Re: [Qemu-devel] [PATCH 10/17] mm: rmap preparation for remap_anon_pages

[Andy Lutomirski]

On Tue, Oct 7, 2014 at 8:52 AM, Andrea Arcangeli <address@hidden> wrote:
> On Tue, Oct 07, 2014 at 04:19:13PM +0200, Andrea Arcangeli wrote:
>> mremap like interface, or file+commands protocol interface. I tend to
>> like mremap more, that's why I opted for a remap_anon_pages syscall
>> kept orthogonal to the userfaultfd functionality (remap_anon_pages
>> could be also used standalone as an accelerated mremap in some
>> circumstances) but nothing prevents to just embed the same mechanism
>
> Sorry for the self followup, but something else comes to mind to
> elaborate this further.
>
> In term of interfaces, the most efficient I could think of to minimize
> the enter/exit kernel, would be to append the "source address" of the
> data received from the network transport, to the userfaultfd_write()
> command (by appending 8 bytes to the wakeup command). Said that,
> mixing the mechanism to be notified about userfaults with the
> mechanism to resolve an userfault to me looks a complication. I kind
> of liked to keep the userfaultfd protocol is very simple and doing
> just its thing. The userfaultfd doesn't need to know how the userfault
> was resolved, even mremap would work theoretically (until we run out
> of vmas). I thought it was simpler to keep it that way. However if we
> want to resolve the fault with a "write()" syscall this may be the
> most efficient way to do it, as we're already doing a write() into the
> pseudofd to wakeup the page fault that contains the destination
> address, I just need to append the source address to the wakeup command.
>
> I probably grossly overestimated the benefits of resolving the
> userfault with a zerocopy page move, sorry. So if we entirely drop the
> zerocopy behavior and the TLB flush of the old page like you
> suggested, the way to keep the userfaultfd mechanism decoupled from
> the userfault resolution mechanism would be to implement an
> atomic-copy syscall. That would work for SIGBUS userfaults too without
> requiring a pseudofd then. It would be enough then to call
> mcopy_atomic(userfault_addr,tmp_addr,len) with the only constraints
> that len must be a multiple of PAGE_SIZE. Of course mcopy_atomic
> wouldn't page fault or call GUP into the destination address (it can't
> otherwise the in-flight partial copy would be visible to the process,
> breaking the atomicity of the copy), but it would fill in the
> pte/trans_huge_pmd with the same strict behavior that remap_anon_pages
> currently has (in turn it would by design bypass the VM_USERFAULT
> check and be ideal for resolving userfaults).

At the risk of asking a possibly useless question, would it make sense
to splice data into a userfaultfd?

--Andy

>
> mcopy_atomic could then be also extended to tmpfs and it would work
> without requiring the source page to be a tmpfs page too without
> having to convert page types on the fly.
>
> If I add mcopy_atomic, the patch in subject (10/17) can be dropped of
> course so it'd be even less intrusive than the current
> remap_anon_pages and it would require zero TLB flush during its
> runtime (it would just require an atomic copy).
>
> So should I try to embed a mcopy_atomic inside userfault_write or can
> I expose it to userland as a standalone new syscall? Or should I do
> something different? Comments?
>
> Thanks,
> Andrea



-- 
Andy Lutomirski
AMA Capital Management, LLC