linux-kernel - Re: [PATCH v4 12/25] mm/memory: Enhance insert_page_into_pte

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Message-ID: <d4d32e17-d8e2-4447-bd33-af41e89a528f@redhat.com>
Date: Fri, 20 Dec 2024 20:01:02 +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: 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
Subject: Re: [PATCH v4 12/25] mm/memory: Enhance insert_page_into_pte_locked()
 to create writable mappings

On 17.12.24 06:12, 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 since v2:
> 
>   - New patch split out from "mm/memory: Add dax_insert_pfn"
> ---
>   mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 06bb29e..cd82952 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2126,19 +2126,47 @@ 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;
>   
> -	if (!pte_none(ptep_get(pte)))
> -		return -EBUSY;
> +	if (!pte_none(entry)) {
> +		if (!mkwrite)
> +			return -EBUSY;
> +
> +		/*
> +		 * For read faults on private mappings the PFN passed in may not
> +		 * match the PFN we have mapped if the mapped PFN is a writeable
> +		 * COW page.  In the mkwrite case we are creating a writable PTE
> +		 * for a shared mapping and we expect the PFNs to match. If they
> +		 * don't match, we are likely racing with block allocation and
> +		 * mapping invalidation so just skip the update.
> +		 */

Would it make sense to instead have here

/* See insert_pfn(). */

But ...

> +		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);

... I am not sure if we want the above at all. Someone inserted a page, 
which is refcounted + mapcounted already.

Now you ignore that and do like the second insertion "worked" ?

No, that feels wrong, I suspect you will run into refcount+mapcount issues.

If there is already something, inserting must fail IMHO. If you want to 
change something to upgrade write permissions, then a different 
interface should be used.

> +		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);
> +		if (mkwrite) {
> +			entry = pte_mkyoung(entry);
 > +			entry = maybe_mkwrite(pte_mkdirty(entry), vma);> +		}
>   		inc_mm_counter(vma->vm_mm, mm_counter_file(folio));
>   		folio_add_file_rmap_pte(folio, page, vma);
>   	}
> @@ -2147,7 +2175,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
>   }
>   
>   static int insert_page(struct vm_area_struct *vma, unsigned long addr,
> -			struct page *page, pgprot_t prot)
> +			struct page *page, pgprot_t prot, bool mkwrite)
>   {
>   	int retval;
>   	pte_t *pte;
> @@ -2160,7 +2188,8 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr,
>   	pte = get_locked_pte(vma->vm_mm, addr, &ptl);
>   	if (!pte)
>   		goto out;
> -	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot);
> +	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot,
> +					mkwrite);

Alignment looks odd. Likely you can also just put it into a single line.



-- 
Cheers,

David / dhildenb