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From:   "Fabio M. De Francesco" <fabio.maria.de.francesco@linux.intel.com>
To:     Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     Ira Weiny <ira.weiny@intel.com>
Subject: Re: [PATCH 1007/1007] mm/zswap: Replace kmap_atomic() with kmap_local_page()
Date:   Mon, 27 Nov 2023 17:00:46 +0100
Message-ID: <4529535.LvFx2qVVIh@fdefranc-mobl3>
Organization: intel
In-Reply-To: <20231127154636.585898-1-fabio.maria.de.francesco@linux.intel.com>
References: <20231127154636.585898-1-fabio.maria.de.francesco@linux.intel.com>
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Please discard this patch for a mistake in the subject.

Fabio

On Monday, 27 November 2023 16:46:22 CET Fabio M. De Francesco 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_aatomic(), 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;
>  	}