linux-kernel - Re: [PATCH v6 13/26] mm/memory: Add vmf_insert_page

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Message-ID: <17d32dc8-39c1-4aa4-ab8c-873c78097fde@redhat.com>
Date: Tue, 14 Jan 2025 17:15:54 +0100
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
 dan.j.williams@...el.com, linux-mm@...ck.org
Cc: alison.schofield@...el.com, lina@...hilina.net, zhang.lyra@...il.com,
 gerald.schaefer@...ux.ibm.com, vishal.l.verma@...el.com,
 dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
 jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com, will@...nel.org,
 mpe@...erman.id.au, npiggin@...il.com, dave.hansen@...ux.intel.com,
 ira.weiny@...el.com, willy@...radead.org, djwong@...nel.org, tytso@....edu,
 linmiaohe@...wei.com, peterx@...hat.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, nvdimm@...ts.linux.dev,
 linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, jhubbard@...dia.com,
 hch@....de, david@...morbit.com, chenhuacai@...nel.org, kernel@...0n.name,
 loongarch@...ts.linux.dev
Subject: Re: [PATCH v6 13/26] mm/memory: Add vmf_insert_page_mkwrite()

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@...dia.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