Re: [RFC v4 0/3] mm: zap pages with read mmap_sem in munmap for large mapping

[Yang Shi]

On 7/11/18 4:10 AM, Kirill A. Shutemov wrote:
> On Wed, Jul 11, 2018 at 07:34:06AM +0800, Yang Shi wrote:
> > Background:
> > Recently, when we ran some vm scalability tests on machines with large memory,
> > we ran into a couple of mmap_sem scalability issues when unmapping large memory
> > space, please refer to https://lkml.org/lkml/2017/12/14/733 and
> > https://lkml.org/lkml/2018/2/20/576.
> >
> >
> > History:
> > Then akpm suggested to unmap large mapping section by section and drop mmap_sem
> > at a time to mitigate it (see https://lkml.org/lkml/2018/3/6/784).
> >
> > V1 patch series was submitted to the mailing list per Andrew's suggestion
> > (see https://lkml.org/lkml/2018/3/20/786). Then I received a lot great feedback
> > and suggestions.
> >
> > Then this topic was discussed on LSFMM summit 2018. In the summit, Michal Hocko
> > suggested (also in the v1 patches review) to try "two phases" approach. Zapping
> > pages with read mmap_sem, then doing via cleanup with write mmap_sem (for
> > discussion detail, see https://lwn.net/Articles/753269/)
> >
> >
> > Approach:
> > Zapping pages is the most time consuming part, according to the suggestion from
> > Michal Hocko [1], zapping pages can be done with holding read mmap_sem, like
> > what MADV_DONTNEED does. Then re-acquire write mmap_sem to cleanup vmas.
> >
> > But, we can't call MADV_DONTNEED directly, since there are two major drawbacks:
> >    * The unexpected state from PF if it wins the race in the middle of munmap.
> >      It may return zero page, instead of the content or SIGSEGV.
> >    * Canât handle VM_LOCKED | VM_HUGETLB | VM_PFNMAP and uprobe mappings, which
> >      is a showstopper from akpm
> >
> > And, some part may need write mmap_sem, for example, vma splitting. So, the
> > design is as follows:
> >          acquire write mmap_sem
> >          lookup vmas (find and split vmas)
> >          set VM_DEAD flags
> >          deal with special mappings
> >          downgrade_write
> >
> >          zap pages
> >          release mmap_sem
> >
> >          retake mmap_sem exclusively
> >          cleanup vmas
> >          release mmap_sem
> >
> > Define large mapping size thresh as PUD size, just zap pages with read mmap_sem
> > for mappings which are >= PUD_SIZE. So, unmapping less than PUD_SIZE area still
> > goes with the regular path.
> >
> > All vmas which will be zapped soon will have VM_DEAD flag set. Since PF may race
> > with munmap, may just return the right content or SIGSEGV before the optimization,
> > but with the optimization, it may return a zero page. Here use this flag to mark
> > PF to this area is unstable, will trigger SIGSEGV, in order to prevent from the
> > unexpected 3rd state.
> >
> > If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are considered
> > as special mappings. They will be dealt with before zapping pages with write
> > mmap_sem held. Basically, just update vm_flags. The actual unmapping is still
> > done with read mmap_sem.
> >
> > And, since they are also manipulated by unmap_single_vma() which is called by
> > zap_page_range() with read mmap_sem held in this case, to prevent from updating
> > vm_flags in read critical section and considering the complexity of coding, just
> > check if VM_DEAD is set, then skip any VM_DEAD area since they should be handled
> > before.
> >
> > When cleaning up vmas, just call do_munmap() without carrying vmas from the above
> > to avoid race condition, since the address space might be already changed under
> > our feet after retaking exclusive lock.
> >
> > For the time being, just do this in munmap syscall path. Other vm_munmap() or
> > do_munmap() call sites (i.e mmap, mremap, etc) remain intact for stability reason.
> > And, make this 64 bit only explicitly per akpm's suggestion.
> I still see VM_DEAD as unnecessary complication. We should be fine without it.
> But looks like I'm in the minority :/
>
> It's okay. I have another suggestion that also doesn't require VM_DEAD
> trick too :)
>
> 1. Take mmap_sem for write;
> 2. Adjust VMA layout (split/remove). After the step all memory we try to
>     unmap is outside any VMA.
> 3. Downgrade mmap_sem to read.
> 4. Zap the page range.
> 5. Drop mmap_sem.
>
> I believe it should be safe.


Yes, it looks so. But, a further question is all the vmas have been 
removed, how zap_page_range could do its job? It depends on the vmas.

One approach is to save all the vmas on a separate list, then 
zap_page_range does unmap with this list.

Yang

> The pages in the range cannot be re-faulted after step 3 as find_vma()
> will not see the corresponding VMA and deliver SIGSEGV.
>
> New VMAs cannot be created in the range before step 5 since we hold the
> semaphore at least for read the whole time.
>
> Do you see problem in this approach?