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

On Tue, Oct 07, 2014 at 11:46:04AM +0100, Dr. David Alan Gilbert wrote:

* Kirill A. Shutemov (kirill@shutemov.name) 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.

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

No. If kernel would ignore MADV_DONTNEED or MADV_DONTDUMP it will not
affect correctness, just behaviour will be suboptimal: more than needed
memory used or wasted space in coredump.

-- 
 Kirill A. Shutemov

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