Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[Florian Weimer]

On 12/08/2017 03:27 PM, Pavel Machek wrote:
> On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
> > If we had a time machine, the right set of flags would be:
> >
> >    - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
> >    - MAP_REPLACE: replace an existing mapping (or force or clobber)

> Actually, if we had a time machine... would we even provide
> MAP_REPLACE functionality?

Probably yes.  ELF loading needs to construct a complex set of mappings 
from a single file.  munmap (to create a hole) followed by mmap would be 
racy because another thread could have reused the gap in the meantime. 
The only alternative to overriding existing mappings would be mremap 
with MREMAP_FIXED, and that doesn't look like an improvement API-wise.

(The glibc dynamic linker uses an mmap call with an increased length to 
reserve address space and then loads additional segments with MAP_FIXED 
at the offsets specified in the program header.)

Thanks,
Florian