Re: [GIT PULL] KVM updates for the 2.6.38 merge window

[Hugh Dickins]

On Thu, Jan 13, 2011 at 7:43 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jan 13, 2011 at 4:53 AM, Gleb Natapov <gleb@xxxxxxxxxx> wrote:
>>
>> I implemented get_user_pages_nowait() on top of your patch. In my testing
>> it works as expected when used inside KVM. Does this looks OK to you?
>
> It looks reasonable, although I suspect the subtle behavior wrt the
> mmap_sem means that you should not expose the magic bare
> FAULT_FLAG_ALLOW_RETRY flag to the __get_user_pages() thing. It's just
> too easy to introduce bugs, methinks.
>
> So I'd suggest
>
> Â- drop FOLL_RETRY
>
> Â- Âmake FOLL_NOWAIT set both (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_NOWAIT)
>
> and that way the get_user_pages() thing will never release the
> mmap_sem, and you never have any subtle locking issues for that
> particular interface.
>
> But some other VM person should look at it too.

I haven't been following and am not really looking yet, but should
express a preference: that Gleb keep it as he had it (if that works),
rather than making FOLL_NOWAIT do a combination.

A couple of months ago I needed to add a FOLL flag myself, and made a
patch to use the same space for FOLL flags and FAULT_FLAGs, to end
such ugliness as you see in this patch:

+                                       ((foll_flags & FOLL_WRITE) ?
+                                       FAULT_FLAG_WRITE : 0) |
+                                       ((foll_flags & FOLL_RETRY) ?
+                                       FAULT_FLAG_ALLOW_RETRY : 0) |
+                                       ((foll_flags & FOLL_NOWAIT) ?
+                                       FAULT_FLAG_RETRY_NOWAIT : 0));

But I missed my window, I think several have been added since, with
more on the way: I intend to put it together again for mmotm once the
dust has settled a little.

__get_user_pages already has wrappers for friendly usage, I think it's
okay for the wrappers to have special knowledge of what's needed.

Hugh
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