Messages in this thread | | | Date | Fri, 27 Aug 2021 19:16:13 +0000 | From | Al Viro <> | Subject | Re: [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable |
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On Fri, Aug 27, 2021 at 11:57:19AM -0700, Linus Torvalds wrote: > On Fri, Aug 27, 2021 at 11:53 AM Al Viro <viro@zeniv.linux.org.uk> wrote: > > > > I really disagree with these calling conventions. "Number not faulted in" > > is bloody useless > > It's what we already have for copy_to/from_user(), so it's actually > consistent with that.
After copy_to/copy_from you've got the data copied and it's not going anywhere. After fault-in you still have to copy, and it still can give you less data than fault-in had succeeded for. So you must handle short copies separately, no matter how much you've got from fault-in.
> And it avoids changing all the existing tests where people really > cared only about the "everything ok" case.
The thing is, the checks tend to be wrong. We can't rely upon the full fault-in to expect the full copy-in/copy-out, so the checks downstream are impossible to avoid anyway. And fault-in failure is always a slow path, so we are not saving time here.
And for the memory poisoining we end up aborting a copy potentially a lot earlier than we should.
> Andreas' first patch did that changed version, and was ugly as hell. > > But if you have a version that avoids the ugliness...
I'll need to dig my notes out...
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