Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

[Zhenyu Zhang]

With the patch applied, I'm unable to hit memory hot-remove failure in
the environment where the issue was initially found.

Tested-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>

On Thu, Nov 24, 2022 at 10:09 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 24.11.22 14:22, David Hildenbrand wrote:
> > On 24.11.22 13:55, Gavin Shan wrote:
> >> On 11/24/22 6:43 PM, David Hildenbrand wrote:
> >>> On 24.11.22 11:21, Gavin Shan wrote:
> >>>> On 11/24/22 6:09 PM, David Hildenbrand wrote:
> >>>>> On 24.11.22 10:55, Gavin Shan wrote:
> >>>>>> The issue is reported when removing memory through virtio_mem device.
> >>>>>> The transparent huge page, experienced copy-on-write fault, is wrongly
> >>>>>> regarded as pinned. The transparent huge page is escaped from being
> >>>>>> isolated in isolate_migratepages_block(). The transparent huge page
> >>>>>> can't be migrated and the corresponding memory block can't be put
> >>>>>> into offline state.
> >>>>>>
> >>>>>> Fix it by replacing page_mapcount() with total_mapcount(). With this,
> >>>>>> the transparent huge page can be isolated and migrated, and the memory
> >>>>>> block can be put into offline state. Besides, The page's refcount is
> >>>>>> increased a bit earlier to avoid the page is released when the check
> >>>>>> is executed.
> >>>>>
> >>>>> Did you look into handling pages that are in the swapcache case as well?
> >>>>>
> >>>>> See is_refcount_suitable() in mm/khugepaged.c.
> >>>>>
> >>>>> Should be easy to reproduce, let me know if you need inspiration.
> >>>>>
> >>>>
> >>>> Nope, I didn't look into the case. Please elaborate the details so that
> >>>> I can reproduce it firstly.
> >>>
> >>>
> >>> A simple reproducer would be (on a system with ordinary swap (not zram))
> >>>
> >>> 1) mmap a region (MAP_ANON|MAP_PRIVATE) that can hold a THP
> >>>
> >>> 2) Enable THP for that region (MADV_HUGEPAGE)
> >>>
> >>> 3) Populate a THP (e.g., write access)
> >>>
> >>> 4) PTE-map the THP, for example, using MADV_FREE on the last subpage
> >>>
> >>> 5) Trigger swapout of the THP, for example, using MADV_PAGEOUT
> >>>
> >>> 6) Read-access to some subpages to fault them in from the swapcache
> >>>
> >>>
> >>> Now you'd have a THP, which
> >>>
> >>> 1) Is partially PTE-mapped into the page table
> >>> 2) Is in the swapcache (each subpage should have one reference from the swapache)
> >>>
> >>>
> >>> Now we could test, if alloc_contig_range() will still succeed (e.g., using virtio-mem).
> >>>
> >>
> >> Thanks for the details. Step (4) and (5) can be actually combined. To swap part of
> >> the THP (e.g. one sub-page) will force the THP to be split.
> >>
> >> I followed your steps in the attached program, there is no issue to do memory hot-remove
> >> through virtio-mem with or without this patch.
> >
> > Interesting. But I don't really see how we could pass this check with a
> > page that's in the swapcache, maybe I'm missing something else.
> >
> > I'll try to see if I can reproduce it.
> >
>
> After some unsuccessful attempts and many head-scratches, I realized
> that it's quite simple why we don't have to worry about swapcache pages
> here:
>
> page_mapping() is != NULL for pages in the swapcache: folio_mapping()
> makes this rather obvious:
>
> if (unlikely(folio_test_swapcache(folio))
>         return swap_address_space(folio_swap_entry(folio));
>
>
> I think the get_page_unless_zero() might also be a fix for the
> page_mapping() call, smells like something could blow up on concurrent
> page freeing. (what about concurrent removal from the swapcache? nobody
> knows :) )
>
>
> Thanks Gavin!
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>
>
> --
> Thanks,
>
> David / dhildenb
>