Re: [Qemu-devel] [PATCH 08/17] mm: madvise MADV_USERFAULT
From: Kirill A. Shutemov <hidden>
Date: 2014-10-07 11:30:48
Also in:
kvm, linux-mm, lkml, qemu-devel
On Tue, Oct 07, 2014 at 12:01:02PM +0100, Dr. David Alan Gilbert wrote:
* Kirill A. Shutemov (kirill@shutemov.name) wrote:quoted
On Tue, Oct 07, 2014 at 11:46:04AM +0100, Dr. David Alan Gilbert wrote:quoted
* 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.That's not how the manpage reads for DONTNEED; it calls it out as a special case near the top, and explicitly says what will happen if you read the area marked as DONTNEED.
Your are right. MADV_DONTNEED doesn't fit the interface too. That's bad and we can't fix it. But it's not a reason to make this mistake again. Read the next sentence: "The kernel is free to ignore the advice." Note, POSIX_MADV_DONTNEED has totally different semantics.
It looks like there are openssl patches that use DONTDUMP to explicitly make sure keys etc don't land in cores.
That's nice to have. But openssl works on systems without the interface, meaning it's not essential for functionality. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>