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

[Peter Xu <peterx@redhat.com>]

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.

Thanks,

-- 
Peter Xu