Re: [PATCH v4 12/12] mm: change the contract for free_pgtables(), update docs

[David Hildenbrand (Arm)]

On 9/22/26 17:35, Lorenzo Stoakes (ARM) wrote:
> Now that page tables are freed after an RCU grace period, it is safe for
> read-only page table walkers to walk page table ranges that are being
> concurrently torn down, provided the mm is kept alive via mmgrab().
> 
> It is however unsafe for writers to do so, as they must obtain an
> appropriate lock to do so safely.
> 
> Update the pte_offset_map_lock()'s comment block to reflect this.
> 
> Similarly update the process addresses documentation.
> 
> Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---
>  Documentation/mm/process_addrs.rst |  6 ++++++
>  mm/pgtable-generic.c               | 15 +++++++++++----
>  2 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
> index a7296f251799..b1f4f44d75eb 100644
> --- a/Documentation/mm/process_addrs.rst
> +++ b/Documentation/mm/process_addrs.rst
> @@ -537,6 +537,12 @@ We establish basic locking rules when interacting with page tables:
>  * When changing a page table entry the page table lock for that page table
>    **must** be held, except if you can safely assume nobody can access the page
>    tables concurrently (such as on invocation of :c:func:`!free_pgtables`).
> +* Page tables may be *walked* under RCU alone, as page tables are freed only
> +  after an RCU grace period has elapsed. However, any entry found must be
> +  revalidated after the page table lock is taken (such as the
> +  :c:func:`!pmd_same` recheck performed by :c:func:`!pte_offset_map_lock`)
> +  before it is acted upon. Changing an entry requires the page table
> +  lock and one of the locks that excludes teardown (mmap or VMA lock).

While we can walk the page tables, I assume there are some limits to what we can
actually do with leaf entries. E.g., Doing a careless pte->folio lookup might be
dangerous.

I wonder if we want to hint at that here: that walking under RCU (traversing
page tables) is something different than actually operating on the leaf entries.

Apart from that LGTM.

-- 
Cheers,

David