linux-kernel - Re: [PATCH 06/13] mm/memory: Add dax_insert

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Date: Thu, 27 Jun 2024 13:33:28 +0200
From: Jan Kara <jack@...e.cz>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.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,
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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
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	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
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	jhubbard@...dia.com, hch@....de, david@...morbit.com
Subject: Re: [PATCH 06/13] mm/memory: Add dax_insert_pfn

On Thu 27-06-24 10:54:21, 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 dax_insert_pfn. 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>

Overall this looks good to me. Some comments below.

> ---
>  include/linux/mm.h |  4 ++-
>  mm/memory.c        | 79 ++++++++++++++++++++++++++++++++++++++++++-----
>  2 files changed, 76 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9a5652c..b84368b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1080,6 +1080,8 @@ int vma_is_stack_for_current(struct vm_area_struct *vma);
>  struct mmu_gather;
>  struct inode;
>  
> +extern void prep_compound_page(struct page *page, unsigned int order);
> +

You don't seem to use this function in this patch?

> diff --git a/mm/memory.c b/mm/memory.c
> index ce48a05..4f26a1f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1989,14 +1989,42 @@ static int validate_page_before_insert(struct page *page)
>  }
>  
>  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);
>  
> -	if (!pte_none(ptep_get(pte)))
> +	if (!pte_none(entry)) {
> +		if (mkwrite) {
> +			/*
> +			 * 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.
> +			 */
> +			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;
> +		}
>  		return -EBUSY;

If you do this like:

		if (!mkwrite)
			return -EBUSY;

You can reduce indentation of the big block and also making the flow more
obvious...

> +	}
> +
>  	/* Ok, finally just insert the thing.. */
>  	folio_get(folio);
> +	if (mkwrite)
> +		entry = maybe_mkwrite(mk_pte(page, prot), vma);
> +	else
> +		entry = mk_pte(page, prot);

I'd prefer:

	entry = mk_pte(page, prot);
	if (mkwrite)
		entry = maybe_mkwrite(entry, vma);

but I don't insist. Also insert_pfn() additionally has pte_mkyoung() and
pte_mkdirty(). Why was it left out here?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR