linux-kernel - Re: [RFC PATCH 0/6] Setting memory policy for restrictedmem file

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Date:   Fri, 14 Apr 2023 17:24:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Ackerley Tng <ackerleytng@...gle.com>, kvm@...r.kernel.org,
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Subject: Re: [RFC PATCH 0/6] Setting memory policy for restrictedmem file

On Fri, Apr 14, 2023, Michal Hocko wrote:
> On Fri 14-04-23 00:11:49, Ackerley Tng wrote:
> > 3. A more generic fbind(): it seems like this new functionality is
> >    really only needed for restrictedmem files, hence a separate,
> >    specific syscall was proposed to avoid complexities with handling
> >    conflicting policies that may be specified via other syscalls like
> >    mbind()
> 
> I do not think it is a good idea to make the syscall restrict mem
> specific.

+1.  IMO, any uAPI that isn't directly related to the fundamental properties of
restricted memory, i.e. isn't truly unique to restrictedmem, should be added as
generic fd-based uAPI.

> History shows that users are much more creative when it comes
> to usecases than us. I do understand that the nature of restricted
> memory is that it is not mapable but memory policies without a mapping
> are a reasonable concept in genereal. After all this just tells where
> the memory should be allocated from. Do we need to implement that for
> any other fs? No, you can safely return EINVAL for anything but
> memfd_restricted fd for now but you shouldn't limit usecases upfront.

I would even go a step further and say that we should seriously reconsider the
design/implemenation of memfd_restricted() if a generic fbind() needs explicit
handling from the restricted memory code.  One of the goals with memfd_restricted()
is to rely on the underlying backing store to handle all of the "normal" behaviors.