On 10-Aug-18 9:35 AM, Drocula wrote:
First, thanks for your suggestions.
When using the MAP_FIXED flag, mmap will return an MMAP_FAILED if
0xf0000000000000 is not available.
In this case, I want mmap to return an address near 0xf0000000000000.
I will submit v2.
How can we be sure there's nothing mapped at that address? I think the
original code was correct - try mapping around that address, and see if
we get *something* close to it with the right bits set. MAP_FIXED seems
dangerous to use without knowing that there's nothing there. Recent
kernels have added a safer version of MAP_FIXED, but obviously it won't
work on the majority of kernel versions we support.
On Fri, Aug 10, 2018, 01:03 Stephen Hemminger <step...@networkplumber.org>
Thanks for the patch, there are some minor style/cleanups that
could be done.
Isn't this going to apply to 64 bit only?
+ * Try to detect whether the system uses 5-level page table.
+ void *page_4k, *mask = (void *)0xf0000000000000;
Magic constants expressed like this seem wrong. Why not use
shift to make it obvious.
Also, you are assuming a particular layout of memory on
Linux which might be problematic. Plus if there is already
some memory in use there, it won't work.
+ page_4k = mmap(mask, 4096, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
Since you are probing maybe MAP_FIXED is what you want.
+ if (page_4k == (void *) -1)
+ return false;
Use MMAP_FAILED here.
+ munmap(page_4k, 4096);
+ if ((unsigned long)page_4k & (unsigned long)mask)
+ return true;
+ return false;
Wouldn't this work the same for what you expect?
return page_4k == mask;
I.e you expect kernel to put page where you want.