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

[Linus Torvalds]

In article <[e-mail]>,
David Woodhouse  <[e-mail]> wrote:
>Why does a _readonly_ mapping fail if the file system has no writepage 
>method? 
>
>do_mmap_pgoff() sets VM_MAYWRITE on the vma and then generic_file_mmap() 
>refuses to allow it. 
>
>Suggested patch below.... or should I just hack fsx-linux to use 
>MAP_PRIVATE for its readonly mappings and ignore it?

This is broken. Since it has VM_MAYWRITE, a subsequent mprotect() may
mark it writable, and you you went boom.

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.

In other words: the code is correct as-is.

		Linus