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.

  #if defined(RTE_ARCH_X86)

Isn't this going to apply to 64 bit only?

+ * Try to detect whether the system uses 5-level page table.
+ */
+static bool
+     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;

+     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.


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