On Fri, Jun 14, 2024 at 09:26:02AM +0000, Pankaj Raghav (Samsung) wrote: > > Hm, but we don't have a reference on this folio. So this isn't safe. > > That is why I added a check for mapping after read_pages(). You are > right, we can make it better. That's not enoughh. > > > + if (mapping != folio->mapping) > > > + nr_pages = min_nrpages; > > > + > > > + VM_BUG_ON_FOLIO(nr_pages < min_nrpages, folio); > > > + ractl->_index += nr_pages; > > > > Why not just: > > ractl->_index += min_nrpages; > > Then we will only move min_nrpages even if the folio we found had a > bigger order. Hannes patches (first patch) made sure we move the > ractl->index by folio_nr_pages instead of 1 and making this change will > defeat the purpose because without mapping order set, min_nrpages will > be 1. Hannes' patch is wrong. It's not safe to call folio_nr_pages() unless you have a reference to the folio. > @@ -266,10 +266,8 @@ void page_cache_ra_unbounded(struct readahead_control *ractl, > * alignment constraint in the page cache. > * > */ > - if (mapping != folio->mapping) > - nr_pages = min_nrpages; > + nr_pages = max(folio_nr_pages(folio), (long)min_nrpages); No. > Now we will still move respecting the min order constraint but if we had > a bigger folio and we do have a reference, then we move folio_nr_pages. You don't have a reference, so it's never safe.
