Re: [PATCH 03/11] MM: improve cleanup when ->readpages doesn't process all pages.

["NeilBrown" <neilb@suse.de>]

On Wed, 02 Mar 2022, Miklos Szeredi wrote:
> On Tue, 22 Feb 2022 at 04:18, NeilBrown <neilb@suse.de> 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@suse.de>
> > ---
> >  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?

   ra->size - ra_async_size 
pages should be submitted reguardless of congestion.

NeilBrown