On Wed, Nov 15, 2023 at 08:29:03PM -0500, Peter Xu wrote: > All the fast-gup functions take a tail page to operate, always need to do > page mask calculations before feeding that into record_subpages(). > > Merge that logic into record_subpages(), so that we always take a head > page, and leave the rest calculation to record_subpages(). This is a bit fragile. You're assuming that pmd_page() always returns a head page, and that's only true today because I looked at the work required vs the reward and decided to cap the large folio size at PMD size. If we allowed 2*PMD_SIZE (eg 4MB on x86), pmd_page() would not return a head page. There is a small amount of demand for > PMD size large folio support, so I suspect we will want to do this eventually. I'm not particularly trying to do these conversions, but it would be good to not add more assumptions that pmd_page() returns a head page. > +static int record_subpages(struct page *head, unsigned long sz, > + unsigned long addr, unsigned long end, > + struct page **pages) > @@ -2870,8 +2873,8 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr, > pages, nr); > } > > - page = nth_page(pmd_page(orig), (addr & ~PMD_MASK) >> PAGE_SHIFT); > - refs = record_subpages(page, addr, end, pages + *nr); > + page = pmd_page(orig); > + refs = record_subpages(page, PMD_SIZE, addr, end, pages + *nr); > > folio = try_grab_folio(page, refs, flags); > if (!folio)
