Re: [RFC PATCH 5/5] Revert "mm/migrate: __unmap_and_move() push good newpage to LRU"

[David Hildenbrand]

On 19.06.23 05:59, Yosry Ahmed wrote:
> On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> > Hi, Yosry,
> >
> > Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
> >
> > > This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
> > >
> > > That commit made sure we immediately add the new page to the LRU before
> > > remove_migration_ptes() is called in migrate_move_folio() (used to be
> > > __unmap_and_move() back then), such that the rmap walk will rebuild the
> > > correct mlock_count for the page again. This was needed because the
> > > mlock_count was lost when the page is isolated. This is no longer the
> > > case since mlock_count no longer overlays page->lru.
> > >
> > > Revert the commit (the code was foliated afterward the commit, so the
> > > revert is updated as such).
> > >
> > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > > ---
> > >   mm/migrate.c | 24 +++++++++---------------
> > >   1 file changed, 9 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 01cac26a3127..68f693731865 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> > >        if (unlikely(!is_lru))
> > >                goto out_unlock_both;
> >
> > The patch itself looks good to me!  Thanks!
> >
> > Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>
> Thanks for taking a look!
>
> > And, it seems that we can remove the above 2 lines and "out_unlock_both"
> > label now.  That can make the code simpler a little.  Right?
>
> I am not familiar with this code. If we remove the above condition
> then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
> page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
> happen without removing the above 2 lines. If this combination is
> impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
> remove the above condition.
>
> It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
> and balloon_page_insert(). The former 2 will never have those pages
> mapped into userspace. I am not familiar with balloon_page_insert(),
> but my gut feeling is that those are pages used by the driver and are
> also not mapped into userspace.

On XEN, there is xen_alloc_ballooned_pages(), which ends up mapping 
balloon-inflated pages into user space (for something like MMIO IIRC). 
But the XEN balloon does not use the balloon compaction framework, so 
__SetPageMovable() does not apply.

The other balloon_page_insert() users (VMware balloon, CMM, 
virtio-balloon) shouldn't be doing something like that.

--
Cheers,

David / dhildenb