On Fri, May 18, 2018 at 06:48:04PM +0200, Christoph Hellwig wrote:
> That way file systems don't have to go spotting for non-contiguous pages
> and work around them. It also kicks off I/O earlier, allowing it to
> finish earlier and reduce latency.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> mm/readahead.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index fa4d4b767130..044ab0c137cc 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -177,8 +177,18 @@ unsigned int __do_page_cache_readahead(struct address_space *mapping,
> rcu_read_lock();
> page = radix_tree_lookup(&mapping->i_pages, page_offset);
> rcu_read_unlock();
> - if (page && !radix_tree_exceptional_entry(page))
> + if (page && !radix_tree_exceptional_entry(page)) {
> + /*
> + * Page already present? Kick off the current batch of
> + * contiguous pages before continuing with the next
> + * batch.
> + */
> + if (nr_pages)
> + read_pages(mapping, filp, &page_pool, nr_pages,
> + gfp_mask);
> + nr_pages = 0;
> continue;
> + }
The comment at the top of this function explicitly states that we don't
submit I/Os before all of the pages are allocated. That probably needs
an update, at least.
That aside, couldn't this introduce that kind of problematic read/write
behavior if the mapping was sparsely populated for whatever reason
(every other page, for example)? Perhaps that's just too unlikely to
matter.
Brian
>
> page = __page_cache_alloc(gfp_mask);
> if (!page)
> --
> 2.17.0
>
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