Re: [RFC v2 07/14] readahead: allocate folios with mapping_min_order in ra_(unbounded|order)

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Feb 13, 2024 at 10:37:06AM +0100, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> Allocate folios with at least mapping_min_order in
> page_cache_ra_unbounded() and page_cache_ra_order() as we need to
> guarantee a minimum order in the page cache.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

Acked-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  mm/readahead.c | 30 ++++++++++++++++++++++++++----
>  1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 13b62cbd3b79..a361fba18674 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -214,6 +214,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  	unsigned long index = readahead_index(ractl);
>  	gfp_t gfp_mask = readahead_gfp_mask(mapping);
>  	unsigned long i = 0;
> +	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
>  
>  	/*
>  	 * Partway through the readahead operation, we will have added
> @@ -235,6 +236,8 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  		struct folio *folio = xa_load(&mapping->i_pages, index + i);
>  
>  		if (folio && !xa_is_value(folio)) {
> +			long nr_pages = folio_nr_pages(folio);
> +
>  			/*
>  			 * Page already present?  Kick off the current batch
>  			 * of contiguous pages before continuing with the
> @@ -244,19 +247,31 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  			 * not worth getting one just for that.
>  			 */
>  			read_pages(ractl);
> -			ractl->_index += folio_nr_pages(folio);
> +
> +			/*
> +			 * Move the ractl->_index by at least min_pages
> +			 * if the folio got truncated to respect the
> +			 * alignment constraint in the page cache.
> +			 *
> +			 */
> +			if (mapping != folio->mapping)
> +				nr_pages = min_nrpages;
> +
> +			VM_BUG_ON_FOLIO(nr_pages < min_nrpages, folio);
> +			ractl->_index += nr_pages;
>  			i = ractl->_index + ractl->_nr_pages - index;
>  			continue;
>  		}
>  
> -		folio = filemap_alloc_folio(gfp_mask, 0);
> +		folio = filemap_alloc_folio(gfp_mask,
> +					    mapping_min_folio_order(mapping));
>  		if (!folio)
>  			break;
>  		if (filemap_add_folio(mapping, folio, index + i,
>  					gfp_mask) < 0) {
>  			folio_put(folio);
>  			read_pages(ractl);
> -			ractl->_index++;
> +			ractl->_index += min_nrpages;
>  			i = ractl->_index + ractl->_nr_pages - index;
>  			continue;
>  		}
> @@ -516,6 +531,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
>  {
>  	struct address_space *mapping = ractl->mapping;
>  	pgoff_t index = readahead_index(ractl);
> +	unsigned int min_order = mapping_min_folio_order(mapping);
>  	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
>  	pgoff_t mark = index + ra->size - ra->async_size;
>  	int err = 0;
> @@ -542,11 +558,17 @@ void page_cache_ra_order(struct readahead_control *ractl,
>  		if (index & ((1UL << order) - 1))
>  			order = __ffs(index);
>  		/* Don't allocate pages past EOF */
> -		while (index + (1UL << order) - 1 > limit)
> +		while (order > min_order && index + (1UL << order) - 1 > limit)
>  			order--;
>  		/* THP machinery does not support order-1 */
>  		if (order == 1)
>  			order = 0;
> +
> +		if (order < min_order)
> +			order = min_order;
> +
> +		VM_BUG_ON(index & ((1UL << order) - 1));
> +
>  		err = ra_alloc_folio(ractl, index, mark, order, gfp);
>  		if (err)
>  			break;
> -- 
> 2.43.0
> 
>