Re: [Qemu-devel] [PATCH 08/17] mm: madvise MADV_USERFAULT

[Kirill A. Shutemov]

On Tue, Oct 07, 2014 at 03:24:58PM +0200, Andrea Arcangeli wrote:
> Hi Kirill,
> 
> On Tue, Oct 07, 2014 at 01:36:45PM +0300, Kirill A. Shutemov wrote:
> > On Fri, Oct 03, 2014 at 07:07:58PM +0200, Andrea Arcangeli wrote:
> > > MADV_USERFAULT is a new madvise flag that will set VM_USERFAULT in the
> > > vma flags. Whenever VM_USERFAULT is set in an anonymous vma, if
> > > userland touches a still unmapped virtual address, a sigbus signal is
> > > sent instead of allocating a new page. The sigbus signal handler will
> > > then resolve the page fault in userland by calling the
> > > remap_anon_pages syscall.
> > 
> > Hm. I wounder if this functionality really fits madvise(2) interface: as
> > far as I understand it, it provides a way to give a *hint* to kernel which
> > may or may not trigger an action from kernel side. I don't think an
> > application will behaive reasonably if kernel ignore the *advise* and will
> > not send SIGBUS, but allocate memory.
> > 
> > I would suggest to consider to use some other interface for the
> > functionality: a new syscall or, perhaps, mprotect().
> 
> I didn't feel like adding PROT_USERFAULT to mprotect, which looks
> hardwired to just these flags:

PROT_NOALLOC may be?

> 
>        PROT_NONE  The memory cannot be accessed at all.
> 
>        PROT_READ  The memory can be read.
> 
>        PROT_WRITE The memory can be modified.
> 
>        PROT_EXEC  The memory can be executed.

To be complete: PROT_GROWSDOWN, PROT_GROWSUP and unused PROT_SEM.

> So here somebody should comment and choose between:
> 
> 1) set VM_USERFAULT with mprotect(PROT_USERFAULT) instead of
>    the current madvise(MADV_USERFAULT)
> 
> 2) drop MADV_USERFAULT and VM_USERFAULT and force the usage of the
>    userfaultfd protocol as the only way for userland to catch
>    userfaults (each userfaultfd must already register itself into its
>    own virtual memory ranges so it's a trivial change for userfaultfd
>    users that deletes just 1 or 2 lines of userland code, but it would
>    prevent to use the SIGBUS behavior with info->si_addr=faultaddr for
>    other users)
> 
> 3) keep things as they are now: use MADV_USERFAULT for SIGBUS
>    userfaults, with optional intersection between the
>    vm_flags&VM_USERFAULT ranges and the userfaultfd registered ranges
>    with vma->vm_userfaultfd_ctx!=NULL to know if to engage the
>    userfaultfd protocol instead of the plain SIGBUS

4) new syscall?
 
> I will update the code accordingly to feedback, so please comment.

I don't have strong points on this. Just *feel* it doesn't fit advice
semantics.

The only userspace interface I've designed was not proven good by time.
I would listen what senior maintainers say. :)
 
-- 
 Kirill A. Shutemov