Re: [PATCH v7 09/24] mm: Put readahead pages in cache earlier

[Christoph Hellwig]

On Wed, Feb 19, 2020 at 01:00:48PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> When populating the page cache for readahead, mappings that use
> ->readpages must populate the page cache themselves as the pages are
> passed on a linked list which would normally be used for the page cache's
> LRU.  For mappings that use ->readpage or the upcoming ->readahead method,
> we can put the pages into the page cache as soon as they're allocated,
> which solves a race between readahead and direct IO.  It also lets us
> remove the gfp argument from read_pages().
> 
> Use the new readahead_page() API to implement the repeated calls to
> ->readpage(), just like most filesystems will.  This iterator also
> supports huge pages, even though none of the filesystems have been
> converted to use them yet.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  include/linux/pagemap.h | 20 +++++++++++++++++
>  mm/readahead.c          | 48 +++++++++++++++++++++++++----------------
>  2 files changed, 49 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 55fcea0249e6..4989d330fada 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -647,8 +647,28 @@ struct readahead_control {
>  /* private: use the readahead_* accessors instead */
>  	pgoff_t _index;
>  	unsigned int _nr_pages;
> +	unsigned int _batch_count;
>  };
>  
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> +	struct page *page;
> +
> +	BUG_ON(rac->_batch_count > rac->_nr_pages);
> +	rac->_nr_pages -= rac->_batch_count;
> +	rac->_index += rac->_batch_count;
> +	rac->_batch_count = 0;
> +
> +	if (!rac->_nr_pages)
> +		return NULL;
> +
> +	page = xa_load(&rac->mapping->i_pages, rac->_index);
> +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +	rac->_batch_count = hpage_nr_pages(page);
> +
> +	return page;
> +}
> +
>  /* The number of pages in this readahead block */
>  static inline unsigned int readahead_count(struct readahead_control *rac)
>  {
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 83df5c061d33..aaa209559ba2 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,15 +113,14 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>  
>  EXPORT_SYMBOL(read_cache_pages);
>  
> -static void read_pages(struct readahead_control *rac, struct list_head *pages,
> -		gfp_t gfp)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages)
>  {
>  	const struct address_space_operations *aops = rac->mapping->a_ops;
> +	struct page *page;
>  	struct blk_plug plug;
> -	unsigned page_idx;
>  
>  	if (!readahead_count(rac))
> -		return;
> +		goto out;
>  
>  	blk_start_plug(&plug);
>  
> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
>  				readahead_count(rac));
>  		/* Clean up the remaining pages */
>  		put_pages_list(pages);
> -		goto out;
> -	}
> -
> -	for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
> -		struct page *page = lru_to_page(pages);
> -		list_del(&page->lru);
> -		if (!add_to_page_cache_lru(page, rac->mapping, page->index,
> -				gfp))
> +		rac->_index += rac->_nr_pages;
> +		rac->_nr_pages = 0;
> +	} else {
> +		while ((page = readahead_page(rac))) {
>  			aops->readpage(rac->file, page);
> -		put_page(page);
> +			put_page(page);
> +		}
>  	}
>  
> -out:
>  	blk_finish_plug(&plug);
>  
>  	BUG_ON(!list_empty(pages));
> -	rac->_nr_pages = 0;
> +	BUG_ON(readahead_count(rac));
> +
> +out:
> +	/* If we were called due to a conflicting page, skip over it */
> +	rac->_index++;
>  }
>  
>  /*
> @@ -165,9 +164,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	LIST_HEAD(page_pool);
>  	loff_t isize = i_size_read(inode);
>  	gfp_t gfp_mask = readahead_gfp_mask(mapping);
> +	bool use_list = mapping->a_ops->readpages;

I find this single use variable a little weird.  Not a dealbreaker,
but just checking the methods would seem a little more obvious to me.

Except for this and the other nitpick the patch looks good to me:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>