linux-kernel - Re: [PATCH] page_io: return proper error codes for swap_read_folio

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Message-ID: <Z8oy0A-vBbGI6ux9@google.com>
Date: Thu, 6 Mar 2025 23:42:08 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Nhat Pham <nphamcs@...il.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, chengming.zhou@...ux.dev,
	linux-mm@...ck.org, kernel-team@...a.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] page_io: return proper error codes for
 swap_read_folio_zeromap()

On Thu, Mar 06, 2025 at 03:00:15PM -0800, Nhat Pham wrote:
> Similar to zswap_load(), also return proper error codes for
> swap_read_folio_zeromap():
> 
> * 0 on success. The folio is unlocked and marked up-to-date.
> * -ENOENT, if the folio is entirely not zeromapped.
> * -EINVAL (with the follio unlocked but not marked to date), if the
>   folio is partially zeromapped. This is not supported, and will SIGBUS
>   the faulting process.
> 
> This patch is purely a clean-up, and should not have any behavioral
> change. It is based on (and should be applied on top of) [1].
> 
> [1]: https://lore.kernel.org/linux-mm/20250306205011.784787-1-nphamcs@gmail.com/
> 
> Suggested-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Suggested-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Nhat Pham <nphamcs@...il.com>
> ---
>  mm/page_io.c | 35 ++++++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 4bce19df557b..48ed1e810392 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -511,7 +511,23 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>  	mempool_free(sio, sio_pool);
>  }
>  
> -static bool swap_read_folio_zeromap(struct folio *folio)
> +/**
> + * swap_read_folio_zeromap - check if the folio was zeromapped, and if so,
> + *                           zero-fill it.
> + * @folio: the folio.
> + *
> + * Return: 0 on success, with the folio zero-filled, unlocked, and marked
> + * up-to-date, or one of the following error codes:
> + *
> + *  -ENOENT: the folio is entirely not zeromapped. The folio remains locked.
> + *
> + *  -EINVAL: some of the subpages in the folio are zeromaped, but not all of
> + *  them. This is an error because we don't currently support a large folio
> + *  that is partially in the zeromap. The folio is unlocked, but NOT marked
> + *  up-to-date, so that an IO error is emitted (e.g. do_swap_page() will
> + *  sigbus).

This is a bit repetitive. Maybe:

 *  -EINVAL: The folio is partially in the zeromap, which is not
 *  currently supported. The folio is unlocked, but NOT marked
 *  up-to-date, so that an IO error is emitted (e.g. do_swap_page() will
 *  sigbus).


> + */
> +static int swap_read_folio_zeromap(struct folio *folio)
>  {
>  	int nr_pages = folio_nr_pages(folio);
>  	struct obj_cgroup *objcg;
> @@ -519,15 +535,17 @@ static bool swap_read_folio_zeromap(struct folio *folio)
>  
>  	/*
>  	 * Swapping in a large folio that is partially in the zeromap is not
> -	 * currently handled. Return true without marking the folio uptodate so
> +	 * currently handled. Return -EINVAL without marking the folio uptodate so
>  	 * that an IO error is emitted (e.g. do_swap_page() will sigbus).
>  	 */

I would drop this whole comment now because it's mostly repeating what's
now documneted above.

With the comments fixed up:

Reviewed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>

>  	if (WARN_ON_ONCE(swap_zeromap_batch(folio->swap, nr_pages,
> -			&is_zeromap) != nr_pages))
> -		return true;
> +			&is_zeromap) != nr_pages)) {
> +		folio_unlock(folio);
> +		return -EINVAL;
> +	}
>  
>  	if (!is_zeromap)
> -		return false;
> +		return -ENOENT;
>  
>  	objcg = get_obj_cgroup_from_folio(folio);
>  	count_vm_events(SWPIN_ZERO, nr_pages);
> @@ -538,7 +556,8 @@ static bool swap_read_folio_zeromap(struct folio *folio)
>  
>  	folio_zero_range(folio, 0, folio_size(folio));
>  	folio_mark_uptodate(folio);
> -	return true;
> +	folio_unlock(folio);
> +	return 0;
>  }
>  
>  static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
> @@ -635,10 +654,8 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
>  	}
>  	delayacct_swapin_start();
>  
> -	if (swap_read_folio_zeromap(folio)) {
> -		folio_unlock(folio);
> +	if (swap_read_folio_zeromap(folio) != -ENOENT)
>  		goto finish;
> -	}
>  
>  	if (zswap_load(folio) != -ENOENT)
>  		goto finish;
> -- 
> 2.43.5
>