Re: [PATCH v6 03/19] mm: Use readahead_control to pass arguments

Dave Chinner
2020-02-18T05:03:08Z

On Mon, Feb 17, 2020 at 10:45:44AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> In this patch, only between __do_page_cache_readahead() and
> read_pages(), but it will be extended in upcoming patches. Also add
> the readahead_count() accessor.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> include/linux/pagemap.h | 17 +++++++++++++++++
> mm/readahead.c | 36 +++++++++++++++++++++---------------
> 2 files changed, 38 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index ccb14b6a16b5..982ecda2d4a2 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -630,6 +630,23 @@ static inline int add_to_page_cache(struct page *page,
> return error;
> }
>
> +/*
> + * Readahead is of a block of consecutive pages.
> + */
> +struct readahead_control {
> + struct file *file;
> + struct address_space *mapping;
> +/* private: use the readahead_* accessors instead */
> + pgoff_t _start;
> + unsigned int _nr_pages;
> +};
> +
> +/* The number of pages in this readahead block */
> +static inline unsigned int readahead_count(struct readahead_control *rac)
> +{
> + return rac->_nr_pages;
> +}
> +
> static inline unsigned long dir_pages(struct inode *inode)
> {
> return (unsigned long)(inode->i_size + PAGE_SIZE - 1) >>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 12d13b7792da..15329309231f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,26 +113,29 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>
> EXPORT_SYMBOL(read_cache_pages);
>
> -static void read_pages(struct address_space *mapping, struct file *filp,
> - struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages,
> + gfp_t gfp)
> {
> + const struct address_space_operations *aops = rac->mapping->a_ops;
> struct blk_plug plug;
> unsigned page_idx;

Splitting out the aops rather than the mapping here just looks
weird, especially as you need the mapping later in the function.
Using aops doesn't even reduce the code side....

>
> blk_start_plug(&plug);
>
> - if (mapping->a_ops->readpages) {
> - mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
> + if (aops->readpages) {
> + aops->readpages(rac->file, rac->mapping, pages,
> + readahead_count(rac));
> /* Clean up the remaining pages */
> put_pages_list(pages);
> goto out;
> }
>
> - for (page_idx = 0; page_idx < nr_pages; page_idx++) {
> + 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, mapping, page->index, gfp))
> - mapping->a_ops->readpage(filp, page);
> + if (!add_to_page_cache_lru(page, rac->mapping, page->index,
> + gfp))
> + aops->readpage(rac->file, page);

... it just makes this less readable by splitting the if() over two
lines...

> put_page(page);
> }
>
> @@ -155,9 +158,13 @@ void __do_page_cache_readahead(struct address_space *mapping,
> unsigned long end_index; /* The last page we want to read */
> LIST_HEAD(page_pool);
> int page_idx;
> - unsigned int nr_pages = 0;
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> + struct readahead_control rac = {
> + .mapping = mapping,
> + .file = filp,
> + ._nr_pages = 0,
> + };

No need to initialise _nr_pages to zero, leaving it out will do the
same thing.

>
> if (isize == 0)
> return;
> @@ -180,10 +187,9 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * contiguous pages before continuing with the next
> * batch.
> */
> - if (nr_pages)
> - read_pages(mapping, filp, &page_pool, nr_pages,
> - gfp_mask);
> - nr_pages = 0;
> + if (readahead_count(&rac))
> + read_pages(&rac, &page_pool, gfp_mask);
> + rac._nr_pages = 0;

Hmmm. Wondering ig it make sense to move the gfp_mask to the readahead
control structure - if we have to pass the gfp_mask down all the
way along side the rac, then I think it makes sense to do that...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com