Re: [PATCH v9 1/6] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

On Thu, Oct 12, 2017 at 6:51 AM, Jan Kara [off-list ref] wrote:

When thinking a bit more about this I've realized one problem: Currently
user can call mmap() with MAP_SHARED type and MAP_SYNC or MAP_DIRECT flags
and he will get the new semantics (if the kernel happens to support it).  I
think that is undesirable [..]

Why?

If you have a performance preference for MAP_DIRECT or something like
that, but you don't want to *enforce* it, you'd use just plain
MAP_SHARED with it.

Ie there may well be "I want this to work, possibly with downsides" issues.

So it seems to be a reasonable model, and disallowing it seems to
limit people and not really help anything.

                 Linus

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