On Tue, 22 Feb 2022 at 04:18, NeilBrown <neilb@xxxxxxx> wrote:
>
> If ->readpages doesn't process all the pages, then it is best to act as
> though they weren't requested so that a subsequent readahead can try
> again.
> So:
> - remove any 'ahead' pages from the page cache so they can be loaded
> with ->readahead() rather then multiple ->read()s
> - update the file_ra_state to reflect the reads that were actually
> submitted.
>
> This allows ->readpages() to abort early due e.g. to congestion, which
> will then allow us to remove the inode_read_congested() test from
> page_Cache_async_ra().
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> mm/readahead.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 73b2bc5302e0..8a97bd408cf6 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -104,7 +104,13 @@
> * for necessary resources (e.g. memory or indexing information) to
> * become available. Pages in the final ``async_size`` may be
> * considered less urgent and failure to read them is more acceptable.
> - * They will eventually be read individually using ->readpage().
> + * In this case it is best to use delete_from_page_cache() to remove the
> + * pages from the page cache as is automatically done for pages that
> + * were not fetched with readahead_page(). This will allow a
> + * subsequent synchronous read ahead request to try them again. If they
> + * are left in the page cache, then they will be read individually using
> + * ->readpage().
> + *
> */
>
> #include <linux/kernel.h>
> @@ -226,8 +232,17 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
>
> if (aops->readahead) {
> aops->readahead(rac);
> - /* Clean up the remaining pages */
> + /*
> + * Clean up the remaining pages. The sizes in ->ra
> + * maybe be used to size next read-ahead, so make sure
> + * they accurately reflect what happened.
> + */
> while ((page = readahead_page(rac))) {
> + rac->ra->size -= 1;
> + if (rac->ra->async_size > 0) {
> + rac->ra->async_size -= 1;
> + delete_from_page_cache(page);
> + }
Does the above imply that filesystem should submit at least ra->size
pages, regardless of congestion?
Thanks,
Miklos
