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[PATCH] x86/mm: pgds getting out of sync after memory hot remove

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After memory hot remove it seems we do not synchronize pgds for kernel
virtual memory range (on vmemmap_free()). This seems bogus to me as it
means we are left with stall entry for process with mm != mm_init

Yet i am puzzle by the fact that i am only now hitting this issue. It
never was an issue with 4.12 or before ie HMM never triggered following
BUG_ON inside sync_global_pgds():

if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))
   BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref));


It seems that Kirill 5 level page table changes play a role in this
behavior change. I could not bisect because HMM is painfull to rebase
for each bisection step so that is just my best guess.


Am i missing something here ? Am i wrong in assuming that should sync
pgd on vmemmap_free() ? If so anyone have a good guess on why i am now
seeing the above BUG_ON ?

Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@suse.de>

Jérôme Glisse (1):
  x86/mm: synchronize pgd in vmemmap_free()

 arch/x86/mm/init_64.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

-- 
2.4.11

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[PATCH] x86/mm: pgds getting out of sync after memory hot remove Jérôme Glisse <jglisse@redhat.com> - 2017-05-19 19:00 +0200
  [PATCH] x86/mm: synchronize pgd in vmemmap_free() Jérôme Glisse <jglisse@redhat.com> - 2017-05-19 19:00 +0200
    Re: [PATCH] x86/mm: synchronize pgd in vmemmap_free() John Hubbard <jhubbard@nvidia.com> - 2017-05-20 02:40 +0200
  Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-22 15:30 +0200
    Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove Jerome Glisse <jglisse@redhat.com> - 2017-05-22 16:20 +0200
      Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-22 16:40 +0200

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