Re: [RFC] find_get_heads_contig

[Kent Overstreet <kent.overstreet@gmail.com>]

On Wed, Oct 21, 2020 at 02:26:30AM +0100, Matthew Wilcox wrote:
> I was going to convert find_get_pages_contig() to only return head pages,
> but I want to change the API to take a pagevec like the other find_*
> functions have or will have.  And it'd be nice if the name of the function
> reminded callers that it only returns head pages.  So comments on this?

Perfect, looks like exactly what we need - at some point I can change my
vectorized pagecache stuff to use this.

Reviewed-by: Kent Overstreet <kent.overstreet@gmail.com>

> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 31ba06409dfa..b7dd2523fe79 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2093,6 +2093,58 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
>  }
>  EXPORT_SYMBOL(find_get_pages_contig);
>  
> +/**
> + * find_get_heads_contig - Return head pages for a contiguous byte range.
> + * @mapping: The address_space to search.
> + * @start: The starting page index.
> + * @end: The final page index (inclusive).
> + * @pvec: Where the resulting pages are placed.
> + *
> + * find_get_heads_contig() will return a batch of head pages from
> + * @mapping.  Pages are returned with an incremented refcount.  Only the
> + * head page of a THP is returned.  In contrast to find_get_entries(),
> + * pages which are partially outside the range are returned.  The head
> + * pages have ascending indices.  The indices may not be consecutive,
> + * but the bytes represented by the pages are contiguous.  If there is
> + * no page at @start, no pages will be returned.
> + *
> + * Return: The number of pages which were found.
> + */
> +unsigned find_get_heads_contig(struct address_space *mapping, pgoff_t start,
> +		pgoff_t end, struct pagevec *pvec)
> +{
> +	XA_STATE(xas, &mapping->i_pages, start);
> +	struct page *page;
> +
> +	rcu_read_lock();
> +	for (page = xas_load(&xas); page; page = xas_next(&xas)) {
> +		if (xas.xa_index > end)
> +			break;
> +		if (xas_retry(&xas, page) || xa_is_sibling(page))
> +			continue;
> +		if (xa_is_value(page))
> +			break;
> +
> +		if (!page_cache_get_speculative(page))
> +			goto retry;
> +
> +		/* Has the page moved or been split? */
> +		if (unlikely(page != xas_reload(&xas)))
> +			goto put_page;
> +
> +		if (!pagevec_add(pvec, page))
> +			break;
> +		continue;
> +put_page:
> +		put_page(page);
> +retry:
> +		xas_reset(&xas);
> +	}
> +	rcu_read_unlock();
> +	return pagevec_count(pvec);
> +}
> +EXPORT_SYMBOL(find_get_heads_contig);
> +
>  /**
>   * find_get_pages_range_tag - Find and return head pages matching @tag.
>   * @mapping:	the address_space to search