linux-kernel - Re: [PATCH v6 12/26] mm/memory: Enhance insert_page_into_pte

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Message-ID: <c3df67b5-47f5-4a2c-ba50-cc0de2b937f5@redhat.com>
Date: Tue, 14 Jan 2025 16:03:29 +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 12/26] mm/memory: Enhance insert_page_into_pte_locked()
 to create writable mappings

On 10.01.25 07:00, Alistair Popple wrote:
> In preparation for using insert_page() for DAX, enhance
> insert_page_into_pte_locked() to handle establishing writable
> mappings.  Recall that DAX returns VM_FAULT_NOPAGE after installing a
> PTE which bypasses the typical set_pte_range() in finish_fault.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> 
> ---
> 
> Changes for v5:
>   - Minor comment/formatting fixes suggested by David Hildenbrand
> 
> Changes since v2:
> 
>   - New patch split out from "mm/memory: Add dax_insert_pfn"
> ---
>   mm/memory.c | 37 +++++++++++++++++++++++++++++--------
>   1 file changed, 29 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 06bb29e..8531acb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2126,19 +2126,40 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
>   }
>   
>   static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
> -			unsigned long addr, struct page *page, pgprot_t prot)
> +				unsigned long addr, struct page *page,
> +				pgprot_t prot, bool mkwrite)
>   {
>   	struct folio *folio = page_folio(page);
> +	pte_t entry = ptep_get(pte);
>   	pte_t pteval;
 >

Just drop "entry" and reuse "pteval"; even saves you from one bug below :)

pte_t pteval = ptep_get(pte);

> -	if (!pte_none(ptep_get(pte)))
> -		return -EBUSY;
> +	if (!pte_none(entry)) {
> +		if (!mkwrite)
> +			return -EBUSY;
> +
> +		/* see insert_pfn(). */
> +		if (pte_pfn(entry) != page_to_pfn(page)) {
> +			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
> +			return -EFAULT;
> +		}
> +		entry = maybe_mkwrite(entry, vma);
> +		entry = pte_mkyoung(entry);
> +		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
> +			update_mmu_cache(vma, addr, pte);
> +		return 0;
> +	}
> +
>   	/* Ok, finally just insert the thing.. */
>   	pteval = mk_pte(page, prot);
>   	if (unlikely(is_zero_folio(folio))) {
>   		pteval = pte_mkspecial(pteval);
>   	} else {
>   		folio_get(folio);
> +		entry = mk_pte(page, prot);

we already do "pteval = mk_pte(page, prot);" above?

And I think your change here does not do what you want, because you
modify the new "entry" but we do

	set_pte_at(vma->vm_mm, addr, pte, pteval);

below ...

> +		if (mkwrite) {
> +			entry = pte_mkyoung(entry);
> +			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		}

So again, better just reuse pteval :)

-- 
Cheers,

David / dhildenb