Re: [PATCH] mm: thp: fix pmd_bad() triggering in code paths holding mmap_sem read mode

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Mar 15, 2012 at 03:45:04PM -0700, Andrew Morton wrote:
> Or do we still need pdm_trans_unstable() checking in
> mem_cgroup_count_precharge_pte_range() and
> mem_cgroup_move_charge_pte_range()?

I think we need a pmd_trans_unstable check before the
pte_offset_map_lock in both places. Otherwise with only the mmap_sem
hold for reading, the pmd may have been transhuge,
mem_cgroup_move_charge_pte_range could be called, and then
MADV_DONTNEED would transform the pmd to none from another thread just
before pmd_trans_huge_lock runs, and we would end up doing
pmd_offset_map_lock on a none pmd (or a transhuge pmd if it becomes
huge again before we get there).

Only if pmd_trans_unstable is false, the pmd can't change from under
us, so we can proceed safely with the pte level walk (and it just need
to be checked with a compiler barrier, as the real pmd changes freely
from under us).

pmd_trans_unstable will never actually trigger unless we're hitting
the race, if the pmd was none when we started the walk we'd abort at
the higher level (method not called), if the pmd was transhuge we'd
stop at the pmd_trans_huge_lock() == 1 branch. So the only way to run
pmd_trans_unstable is when the result is undefined, i.e. the pmd was
not none initially but it become none or transhuge or none again at
some point, so we can just simply consider it still none and skip for
the undefined case.

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