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>
