Re: [PATCH RFC 07/12] mm/gup: Refactor record_subpages() to find 1st small page

Matthew Wilcox
2023-11-16T19:41:38Z

On Thu, Nov 16, 2023 at 02:40:21PM -0500, Peter Xu wrote:
> On Thu, Nov 16, 2023 at 02:51:52PM +0000, Matthew Wilcox wrote:
> > 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.
>
> Makes sense. Actually, IIUC arm64's CONT_PMD pages can already make that
> not a head page.
>
> The code should still be correct, though. AFAIU what I need to do then is
> renaming the first field of record_subpages() (s/head/base/) in the next
> version, or just keep it the old one ("page"), then update the commit
> message.

Yeah, I think just leave it as 'page' would be best. Thanks.