Re: [PATCH RFC] mm: Let readahead submit larger batches of pages in case of ra->ra_pages == 0

[Andrew Morton]

On Fri,  4 Sep 2020 16:48:07 +0200 Bean Huo <huobean@xxxxxxxxx> wrote:

> From: Bean Huo <beanhuo@xxxxxxxxxx>
> 
> Current generic_file_buffered_read() will break up the larger batches of pages
> and read data in single page length in case of ra->ra_pages == 0. This patch is
> to allow it to pass the batches of pages down to the device if the supported
> maximum IO size >= the requested size.
> 
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2062,6 +2062,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  	struct file *filp = iocb->ki_filp;
>  	struct address_space *mapping = filp->f_mapping;
>  	struct inode *inode = mapping->host;
> +	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
>  	struct file_ra_state *ra = &filp->f_ra;
>  	loff_t *ppos = &iocb->ki_pos;
>  	pgoff_t index;
> @@ -2098,9 +2099,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  		if (!page) {
>  			if (iocb->ki_flags & IOCB_NOIO)
>  				goto would_block;
> -			page_cache_sync_readahead(mapping,
> -					ra, filp,
> -					index, last_index - index);
> +
> +			if (!ra->ra_pages && bdi->io_pages >= last_index - index)
> +				__do_page_cache_readahead(mapping, filp, index,
> +							  last_index - index, 0);
> +			else
> +				page_cache_sync_readahead(mapping, ra, filp,
> +							  index,
> +							  last_index - index);
>  			page = find_get_page(mapping, index);
>  			if (unlikely(page == NULL))
>  				goto no_cached_page;

I assume this is a performance patch.  What are the observed changes in
behaviour?

What is special about ->ra_pages==0?  Wouldn't this optimization still
be valid if ->ra_pages==2?

Doesn't this defeat the purpose of having ->ra_pages==0?