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

[Alistair Popple <apopple@nvidia.com>]

Dan Williams <dan.j.williams@intel.com> writes:

> 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?

No problem, although I notice Andrew has already merged it into
mm-unstable. If you end up rebasing your series on top of mine I think
all that's needed is a patch somewhere in your series to drop the
various `if (pgmap->type == MEMORY_DEVICE_*)` I added to (hopefully)
avoid breaking DAX. Assuming DAX takes a pagemap reference on struct
page allocation something like below.

---

diff --git a/mm/memremap.c b/mm/memremap.c
index 421bec3a29ee..da1a0e0abb8b 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -507,15 +507,7 @@ void free_zone_device_page(struct page *page)
 	page->mapping = NULL;
 	page->pgmap->ops->page_free(page);

-	if (page->pgmap->type != MEMORY_DEVICE_PRIVATE &&
-	    page->pgmap->type != MEMORY_DEVICE_COHERENT)
-		/*
-		 * Reset the page count to 1 to prepare for handing out the page
-		 * again.
-		 */
-		set_page_count(page, 1);
-	else
-		put_dev_pagemap(page->pgmap);
+	put_dev_pagemap(page->pgmap);
 }

 void zone_device_page_init(struct page *page)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 014dbdf54d62..3e5ff06700ca 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6816,9 +6816,7 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
 	 * ZONE_DEVICE pages are released directly to the driver page allocator
 	 * which will set the page count to 1 when allocating the page.
 	 */
-	if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
-	    pgmap->type == MEMORY_DEVICE_COHERENT)
-		set_page_count(page, 0);
+	set_page_count(page, 0);
 }

 /*