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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
