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






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.
   * Can’t 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?