Re: [PATCH v6 13/26] mm/memory: Add vmf_insert_page_mkwrite()

David Hildenbrand
2025-01-14T16:17:37Z

On 10.01.25 07:00, Alistair Popple wrote:
> Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This
> creates a special devmap PTE entry for the pfn but does not take a
> reference on the underlying struct page for the mapping. This is
> because DAX page refcounts are treated specially, as indicated by the
> presence of a devmap entry.
>
> To allow DAX page refcounts to be managed the same as normal page
> refcounts introduce vmf_insert_page_mkwrite(). This will take a
> reference on the underlying page much the same as vmf_insert_page,
> except it also permits upgrading an existing mapping to be writable if
> requested/possible.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>
> ---
>
> Updates from v2:
>
> - Rename function to make not DAX specific
>
> - Split the insert_page_into_pte_locked() change into a separate
> patch.
>
> Updates from v1:
>
> - Re-arrange code in insert_page_into_pte_locked() based on comments
> from Jan Kara.
>
> - Call mkdrity/mkyoung for the mkwrite case, also suggested by Jan.
> ---
> include/linux/mm.h | 2 ++
> mm/memory.c | 36 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 38 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e790298..f267b06 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3620,6 +3620,8 @@ int vm_map_pages(struct vm_area_struct *vma, struct page **pages,
> unsigned long num);
> int vm_map_pages_zero(struct vm_area_struct *vma, struct page **pages,
> unsigned long num);
> +vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,
> + bool write);
> vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
> unsigned long pfn);
> vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
> diff --git a/mm/memory.c b/mm/memory.c
> index 8531acb..c60b819 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2624,6 +2624,42 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
> return VM_FAULT_NOPAGE;
> }
>
> +vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,
> + bool write)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + pgprot_t pgprot = vma->vm_page_prot;
> + unsigned long pfn = page_to_pfn(page);
> + unsigned long addr = vmf->address;
> + int err;
> +
> + if (addr < vma->vm_start || addr >= vma->vm_end)
> + return VM_FAULT_SIGBUS;
> +
> + track_pfn_insert(vma, &pgprot, pfn_to_pfn_t(pfn));

I think I raised this before: why is this track_pfn_insert() in here? It
only ever does something to VM_PFNMAP mappings, and that cannot possibly
be the case here (nothing in VM_PFNMAP is refcounted, ever)?

> +
> + if (!pfn_modify_allowed(pfn, pgprot))
> + return VM_FAULT_SIGBUS;

Why is that required? Why are we messing so much with PFNs? :)

Note that x86 does in there

/* If it's real memory always allow */
if (pfn_valid(pfn))
return true;

See below, when would we ever have a "struct page *" but !pfn_valid() ?

> +
> + /*
> + * We refcount the page normally so make sure pfn_valid is true.
> + */
> + if (!pfn_valid(pfn))
> + return VM_FAULT_SIGBUS;

Somebody gave us a "struct page", how could the pfn ever by invalid (not
have a struct page)?

I think all of the above regarding PFNs should be dropped -- unless I am
missing something important.

> +
> + if (WARN_ON(is_zero_pfn(pfn) && write))
> + return VM_FAULT_SIGBUS;

is_zero_page() if you already have the "page". But note that in
validate_page_before_insert() we do have a check that allows for
conditional insertion of the shared zeropage.

So maybe this hunk is also not required.

> +
> + err = insert_page(vma, addr, page, pgprot, write);
> + if (err == -ENOMEM)
> + return VM_FAULT_OOM;
> + if (err < 0 && err != -EBUSY)
> + return VM_FAULT_SIGBUS;
> +
> + return VM_FAULT_NOPAGE;
> +}
> +EXPORT_SYMBOL_GPL(vmf_insert_page_mkwrite);

--
Cheers,

David / dhildenb