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SubjectRe: madvise(MADV_REMOVE) deadlocks on shmem THP
On (21/01/13 20:31), Hugh Dickins wrote:
> > We are running into lockups during the memory pressure tests on our
> > boards, which essentially NMI panic them. In short the test case is
> >
> > - THP shmem
> > echo advise > /sys/kernel/mm/transparent_hugepage/shmem_enabled
> >
> > - And a user-space process doing madvise(MADV_HUGEPAGE) on new mappings,
> > and madvise(MADV_REMOVE) when it wants to remove the page range
> >
> > The problem boils down to the reverse locking chain:
> > kswapd does
> >
> > lock_page(page) -> down_read(page->mapping->i_mmap_rwsem)
> >
> > madvise() process does
> >
> > down_write(page->mapping->i_mmap_rwsem) -> lock_page(page)
> >
> >
> >
> > CPU0 CPU1
> >
> > kswapd vfs_fallocate()
> > shrink_node() shmem_fallocate()
> > shrink_active_list() unmap_mapping_range()
> > page_referenced() << lock page:PG_locked >> unmap_mapping_pages() << down_write(mapping->i_mmap_rwsem) >>
> > rmap_walk_file() zap_page_range_single()
> > down_read(mapping->i_mmap_rwsem) << W-locked on CPU1>> unmap_page_range()
> > rwsem_down_read_failed() __split_huge_pmd()
> > __rwsem_down_read_failed_common() __lock_page() << PG_locked on CPU0 >>
> > schedule() wait_on_page_bit_common()
> > io_schedule()
>
> Very interesting, Sergey: many thanks for this report.

Thanks for the quick feedback.

> There is no doubt that kswapd is right in its lock ordering:
> __split_huge_pmd() is in the wrong to be attempting lock_page().
>
> Which used not to be done, but was added in 5.8's c444eb564fb1 ("mm:
> thp: make the THP mapcount atomic against __split_huge_pmd_locked()").

Hugh, I forgot to mention, we are facing these issues on 4.19.
Let me check if (maybe) we have cherry picked c444eb564fb1.

-ss

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