Re: RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.

[David Woodhouse <dwmw2@infradead.org>]

torvalds@transmeta.com said:
>  This is broken. Since it has VM_MAYWRITE, a subsequent mprotect() may
> mark it writable, and you you went boom.

Er, we clear VM_MAYWRITE...

+               if (vma->vm_flags & VM_WRITE)
                        return -EINVAL;
+               else
+                       vma->vm_flags &= ~VM_MAYWRITE;

> If you really want a shared mapping, you'd better open with O_RDONLY,
> at which point the existing code should be perfectly happy and does
> the right thing. 

It's a read-only mapping. Whether it's shared or private is not relevant,
surely, since those affect only the behaviour if we write to it -- which we 
can't. 

I don't _really_ want a shared mapping; all I want is for the fsx-linux
stress test to run, and find interesting breakage on my file system to keep
me from getting bored (what are Friday nights for, after all?).

As shipped, fsx-linux uses PROT_READ, MAP_SHARED on its test file, which
definitely needs to be opened for write. For now, I've just changed it to
use MAP_PRIVATE. I'm just a bit concerned about having to change the test to
get it to work though, and don't see why a _readonly_ mmap should fail due 
to lack of writepage.

--
dwmw2