Re: [PATCH v7 03/14] mm: handling Non-LRU pages returned by vm_normal_pages

David Hildenbrand
2022-06-29T09:59:36Z

On 29.06.22 05:54, Alex Sierra wrote:
> With DEVICE_COHERENT, we'll soon have vm_normal_pages() return
> device-managed anonymous pages that are not LRU pages. Although they
> behave like normal pages for purposes of mapping in CPU page, and for
> COW. They do not support LRU lists, NUMA migration or THP.
>
> Callers to follow_page that expect LRU pages, are also checked for
> device zone pages due to DEVICE_COHERENT type.

Can we rephrase that to (because zeropage)

"Callers to follow_page() currently don't expect ZONE_DEVICE pages,
however, with DEVICE_COHERENT we might now return ZONE_DEVICE. Check for
ZONE_DEVICE pages in applicable users of follow_page() as well."

[...]

> /*
> diff --git a/mm/memory.c b/mm/memory.c
> index 7a089145cad4..e18555af9024 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -624,6 +624,13 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> if (is_zero_pfn(pfn))
> return NULL;
> if (pte_devmap(pte))
> +/*
> + * NOTE: New uers of ZONE_DEVICE will not set pte_devmap() and will have

s/uers/users/

> + * refcounts incremented on their struct pages when they are inserted into
> + * PTEs, thus they are safe to return here. Legacy ZONE_DEVICE pages that set
> + * pte_devmap() do not have refcounts. Example of legacy ZONE_DEVICE is
> + * MEMORY_DEVICE_FS_DAX type in pmem or virtio_fs drivers.
> + */

[...]

> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index ba5592655ee3..e034aae2a98b 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -95,7 +95,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> continue;
>
> page = vm_normal_page(vma, addr, oldpte);
> - if (!page || PageKsm(page))
> + if (!page || is_zone_device_page(page) || PageKsm(page))
> continue;
>
> /* Also skip shared copy-on-write pages */

In -next/-mm there is now an additional can_change_pte_writable() that
calls vm_normal_page() -- added by me. I assume that that is indeed
fine because we can simply map device coherent pages writable.

Besides the nits, LGTM

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb