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

[Dr. David Alan Gilbert]

* Kirill A. Shutemov (address@hidden) 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.

Aren't DONTNEED and DONTDUMP  similar cases of madvise operations that are
expected to do what they say ?

> I would suggest to consider to use some other interface for the
> functionality: a new syscall or, perhaps, mprotect().

Dave

> -- 
>  Kirill A. Shutemov
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK