Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Kirill A. Shutemov]

On Mon, May 22, 2017 at 04:36:00PM +0300, Mike Rapoport wrote:
> On Mon, May 22, 2017 at 02:42:43PM +0300, Kirill A. Shutemov wrote:
> > On Mon, May 22, 2017 at 09:12:42AM +0300, Mike Rapoport wrote:
> > > Currently applications can explicitly enable or disable THP for a memory
> > > region using MADV_HUGEPAGE or MADV_NOHUGEPAGE. However, once either of
> > > these advises is used, the region will always have
> > > VM_HUGEPAGE/VM_NOHUGEPAGE flag set in vma->vm_flags.
> > > The MADV_CLR_HUGEPAGE resets both these flags and allows managing THP in
> > > the region according to system-wide settings.
> > 
> > Seems reasonable. But could you describe an use-case when it's useful in
> > real world.
> 
> My use-case was combination of pre- and post-copy migration of containers
> with CRIU.
> In this case we populate a part of a memory region with data that was saved
> during the pre-copy stage. Afterwards, the region is registered with
> userfaultfd and we expect to get page faults for the parts of the region
> that were not yet populated. However, khugepaged collapses the pages and
> the page faults we would expect do not occur.
> 
> We could have used MADV_NOHUGEPAGE before populating the region with the
> pre-copy data, but then, in the end, the restored application will be resumed
> with vma->vm_flags different from the ones it had when it was frozen.
> 
> Another possibility I've considered was to register the region with
> userfaultfd before populating it with data, but in that case we get the
> overhead of UFFD_EVENT_PAGEFAULT + UFFDIO_{COPY,ZEROPAGE} for nothing :(

Okay. Makes sense. Feel free to use my Acked-by (with change to RESET).

-- 
 Kirill A. Shutemov