = open("/dev/mem", O_RDWR)) < 0) > return 1; > > p = mmap(0, 128 * 1024 * 1024, PROT_READ|PROT_WRITE, MAP_SHARED, fm, > 0); > printf("Mapped: %p\n", p); > > lseek(fm, add, SEEK_SET); > read(fm, &v, sizeof(v)); > printf("V: %x\n", v); > > pi = (uint*)(p + add); > printf("Vmmap: %p %x\n", pi, *pi); > > close(fm); > return 0; > } > > The value of add is hardcoded to the value printed for the physical > address in the drivers prink routine. > The lseek/read from the /dev/mem device yields the value 0x11223344. > However the mmap method also on /dev/mem yields the value 0. > > Whats wrong with my mmap() or kalloc() ?
Executive summary:
mmap of /dev/mem gives different values than lseek/read of /dev/mem
Using lseek/read gives back bits of physical memory, but dereferencing a pointer into the mmaped area mostly gives zeroes.
The file-system specific mmap routine, mmap_mem, calls remap_page_range to do the work of remapping the physical pages into user space. But eventually in remap_pte_range there's a check
if ((!VALID_PAGE(page)) || PageReserved(page)) set_pte(pte, mk_pte_phys(phys_addr, prot));
And the effect is that only the reserved pages and those outside of the physical memory space get mapped. This isn't intuitive for /dev/mem, but is it the intended behavior?
You could replicate those 80 odd lines of remap_page_range and helpers to get a version without the PageReserved test. Yuk.
Quick hack: SetPageReserved(virt_to_page(kva));
but you may want to undo that (Clear...) before kfree-ing the page, as I'm not sure what will be confused by that.
Longer term solution: write your own mmap routine in the driver, have the user code open /dev/my-driver, and mmap() on that instead of going through /dev/mem with a hardcoded address.
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