Re: [PATCH v3 08/16] mm/rmap: drop "compound" parameter from page_add_new_anon_rmap()

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 13, 2022 at 02:28:38PM +0200, David Hildenbrand wrote:
> On 13.04.22 14:26, Matthew Wilcox wrote:
> > On Tue, Apr 12, 2022 at 11:37:09AM +0200, David Hildenbrand wrote:
> >> On 12.04.22 10:47, Vlastimil Babka wrote:
> >>> There's a VM_BUG_ON_PAGE(PageTransCompound(page), page); later in a
> >>> !compound branch. Since compound is now determined by the same check, could
> >>> be deleted.
> >>
> >> Yes, eventually we could get rid of both VM_BUG_ON_PAGE() on both
> >> branches and add a single VM_BUG_ON_PAGE(PageTail(page), page) check on
> >> the compound branch. (we could also make sure that we're not given a
> >> hugetlb page)
> > 
> > As a rule of thumb, if you find yourself wanting to add
> > VM_BUG_ON_PAGE(PageTail(page), page), you probably want to change the
> > interface to take a folio.
> 
> Yeah, I had the same in mind. Might be a reasonable addon on top --
> although it would stick out in the rmap code a bit because most
> functions deal with both, folios and subpages.

I have the start of a series which starts looking at the fault path
to see where it makes sense to use folios and where it makes sense to
use pages.

We're (generally) faulting on a PTE, so we need the precise page to
be returned in vmf->page.  However vmf->cow_page can/should be a
folio (because it's definitely not a tail page).  That trickles
down into copy_present_page() (new_page and prealloc both become folios)
and so page_add_new_anon_rmap() then looks like a good target to
take a folio.

The finish_fault() -> do_set_pte() -> page_add_new_anon_rmap() looks
like the only kind of strange place where we don't necessarily have a
folio (all the others we just allocated it).