Re: [PATCH v6 07/19] mm: Put readahead pages in cache earlier

John Hubbard
2020-02-19T00:01:47Z

On 2/17/20 10:45 AM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> At allocation time, put the pages in the cache unless we're using
> ->readpages. Add the readahead_for_each() iterator for the benefit of
> the ->readpage fallback. This iterator supports huge pages, even though
> none of the filesystems to be converted do yet.
>

"Also, remove the gfp argument from read_pages(), now that read_pages()
no longer does allocation."

Generally looks accurate, just a few notes below:

> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> include/linux/pagemap.h | 24 ++++++++++++++++++++++++
> mm/readahead.c | 34 +++++++++++++++++-----------------
> 2 files changed, 41 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 982ecda2d4a2..3613154e79e4 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -639,8 +639,32 @@ struct readahead_control {
> /* private: use the readahead_* accessors instead */
> pgoff_t _start;
> unsigned int _nr_pages;
> + unsigned int _batch_count;
> };
>
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> + struct page *page;
> +
> + if (!rac->_nr_pages)
> + return NULL;
> +
> + page = xa_load(&rac->mapping->i_pages, rac->_start);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + rac->_batch_count = hpage_nr_pages(page);
> +
> + return page;
> +}
> +
> +static inline void readahead_next(struct readahead_control *rac)
> +{
> + rac->_nr_pages -= rac->_batch_count;
> + rac->_start += rac->_batch_count;
> +}
> +
> +#define readahead_for_each(rac, page) \
> + for (; (page = readahead_page(rac)); readahead_next(rac))
> +

How about this instead? It uses the "for" loop fully and more naturally,
and is easier to read. And it does the same thing:

static inline struct page *readahead_page(struct readahead_control *rac)
{
struct page *page;

if (!rac->_nr_pages)
return NULL;

page = xa_load(&rac->mapping->i_pages, rac->_start);
VM_BUG_ON_PAGE(!PageLocked(page), page);
rac->_batch_count = hpage_nr_pages(page);

return page;
}

static inline struct page *readahead_next(struct readahead_control *rac)
{
rac->_nr_pages -= rac->_batch_count;
rac->_start += rac->_batch_count;

return readahead_page(rac);
}

#define readahead_for_each(rac, page) \
for (page = readahead_page(rac); page != NULL; \
page = readahead_page(rac))

> /* 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 bdc5759000d3..9e430daae42f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,12 +113,11 @@ 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;
>
> blk_start_plug(&plug);
>
> @@ -127,19 +126,13 @@ 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))
> + } else {
> + readahead_for_each(rac, page) {
> aops->readpage(rac->file, page);
> - put_page(page);
> + put_page(page);
> + }
> }
>
> -out:
> blk_finish_plug(&plug);
> }
>
> @@ -159,6 +152,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> unsigned long i;
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> + bool use_list = mapping->a_ops->readpages;

fwiw, "bool have_readpages" seems like a better name (after all, that's how read_pages()
effectively is written: "if you have .readpages, then..."), but I can see both sides
of that bikeshed. :)

> struct readahead_control rac = {
> .mapping = mapping,
> .file = filp,
> @@ -196,8 +190,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
> page = __page_cache_alloc(gfp_mask);
> if (!page)
> break;
> - page->index = offset;
> - list_add(&page->lru, &page_pool);
> + if (use_list) {
> + page->index = offset;
> + list_add(&page->lru, &page_pool);
> + } else if (add_to_page_cache_lru(page, mapping, offset,
> + gfp_mask) < 0) {

It would be a little safer from a maintenance point of view, to check for !=0, rather
than checking for < 0. Most (all?) existing callers check that way, and it's good
to stay with the pack there.

> + put_page(page);
> + goto read;
> + }
> if (i == nr_to_read - lookahead_size)
> SetPageReadahead(page);
> rac._nr_pages++;
> @@ -205,7 +205,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> continue;
> read:
> if (readahead_count(&rac))
> - read_pages(&rac, &page_pool, gfp_mask);
> + read_pages(&rac, &page_pool);
> rac._nr_pages = 0;
> rac._start = ++offset;
> }
> @@ -216,7 +216,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * will then handle the error.
> */
> if (readahead_count(&rac))
> - read_pages(&rac, &page_pool, gfp_mask);
> + read_pages(&rac, &page_pool);
> BUG_ON(!list_empty(&page_pool));
> }
>
>

thanks,
--
John Hubbard
NVIDIA