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Subject: Re: [PATCH v2 04/32] mm/pgtable: allow pte_offset_map[_lock]() to
 fail
Date:   Mon, 10 Jul 2023 21:23:19 -0400
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Content-Type: text/plain

On 8 Jun 2023, at 21:10, Hugh Dickins wrote:

> Make pte_offset_map() a wrapper for __pte_offset_map() (optionally
> outputs pmdval), pte_offset_map_lock() a sparse __cond_lock wrapper for
> __pte_offset_map_lock(): those __funcs added in mm/pgtable-generic.c.
>
> __pte_offset_map() do pmdval validation (including pmd_clear_bad()
> when pmd_bad()), returning NULL if pmdval is not for a page table.
> __pte_offset_map_lock() verify pmdval unchanged after getting the
> lock, trying again if it changed.
>
> No #ifdef CONFIG_TRANSPARENT_HUGEPAGE around them: that could be done
> to cover the imminent case, but we expect to generalize it later, and
> it makes a mess of where to do the pmd_bad() clearing.
>
> Add pte_offset_map_nolock(): outputs ptl like pte_offset_map_lock(),
> without actually taking the lock.  This will be preferred to open uses of
> pte_lockptr(), because (when split ptlock is in page table's struct page)
> it points to the right lock for the returned pte pointer, even if *pmd
> gets changed racily afterwards.
>
> Update corresponding Documentation.
>
> Do not add the anticipated rcu_read_lock() and rcu_read_unlock()s yet:
> they have to wait until all architectures are balancing pte_offset_map()s
> with pte_unmap()s (as in the arch series posted earlier).  But comment
> where they will go, so that it's easy to add them for experiments.  And
> only when those are in place can transient racy failure cases be enabled.
> Add more safety for the PAE mismatched pmd_low pmd_high case at that time.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>  Documentation/mm/split_page_table_lock.rst | 17 ++++---
>  include/linux/mm.h                         | 27 +++++++----
>  include/linux/pgtable.h                    | 22 ++++++---
>  mm/pgtable-generic.c                       | 56 ++++++++++++++++++++++
>  4 files changed, 101 insertions(+), 21 deletions(-)
LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi

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