Re: [PATCH 08/17] mm: madvise MADV_USERFAULT

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:

quoted

On Fri, Oct 03, 2014 at 07:07:58PM +0200, Andrea Arcangeli wrote:

quoted

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

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