Re: [PATCH v2] mm/memory: Don't require head page for do_set_pmd()

[Hugh Dickins <hughd@google.com>]

On Tue, 11 Jun 2024, Andrew Morton wrote:
> On Tue, 11 Jun 2024 19:22:03 +0100 Matthew Wilcox <willy@infradead.org> 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@redhat.com> 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@rivosinc.com>
> > > > > ---
> > > > >   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@redhat.com>

Acked-by: Hugh Dickins <hughd@google.com>

> > > 
> > > 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.
> 
> OK, thanks, I pasted the above text and added the cc:stable.

Yes please. My interest in this being that yesterday I discovered
the large drop in ShmemPmdMapped between v6.7 and v6.8, bisected,
and was testing overnight with a patch very much like this one of
Andrew's. I'd been hoping to send mine today, but now no need.

> 
> I didn't move it into the hotfixes queue - it's a non-trivial
> behavioral change and extra test time seems prudent(?).

It is certainly worth some test soak time, and the bug might have
been masking other issues which may now emerge; but the fix is
just reverting to the old pre-v6.8 behaviour.

Thanks,
Hugh