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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 16 Mar 2012 00:15:56 +0100
Andrea Arcangeli <aarcange@redhat.com> wrote:

> 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).

page_table_lock doesn't prevent the race?  pmd_trans_huge_lock()
rechecks after taking that lock...

> 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.

Naoya, could you please take a look into this?

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