On 22.02.21 10:38, David Hildenbrand wrote:
On 17.02.21 17:19, James Bottomley wrote:
On Tue, 2021-02-16 at 18:16 +0100, David Hildenbrand wrote: [...]
The discussion regarding migratability only really popped upbecause this is a user-visible thing and not being able to migrate can be a real problem (fragmentation, ZONE_MOVABLE, ...).
I think the biggest use will potentially come from hardware acceleration. If it becomes simple to add say encryption to a secret page with no cost, then no flag needed. However, if we only have a limited number of keys so once we run out no more encrypted memory then it becomes a costly resource and users might want a choice of being backed by encryption or not.
Right. But wouldn't HW support with configurable keys etc. need more syscall parameters (meaning, even memefd_secret() as it is would not be sufficient?). I suspect the simplistic flag approach might not be sufficient. I might be wrong because I have no clue about MKTME and friends.
The theory I was operating under is key management is automatic and hidden, but key scarcity can't be, so if you flag requesting hardware backing then you either get success (the kernel found a key) or failure (the kernel is out of keys). If we actually want to specify the key then we need an extra argument and we *must* have a new system call.
Anyhow, I still think extending memfd_create() might just be good enough - at least for now.
I really think this is the wrong approach for a user space ABI. If we think we'll ever need to move to a separate syscall, we should begin with one. The pain of trying to shift userspace from memfd_create to a new syscall would be enormous. It's not impossible (see clone3) but it's a pain we should avoid if we know it's coming.
Sorry for the late reply, there is just too much going on :)
*If* we ever realize we need to pass more parameters we can easily have a new syscall for that purpose. *Then*, we know how that syscall will look like. Right now, it's just pure speculation.
Until then, going with memfd_create() works just fine IMHO.
The worst think that could happen is that we might not be able to create all fancy sectremem flavors in the future via memfd_create() but only via different, highly specialized syscall. I don't see a real problem with that.
Adding to that, I'll give up arguing now as I have more important things to do. It has been questioned by various people why we need a dedicate syscall and at least for me, without a satisfying answer.
Worst thing is that we end up with a syscall that could have been avoided, for example, because 1. We add existing/future memfd_create() flags to memfd_secret() as well when we need them (sealing, hugetlb., ..). 2. We decide in the future to still add MFD_SECRET support to memfd_secret().
So be it.