On Tue, Jun 11, 2024 at 07:22:03PM +0100, Matthew Wilcox wrote:
> On Tue, Jun 11, 2024 at 11:06:22AM -0700, Andrew Morton wrote:
> > On Tue, 11 Jun 2024 17:33:17 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > > On 11.06.24 17:32, Andrew Bresticker wrote:
> > > > The requirement that the head page be passed to do_set_pmd() was added
> > > > in commit ef37b2ea08ac ("mm/memory: page_add_file_rmap() ->
> > > > folio_add_file_rmap_[pte|pmd]()") and prevents pmd-mapping in the
> > > > finish_fault() and filemap_map_pages() paths if the page to be inserted
> > > > is anything but the head page for an otherwise suitable vma and pmd-sized
> > > > page.
> > > >
> > > > Fixes: ef37b2ea08ac ("mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()")
> > > > Signed-off-by: Andrew Bresticker <abrestic@xxxxxxxxxxxx>
> > > > ---
> > > > mm/memory.c | 3 ++-
> > > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 0f47a533014e..a1fce5ddacb3 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -4614,8 +4614,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> > > > if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> > > > return ret;
> > > >
> > > > - if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
> > > > + if (folio_order(folio) != HPAGE_PMD_ORDER)
> > > > return ret;
> > > > + page = &folio->page;
> > > >
> > > > /*
> > > > * Just backoff if any subpage of a THP is corrupted otherwise
> > >
> > > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> >
> > You know what I'm going to ask ;) I'm assuming that the runtime effects
> > are "small performance optimization" and that "should we backport the
> > fix" is "no".
>
> We're going to stop using PMDs to map large folios unless the fault is
> within the first 4KiB of the PMD. No idea how many workloads that
> affects, but it only needs to be backported as far as v6.8, so we
> may as well backport it.
Hi, I am reviving this thread after noticing this comment attached
to the fix.
If you intend to install PTE level mappings for faults that happen outside of
the first 4KiB, I believe this will make THP support for KVM ineffective.
