On Mon, Jun 17, 2024 at 01:32:42PM +0100, Matthew Wilcox wrote: > 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. I am hitting my head now because you have literally mentioned that in the comment: * next batch. This page may be the one we would * have intended to mark as Readahead, but we don't * have a stable reference to this page, and it's * not worth getting one just for that. I will move it by min_nrpages as follows: > ractl->_index += min_nrpages; So the following can still be there from Hannes patch as we have a stable reference: ractl->_workingset |= folio_test_workingset(folio); - ractl->_nr_pages++; + ractl->_nr_pages += folio_nr_pages(folio); + i += folio_nr_pages(folio); } Thanks for the clarification. -- Pankaj
