Re: [PATCH v1 2/2] mm: remove extra ZONE_DEVICE struct page refcount

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Oct 14, 2021 at 10:39:28AM -0500, Alex Sierra wrote:
> From: Ralph Campbell <rcampbell@nvidia.com>
> 
> ZONE_DEVICE struct pages have an extra reference count that complicates the
> code for put_page() and several places in the kernel that need to check the
> reference count to see that a page is not being used (gup, compaction,
> migration, etc.). Clean up the code so the reference count doesn't need to
> be treated specially for ZONE_DEVICE.
> 
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Signed-off-by: Alex Sierra <alex.sierra@amd.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> v2:
> AS: merged this patch in linux 5.11 version
> 
> v5:
> AS: add condition at try_grab_page to check for the zone device type, while
> page ref counter is checked less/equal to zero. In case of device zone, pages
> ref counter are initialized to zero.
> 
> v7:
> AS: fix condition at try_grab_page added at v5, is invalid. It supposed
> to fix xfstests/generic/413 test, however, there's a known issue on
> this test where DAX mapped area DIO to non-DAX expect to fail.
> https://patchwork.kernel.org/project/fstests/patch/1489463960-3579-1-git-send-email-xzhou@redhat.com
> This condition was removed after rebase over patch series
> https://lore.kernel.org/r/20210813044133.1536842-4-jhubbard@nvidia.com
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c     |  2 +-
>  drivers/gpu/drm/nouveau/nouveau_dmem.c |  2 +-
>  fs/dax.c                               |  4 +-
>  include/linux/dax.h                    |  2 +-
>  include/linux/memremap.h               |  7 +--
>  include/linux/mm.h                     | 11 ----
>  lib/test_hmm.c                         |  2 +-
>  mm/internal.h                          |  8 +++
>  mm/memcontrol.c                        |  6 +--
>  mm/memremap.c                          | 69 +++++++-------------------
>  mm/migrate.c                           |  5 --
>  mm/page_alloc.c                        |  3 ++
>  mm/swap.c                              | 45 ++---------------
>  13 files changed, 46 insertions(+), 120 deletions(-)

Has anyone tested this with FSDAX? Does get_user_pages() on fsdax
backed memory still work?

What refcount value does the struct pages have when they are installed
in the PTEs? Remember a 0 refcount will make all the get_user_pages()
fail.

I'm looking at the call path starting in ext4_punch_hole() and I would
expect to see something manipulating the page ref count before
the ext4_break_layouts() call path gets to the dax_page_unused() test.

All I see is we go into unmap_mapping_pages() - that would normally
put back the page references held by PTEs but insert_pfn() has this:

	if (pfn_t_devmap(pfn))
		entry = pte_mkdevmap(pfn_t_pte(pfn, prot));

And:

static inline pte_t pte_mkdevmap(pte_t pte)
{
	return pte_set_flags(pte, _PAGE_SPECIAL|_PAGE_DEVMAP);
}

Which interacts with vm_normal_page():

		if (pte_devmap(pte))
			return NULL;

To disable that refcounting?

So... I have a feeling this will have PTEs pointing to 0 refcount
pages? Unless FSDAX is !pte_devmap which is not the case, right?

This seems further confirmed by this comment:

	/*
	 * If we race get_user_pages_fast() here either we'll see the
	 * elevated page count in the iteration and wait, or
	 * get_user_pages_fast() will see that the page it took a reference
	 * against is no longer mapped in the page tables and bail to the
	 * get_user_pages() slow path.  The slow path is protected by
	 * pte_lock() and pmd_lock(). New references are not taken without
	 * holding those locks, and unmap_mapping_pages() will not zero the
	 * pte or pmd without holding the respective lock, so we are
	 * guaranteed to either see new references or prevent new
	 * references from being established.
	 */

Which seems to explain this scheme relies on unmap_mapping_pages() to
fence GUP_fast, not on GUP_fast observing 0 refcounts when it should
stop.

This seems like it would be properly fixed by using normal page
refcounting for PTEs - ie stop using special for these pages?

Does anyone know why devmap is pte_special anyhow?

> +void free_zone_device_page(struct page *page)
> +{
> +	switch (page->pgmap->type) {
> +	case MEMORY_DEVICE_PRIVATE:
> +		free_device_page(page);
> +		return;
> +	case MEMORY_DEVICE_FS_DAX:
> +		/* notify page idle */
> +		wake_up_var(&page->_refcount);
> +		return;

It is not for this series, but I wonder if we should just always call
ops->page_free and have free_device_page() logic in that callback for
the non-fs-dax cases?

For instance where is the mem_cgroup_charge() call to pair with the
mem_cgroup_uncharge() in free_device_page()?

Isn't cgroup charging (or not) the responsibility of the "allocator"
eg the pgmap_ops owner?

Jason