On Tue, Oct 07, 2014 at 12:01:02PM +0100, Dr. David Alan Gilbert wrote: > * Kirill A. Shutemov (kirill@xxxxxxxxxxxxx) wrote: > > On Tue, Oct 07, 2014 at 11:46:04AM +0100, Dr. David Alan Gilbert wrote: > > > * Kirill A. Shutemov (kirill@xxxxxxxxxxxxx) 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 ? > > > > 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 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
