linux-kernel - Re: [PATCH 1006/1007] mm/swapfile: Replace kmap_atomic() with kmap_local

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Message-ID: <12332144.O9o76ZdvQC@fdefranc-mobl3>
Date:   Mon, 27 Nov 2023 16:51:07 +0100
From:   "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 1006/1007] mm/swapfile: Replace kmap_atomic() with
 kmap_local_page()

Please discard this patch for a mistake in the subject.

Fabio

On Monday, 27 November 2023 16:45:53 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
> swapfile.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/swapfile.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@...el.com>
> Signed-off-by: Fabio M. De Francesco
> <fabio.maria.de.francesco@...ux.intel.com> ---
>  mm/swapfile.c | 34 +++++++++++++++++-----------------
>  1 file changed, 17 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4bc70f459164..8be70912e298 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1495,9 +1495,9 @@ int swp_swapcount(swp_entry_t entry)
> 
>  	do {
>  		page = list_next_entry(page, lru);
> -		map = kmap_atomic(page);
> +		map = kmap_local_page(page);
>  		tmp_count = map[offset];
> -		kunmap_atomic(map);
> +		kunmap_local(map);
> 
>  		count += (tmp_count & ~COUNT_CONTINUED) * n;
>  		n *= (SWAP_CONT_MAX + 1);
> @@ -3477,9 +3477,9 @@ int add_swap_count_continuation(swp_entry_t entry,
> gfp_t gfp_mask) if (!(count & COUNT_CONTINUED))
>  			goto out_unlock_cont;
> 
> -		map = kmap_atomic(list_page) + offset;
> +		map = kmap_local_page(list_page) + offset;
>  		count = *map;
> -		kunmap_atomic(map);
> +		kunmap_local(map);
> 
>  		/*
>  		 * If this continuation count now has some space in it,
> @@ -3529,7 +3529,7 @@ static bool swap_count_continued(struct
> swap_info_struct *si, spin_lock(&si->cont_lock);
>  	offset &= ~PAGE_MASK;
>  	page = list_next_entry(head, lru);
> -	map = kmap_atomic(page) + offset;
> +	map = kmap_local_page(page) + offset;
> 
>  	if (count == SWAP_MAP_MAX)	/* initial increment from swap_map */
>  		goto init_map;		/* jump over SWAP_CONT_MAX 
checks */
> @@ -3539,27 +3539,27 @@ static bool swap_count_continued(struct
> swap_info_struct *si, * Think of how you add 1 to 999
>  		 */
>  		while (*map == (SWAP_CONT_MAX | COUNT_CONTINUED)) {
> -			kunmap_atomic(map);
> +			kunmap_local(map);
>  			page = list_next_entry(page, lru);
>  			BUG_ON(page == head);
> -			map = kmap_atomic(page) + offset;
> +			map = kmap_local_page(page) + offset;
>  		}
>  		if (*map == SWAP_CONT_MAX) {
> -			kunmap_atomic(map);
> +			kunmap_local(map);
>  			page = list_next_entry(page, lru);
>  			if (page == head) {
>  				ret = false;	/* add count continuation */
>  				goto out;
>  			}
> -			map = kmap_atomic(page) + offset;
> +			map = kmap_local_page(page) + offset;
>  init_map:		*map = 0;		/* we didn't zero the page */
>  		}
>  		*map += 1;
> -		kunmap_atomic(map);
> +		kunmap_local(map);
>  		while ((page = list_prev_entry(page, lru)) != head) {
> -			map = kmap_atomic(page) + offset;
> +			map = kmap_local_page(page) + offset;
>  			*map = COUNT_CONTINUED;
> -			kunmap_atomic(map);
> +			kunmap_local(map);
>  		}
>  		ret = true;			/* incremented */
> 
> @@ -3569,21 +3569,21 @@ init_map:		*map = 0;		/* we didn't 
zero the page */
>  		 */
>  		BUG_ON(count != COUNT_CONTINUED);
>  		while (*map == COUNT_CONTINUED) {
> -			kunmap_atomic(map);
> +			kunmap_local(map);
>  			page = list_next_entry(page, lru);
>  			BUG_ON(page == head);
> -			map = kmap_atomic(page) + offset;
> +			map = kmap_local_page(page) + offset;
>  		}
>  		BUG_ON(*map == 0);
>  		*map -= 1;
>  		if (*map == 0)
>  			count = 0;
> -		kunmap_atomic(map);
> +		kunmap_local(map);
>  		while ((page = list_prev_entry(page, lru)) != head) {
> -			map = kmap_atomic(page) + offset;
> +			map = kmap_local_page(page) + offset;
>  			*map = SWAP_CONT_MAX | count;
>  			count = COUNT_CONTINUED;
> -			kunmap_atomic(map);
> +			kunmap_local(map);
>  		}
>  		ret = count == COUNT_CONTINUED;
>  	}