Re: pthread_getattr_np doing loads of mremaps on ARM, MIPS under QEMU user-mode

[Tobias Koch <tobias.koch@nonterra.com>]

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Hi,

thank you! And yes, you are absolutely right. QEMU always fails in 
mmap.c after looking at page flags in the extended memory range:

         int prot = 0;
         if (reserved_va && old_size < new_size) {
             abi_ulong addr;
             for (addr = old_addr + old_size;
                  addr < old_addr + new_size;
                  addr++) {
                 prot |= page_get_flags(addr);
             }
         }
         if (prot == 0) {
             host_addr = mremap(g2h(old_addr), old_size, new_size, flags);
             if (host_addr != MAP_FAILED && reserved_va && old_size > new_size) {
                 mmap_reserve(old_addr + old_size, new_size - old_size);
             }
         } else {
	    /* musl's pthread_getattr_np always fails here doing mremaps: */
  	    errno = ENOMEM;
             host_addr = MAP_FAILED;
         }

I wonder why it cannot just try the remap directly and use the result of 
that, but that's clearly a question for the QEMU people.

Tobias


On 21/06/2017 03:41, Rich Felker wrote:
> [...]
> OK, qemu-user is just emulating mremap very poorly. The man page
> documents that it fails with EFAULT when the address passed is not
> valid, but qemu seems to be giving ENOMEM, which has a different
> meaning. I remember we looked hard when this was first implemented to
> find an acceptable/safe way to probe for valid pages, so I doubt we'll
> find a better or equally good way that both works around the bug in
> qemu-user and does the right thing on all real kernels. I think the
> qemu bug should just be reported and fixed.
>
> Rich


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