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Date: Tue, 13 Feb 2024 16:00:23 +0100
From: Hannes Reinecke <hare@...e.de>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc: mcgrof@...nel.org, gost.dev@...sung.com, akpm@...ux-foundation.org,
 kbusch@...nel.org, djwong@...nel.org, chandan.babu@...cle.com,
 p.raghav@...sung.com, linux-kernel@...r.kernel.org, willy@...radead.org,
 linux-mm@...ck.org, david@...morbit.com
Subject: Re: [RFC v2 05/14] readahead: align index to mapping_min_order in
 ondemand_ra and force_ra

On 2/13/24 10:37, Pankaj Raghav (Samsung) wrote:
> From: Luis Chamberlain <mcgrof@...nel.org>
> 
> Align the ra->start and ra->size to mapping_min_order in
> ondemand_readahead(), and align the index to mapping_min_order in
> force_page_cache_ra(). This will ensure that the folios allocated for
> readahead that are added to the page cache are aligned to
> mapping_min_order.
> 
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> ---
>   mm/readahead.c | 48 ++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 40 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 4fa7d0e65706..5e1ec7705c78 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -315,6 +315,7 @@ void force_page_cache_ra(struct readahead_control *ractl,
>   	struct file_ra_state *ra = ractl->ra;
>   	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
>   	unsigned long max_pages, index;
> +	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
>   
>   	if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
>   		return;
> @@ -324,6 +325,13 @@ void force_page_cache_ra(struct readahead_control *ractl,
>   	 * be up to the optimal hardware IO size
>   	 */
>   	index = readahead_index(ractl);
> +	if (!IS_ALIGNED(index, min_nrpages)) {
> +		unsigned long old_index = index;
> +
> +		index = round_down(index, min_nrpages);
> +		nr_to_read += (old_index - index);
> +	}
> +
>   	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
>   	nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
>   	while (nr_to_read) {
> @@ -332,6 +340,7 @@ void force_page_cache_ra(struct readahead_control *ractl,
>   		if (this_chunk > nr_to_read)
>   			this_chunk = nr_to_read;
>   		ractl->_index = index;
> +		VM_BUG_ON(!IS_ALIGNED(index, min_nrpages));
>   		do_page_cache_ra(ractl, this_chunk, 0);
>   
>   		index += this_chunk;
> @@ -344,11 +353,20 @@ void force_page_cache_ra(struct readahead_control *ractl,
>    * for small size, x 4 for medium, and x 2 for large
>    * for 128k (32 page) max ra
>    * 1-2 page = 16k, 3-4 page 32k, 5-8 page = 64k, > 8 page = 128k initial
> + *
> + * For higher order address space requirements we ensure no initial reads
> + * are ever less than the min number of pages required.
> + *
> + * We *always* cap the max io size allowed by the device.
>    */
> -static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
> +static unsigned long get_init_ra_size(unsigned long size,
> +				      unsigned int min_nrpages,
> +				      unsigned long max)
>   {
>   	unsigned long newsize = roundup_pow_of_two(size);
>   
> +	newsize = max_t(unsigned long, newsize, min_nrpages);
> +
>   	if (newsize <= max / 32)
>   		newsize = newsize * 4;
>   	else if (newsize <= max / 4)
> @@ -356,6 +374,8 @@ static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
>   	else
>   		newsize = max;
>   
> +	VM_BUG_ON(newsize & (min_nrpages - 1));
> +
>   	return newsize;
>   }
>   
> @@ -364,14 +384,16 @@ static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
>    *  return it as the new window size.
>    */
>   static unsigned long get_next_ra_size(struct file_ra_state *ra,
> +				      unsigned int min_nrpages,
>   				      unsigned long max)
>   {
> -	unsigned long cur = ra->size;
> +	unsigned long cur = max(ra->size, min_nrpages);
>   
>   	if (cur < max / 16)
>   		return 4 * cur;
>   	if (cur <= max / 2)
>   		return 2 * cur;
> +
>   	return max;
>   }
>   
> @@ -561,7 +583,11 @@ static void ondemand_readahead(struct readahead_control *ractl,
>   	unsigned long add_pages;
>   	pgoff_t index = readahead_index(ractl);
>   	pgoff_t expected, prev_index;
> -	unsigned int order = folio ? folio_order(folio) : 0;
> +	unsigned int min_order = mapping_min_folio_order(ractl->mapping);
> +	unsigned int min_nrpages = mapping_min_folio_nrpages(ractl->mapping);
> +	unsigned int order = folio ? folio_order(folio) : min_order;
> +
> +	VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));
>   
>   	/*
>   	 * If the request exceeds the readahead window, allow the read to
> @@ -583,8 +609,8 @@ static void ondemand_readahead(struct readahead_control *ractl,
>   	expected = round_down(ra->start + ra->size - ra->async_size,
>   			1UL << order);
>   	if (index == expected || index == (ra->start + ra->size)) {
> -		ra->start += ra->size;
> -		ra->size = get_next_ra_size(ra, max_pages);
> +		ra->start += round_down(ra->size, min_nrpages);
> +		ra->size = get_next_ra_size(ra, min_nrpages, max_pages);
>   		ra->async_size = ra->size;
>   		goto readit;
>   	}
> @@ -603,13 +629,18 @@ static void ondemand_readahead(struct readahead_control *ractl,
>   				max_pages);
>   		rcu_read_unlock();
>   
> +		start = round_down(start, min_nrpages);
> +
> +		VM_BUG_ON(folio->index & (folio_nr_pages(folio) - 1));
> +
>   		if (!start || start - index > max_pages)
>   			return;
>   
>   		ra->start = start;
>   		ra->size = start - index;	/* old async_size */
> +

Stale whitespace.

>   		ra->size += req_size;
> -		ra->size = get_next_ra_size(ra, max_pages);
> +		ra->size = get_next_ra_size(ra, min_nrpages, max_pages);
>   		ra->async_size = ra->size;
>   		goto readit;
>   	}
> @@ -646,7 +677,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
>   
>   initial_readahead:
>   	ra->start = index;
> -	ra->size = get_init_ra_size(req_size, max_pages);
> +	ra->size = get_init_ra_size(req_size, min_nrpages, max_pages);
>   	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
>   
>   readit:
> @@ -657,7 +688,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
>   	 * Take care of maximum IO pages as above.
>   	 */
>   	if (index == ra->start && ra->size == ra->async_size) {
> -		add_pages = get_next_ra_size(ra, max_pages);
> +		add_pages = get_next_ra_size(ra, min_nrpages, max_pages);
>   		if (ra->size + add_pages <= max_pages) {
>   			ra->async_size = add_pages;
>   			ra->size += add_pages;
> @@ -668,6 +699,7 @@ static void ondemand_readahead(struct readahead_control *ractl,
>   	}
>   
>   	ractl->_index = ra->start;
> +	VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));
>   	page_cache_ra_order(ractl, ra, order);
>   }
>   
Otherwise looks good.

Cheers,

Hannes


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