On Wed, Mar 27, 2024 at 6:05 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <soma.nakata01@xxxxxxxxx> wrote: > > > Functions such as __filemap_get_folio() check the truncation of > > folios based on the mapping field. Therefore setting this field to NULL > > earlier prevents unnecessary operations on already removed folios. > > > > ... > > > > --- a/mm/filemap.c > > +++ b/mm/filemap.c > > @@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping, > > > > VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); > > > > + folio->mapping = NULL; > > + /* Leave page->index set: truncation lookup relies upon it */ > > + > > xas_store(&xas, shadow); > > xas_init_marks(&xas); > > > > - folio->mapping = NULL; > > - /* Leave page->index set: truncation lookup relies upon it */ > > mapping->nrpages -= nr; > > } > > Seems at least harmless, but I wonder if it can really make any > difference. Don't readers of folio->mapping lock the folio first? Yes, the reader locks the folio. Only __filemap_remove_folio() calls page_cache_delete(), and it says the caller has to lock the folio or make sure that usage is safe. In the latter case, this patch improves efficiency a little bit. However, I found that there is not any latter case actually, so discard it or apply, also to make the order of operations in page_cache_delete() and page_cache_delete_batch() the same for a cleanup. Thanks,
