>> >Apropriate placement of mappings in the address space isn't always possible.
>> >MAP_FIXED is one example. Aliases in the page cache are harder to handle.
>> >If one of the page cache mappings is writable then readers may even observe
>> >stale data or in worst case stale data being written to disk.
>> mmap() is allowed to fail. I would think that, if someone tries to force an
>> unsafe mapping, one should give them EINVAL if one doesn't want to deal
>> with the special case otherwise, or create a copy-on-write clone in a safe
>> physical page if one wants to be extra-specially nice...
>I'm only worried because I don't know how much software such a change
But it's already broken - it just doesn't know it. The difference is that
now the software will fail in a systematic and recoverable way, whereas
before it would simply be randomly corrupt. I agree that it's regrettable,
but the job of the OS (IMHO) is to provide a known-reliable access
to the underlying hardware, and to refuse accesses that compromise
the integrity of the system and the application.
>IRIX uses something they call page ownership switching. Essentially they
>ensure that only mappings of one colour are accessible at any time.
>Accessing a page's mapping of a different colour will make the mm flush
>caches, make the old colour inaccessible and the new colour accessible
>in the page tables. That requires a reverse mapping of physical to virtual
>addresses, something that Linus so far has always refused to accept.
Just what has he refused to accept, and what was his rationale?