Re: [PATCH V1 1/2] docs/mm: explain when and why rmap locks need to be taken during mremap()

[Jonathan Corbet]

Harry Yoo <harry.yoo@xxxxxxxxxx> writes:

> While move_ptes() has a comment explaining why rmap locks are needed,
> Documentation/mm/process_addrs.rst does not. Without being aware of that
> comment, I spent hours figuring out how things could go wrong and why,
> in some cases, rmap locks can be safely skipped.
>
> Add a more comprehensive explanation to the documentation to save time
> for others.
>
> Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> ---
>  Documentation/mm/process_addrs.rst | 32 ++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
> index be49e2a269e4..ee7c0dba339e 100644
> --- a/Documentation/mm/process_addrs.rst
> +++ b/Documentation/mm/process_addrs.rst
> @@ -744,6 +744,38 @@ You can observe this in the :c:func:`!mremap` implementation in the functions
>  :c:func:`!take_rmap_locks` and :c:func:`!drop_rmap_locks` which perform the rmap
>  side of lock acquisition, invoked ultimately by :c:func:`!move_page_tables`.
>  
> +.. note:: If :c:func:`!mremap()` -> :c:func:`!move_ptes()` does not take rmap
> +          locks, :c:func:`!rmap_walk()` may miss a pte for the folio.
> +
> +          The problematic sequence is as follows:

Please don't use :c:func: - just write function() and all the right
things will happen.  (For extra credit, fix the existing usages :)

Thanks,

jon