On Fri 2017-12-08 22:08:07, Michael Ellerman wrote: > Matthew Wilcox <wi...@infradead.org> writes: > > > On Thu, Dec 07, 2017 at 11:14:27AM -0800, Kees Cook wrote: > >> On Wed, Dec 6, 2017 at 9:46 PM, Michael Ellerman <m...@ellerman.id.au> > >> wrote: > >> > Matthew Wilcox <wi...@infradead.org> writes: > >> >> So, just like we currently say "exactly one of MAP_SHARED or > >> >> MAP_PRIVATE", > >> >> we could add a new paragraph saying "at most one of MAP_FIXED or > >> >> MAP_REQUIRED" and "any of the following values". > >> > > >> > MAP_REQUIRED doesn't immediately grab me, but I don't actively dislike > >> > it either :) > >> > > >> > What about MAP_AT_ADDR ? > >> > > >> > It's short, and says what it does on the tin. The first argument to mmap > >> > is actually called "addr" too. > >> > >> "FIXED" is supposed to do this too. > >> > >> Pavel suggested: > >> > >> MAP_ADD_FIXED > >> > >> (which is different from "use fixed", and describes why it would fail: > >> can't add since it already exists.) > >> > >> Perhaps "MAP_FIXED_NEW"? > >> > >> There has been a request to drop "FIXED" from the name, so these: > >> > >> MAP_FIXED_NOCLOBBER > >> MAP_FIXED_NOREPLACE > >> MAP_FIXED_ADD > >> MAP_FIXED_NEW > >> > >> Could be: > >> > >> MAP_NOCLOBBER > >> MAP_NOREPLACE > >> MAP_ADD > >> MAP_NEW > >> > >> and we still have the unloved, but acceptable: > >> > >> MAP_REQUIRED > >> > >> My vote is still for "NOREPLACE" or "NOCLOBBER" since it's very > >> specific, though "NEW" is pretty clear too. > > > > How about MAP_NOFORCE? > > It doesn't tell me that addr is not a hint. That's a crucial detail. > > Without MAP_FIXED mmap never "forces/replaces/clobbers", so why would I > need MAP_NOFORCE if I don't have MAP_FIXED? > > So it needs something in there to indicate that the addr is not a hint, > that's the only thing that flag actually *does*. > > > 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?
> But the two were conflated for some reason in the current MAP_FIXED.
>
> Given we can't go back and fix it, the closest we can get is to add a
> variant of MAP_FIXED which subtracts the "REPLACE" semantic.
>
> ie: MAP_FIXED_NOREPLACE
I like MAP_FIXED_NOREPLACE.
Pavel
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