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From: Mel Gorman on 26 Apr 2010 18:40
After digging around a lot, I believe the following two patches are the
best way to close the race that allows a migration PTE to be left behind
triggering a BUG check in migration_entry_to_page().

Patch one alters has fork() wait for migration to complete. Patch two has
vma_adjust() acquire the anon_vma lock it is aware of and makes rmap_walk()
aware that different VMAs can be encountered during the walk.

I dropped the use of the seq counter because there were still races in
place. For example, while the seq counter would catch when vma_adjust()
and rmap_walk() were looking at the same VMA, there was still insufficient
protection on the VMA list being modified.

The reproduction case was as follows;

1. Run kernel compilation in a loop
2. Start two processes that repeatedly fork()ed and manipulated mappings
3. Constantly compact memory using /proc/sys/vm/compact_memory
4. Optionally add/remove swap

With these two patches applied, I was unable to trigger the bug check
in migration_entry_to_page() but it would be really helpful if Rik could
comment on the anon_vma locking requirements and whether patch 2 is 100%
safe or not. The tests have only been running 8 hours but I'm posting now
anyway and will see how it survives running for a few days.

The other issues raised about expand_downwards will need to be re-examined to
see if they still exist and transparent hugepage support will need further
thinking to see if split_huge_page() can deal with these situations.

mm/ksm.c | 13 +++++++++++++
mm/memory.c | 25 ++++++++++++++++---------
mm/mmap.c | 6 ++++++
mm/rmap.c | 23 ++++++++++++++++++++---
4 files changed, 55 insertions(+), 12 deletions(-)

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