Re: [PATCH] mm/zswap: Replace kmap_atomic() with kmap_local_page()

[Nhat Pham <nphamcs@gmail.com>]

On Mon, Nov 27, 2023 at 8:03 AM Fabio M. De Francesco
<fabio.maria.de.francesco@linux.intel.com> wrote:
>
> kmap_atomic() has been deprecated in favor of kmap_local_page().
>
> Therefore, replace kmap_atomic() with kmap_local_page() in
> zswap.c.
>
> kmap_atomic() is implemented like a kmap_local_page() which also
> disables page-faults and preemption (the latter only in !PREEMPT_RT
> kernels). The kernel virtual addresses returned by these two API are
> only valid in the context of the callers (i.e., they cannot be handed to
> other threads).
>
> With kmap_local_page() the mappings are per thread and CPU local like
> in kmap_atomic(); however, they can handle page-faults and can be called
> from any context (including interrupts). The tasks that call
> kmap_local_page() can be preempted and, when they are scheduled to run
> again, the kernel virtual addresses are restored and are still valid.
>
> In mm/zswap.c, the blocks of code between the mappings and un-mappings
> do not depend on the above-mentioned side effects of kmap_atomic(), so
> that the mere replacements of the old API with the new one is all that is
> required (i.e., there is no need to explicitly call pagefault_disable()
> and/or preempt_disable()).
>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
> ---
>  mm/zswap.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 74411dfdad92..699c6ee11222 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1267,16 +1267,16 @@ bool zswap_store(struct folio *folio)
>         }
>
>         if (zswap_same_filled_pages_enabled) {
> -               src = kmap_atomic(page);
> +               src = kmap_local_page(page);
>                 if (zswap_is_page_same_filled(src, &value)) {
> -                       kunmap_atomic(src);
> +                       kunmap_local(src);
>                         entry->swpentry = swp_entry(type, offset);
>                         entry->length = 0;
>                         entry->value = value;
>                         atomic_inc(&zswap_same_filled_pages);
>                         goto insert_entry;
>                 }
> -               kunmap_atomic(src);
> +               kunmap_local(src);
>         }
>
>         if (!zswap_non_same_filled_pages_enabled)
> @@ -1422,9 +1422,9 @@ bool zswap_load(struct folio *folio)
>         spin_unlock(&tree->lock);
>
>         if (!entry->length) {
> -               dst = kmap_atomic(page);
> +               dst = kmap_local_page(page);
>                 zswap_fill_page(dst, entry->value);
> -               kunmap_atomic(dst);
> +               kunmap_local(dst);
>                 ret = true;
>                 goto stats;
>         }
> --
> 2.42.0
>
>
Probably worth running a couple rounds of stress tests, but otherwise
LGTM. FWIW, I've wanted to do this ever since I worked on the
storing-uncompressed-pages patch.


Reviewed-by: Nhat Pham <nphamcs@gmail.com>