Re: [PATCH 2/7] mm: Free device private pages have zero refcount

Dan Williams
2022-09-29T20:18:30Z

Alistair Popple wrote:
>
> Jason Gunthorpe <jgg@nvidia.com> writes:
>
> > On Mon, Sep 26, 2022 at 04:03:06PM +1000, Alistair Popple wrote:
> >> Since 27674ef6c73f ("mm: remove the extra ZONE_DEVICE struct page
> >> refcount") device private pages have no longer had an extra reference
> >> count when the page is in use. However before handing them back to the
> >> owning device driver we add an extra reference count such that free
> >> pages have a reference count of one.
> >>
> >> This makes it difficult to tell if a page is free or not because both
> >> free and in use pages will have a non-zero refcount. Instead we should
> >> return pages to the drivers page allocator with a zero reference count.
> >> Kernel code can then safely use kernel functions such as
> >> get_page_unless_zero().
> >>
> >> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> >> ---
> >> arch/powerpc/kvm/book3s_hv_uvmem.c | 1 +
> >> drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 +
> >> drivers/gpu/drm/nouveau/nouveau_dmem.c | 1 +
> >> lib/test_hmm.c | 1 +
> >> mm/memremap.c | 5 -----
> >> mm/page_alloc.c | 6 ++++++
> >> 6 files changed, 10 insertions(+), 5 deletions(-)
> >
> > I think this is a great idea, but I'm surprised no dax stuff is
> > touched here?
>
> free_zone_device_page() shouldn't be called for pgmap->type ==
> MEMORY_DEVICE_FS_DAX so I don't think we should have to worry about DAX
> there. Except that the folio code looks like it might have introduced a
> bug. AFAICT put_page() always calls
> put_devmap_managed_page(&folio->page) but folio_put() does not (although
> folios_put() does!). So it seems folio_put() won't end up calling
> __put_devmap_managed_page_refs() as I think it should.
>
> I think you're right about the change to __init_zone_device_page() - I
> should limit it to DEVICE_PRIVATE/COHERENT pages only. But I need to
> look at Dan's patch series more closely as I suspect it might be better
> to rebase this patch on top of that.

Apologies for the delay I was travelling the past few days. Yes, I think
this patch slots in nicely to avoid the introduction of an init_mode
[1]:

https://lore.kernel.org/nvdimm/166329940343.2786261.6047770378829215962.stgit@dwillia2-xfh.jf.intel.com/

Mind if I steal it into my series?