Re: [PATCH 04/13] mm: handle readahead in generic_file_buffered_read_pagenotuptodate

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Sat, Oct 31, 2020 at 09:59:55AM +0100, Christoph Hellwig wrote:
> Move the calculation of the per-page variables and the readahead handling
> from the only caller into generic_file_buffered_read_pagenotuptodate,
> which now becomes a routine to handle everything related to bringing
> one page uptodate and thus is renamed to filemap_read_one_page.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  mm/filemap.c | 63 +++++++++++++++++++++++-----------------------------
>  1 file changed, 28 insertions(+), 35 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index bae5b905aa7bdc..5cdf8090d4e12c 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2217,13 +2217,26 @@ static int filemap_readpage(struct kiocb *iocb, struct page *page)
>  	return error;
>  }
>  
> -static int generic_file_buffered_read_pagenotuptodate(struct kiocb *iocb,
> -		struct iov_iter *iter, struct page *page, loff_t pos,
> -		loff_t count, bool first)
> +static int filemap_make_page_uptodate(struct kiocb *iocb, struct iov_iter *iter,
> +		struct page *page, pgoff_t pg_index, bool first)

I prefer "filemap_update_page".

I don't understand why you pass in pg_index instead of using page->index.
We dereferenced the page pointer already to check PageReadahead(), so
there's no performance issue here.

Also, if filemap_find_get_pages() stops on the first !Uptodate or
Readahead page, as I had it in my patchset, then we don't need the loop
at all -- filemap_read_pages() looks like:

        nr_pages = filemap_find_get_pages(iocb, iter, pages, nr);
	if (!nr_pages) {
		pages[0] = filemap_create_page(iocb, iter);
		if (!IS_ERR_OR_NULL(pages[0]))
			return 1;
		if (!pages[0])
			goto retry;
		return PTR_ERR(pages[0]);
	}

	page = pages[nr_pages - 1];
	if (PageUptodate(page) && !PageReadahead(page))
		return nr_pages;
	err = filemap_update_page(iocb, iter, page);
	if (!err)
		return nr_pages;
	nr_pages -= 1;
	if (nr_pages)
		return nr_pages;
	return err;