Re: [PATCH v6 3/5] mm: introduce mmap3 for safely defining new mmap flags

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Thu, Aug 24, 2017 at 10:36:02AM -0700, Dan Williams wrote:
> I'll let Andy and Kirill restate their concerns, but one of the
> arguments that swayed me is that any new mmap flag with this hack must
> be documented to only work with MAP_SHARED and that MAP_PRIVATE is
> silently ignored. I agree with the mess and delays it causes for other
> archs and libc, but at the same time this is for new applications and
> libraries that know to look for the new flag, so they need to do the
> extra work to check for the new syscall.

True.  That is for the original hack, but I spent some more time
looking at the mmap code, and there is one thing I noticed:

include/uapi/asm-generic/mman-common.h:

#define MAP_SHARED      0x01            /* Share changes */
#define MAP_PRIVATE     0x02            /* Changes are private */
#define MAP_TYPE        0x0f            /* Mask for type of mapping */

mm/mmap.c:

	if (file) {
		struct inode *inode = file_inode(file);

		switch (flags & MAP_TYPE) {
                case MAP_SHARED:
			...
		case MAP_PRIVATE:
			...
		default:
			return -EINVAL;
		}

and very similar for the anonymous and nommu cases.

So if we pick e.g. 0x4 as the valid bit we don't even need to overload
the MAP_SHARED and MAP_PRIVATE meaning.

> 
> However, if the fcntl lease approach works for the DMA cases then we
> only have the one mmap flag to add for now, so maybe the weird
> MAP_{SHARED|PRIVATE} semantics are tolerable.
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