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

[JÃrÃme Glisse]

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@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>

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

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

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2.4.11