On Fri, 4 Sep 2020 16:48:07 +0200 Bean Huo <huobean@xxxxxxxxx> wrote:
> From: Bean Huo <beanhuo@xxxxxxxxxx>
>
> Current generic_file_buffered_read() will break up the larger batches of pages
> and read data in single page length in case of ra->ra_pages == 0. This patch is
> to allow it to pass the batches of pages down to the device if the supported
> maximum IO size >= the requested size.
>
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2062,6 +2062,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
> struct file *filp = iocb->ki_filp;
> struct address_space *mapping = filp->f_mapping;
> struct inode *inode = mapping->host;
> + struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> struct file_ra_state *ra = &filp->f_ra;
> loff_t *ppos = &iocb->ki_pos;
> pgoff_t index;
> @@ -2098,9 +2099,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
> if (!page) {
> if (iocb->ki_flags & IOCB_NOIO)
> goto would_block;
> - page_cache_sync_readahead(mapping,
> - ra, filp,
> - index, last_index - index);
> +
> + if (!ra->ra_pages && bdi->io_pages >= last_index - index)
> + __do_page_cache_readahead(mapping, filp, index,
> + last_index - index, 0);
> + else
> + page_cache_sync_readahead(mapping, ra, filp,
> + index,
> + last_index - index);
> page = find_get_page(mapping, index);
> if (unlikely(page == NULL))
> goto no_cached_page;
I assume this is a performance patch. What are the observed changes in
behaviour?
What is special about ->ra_pages==0? Wouldn't this optimization still
be valid if ->ra_pages==2?
Doesn't this defeat the purpose of having ->ra_pages==0?
