Re: [PATCH v2 02/18] mm/gup: factor out duplicate code from four routines

[Jerome Glisse <jglisse@redhat.com>]

On Sun, Nov 03, 2019 at 01:17:57PM -0800, John Hubbard wrote:
> There are four locations in gup.c that have a fair amount of code
> duplication. This means that changing one requires making the same
> changes in four places, not to mention reading the same code four
> times, and wondering if there are subtle differences.
> 
> Factor out the common code into static functions, thus reducing the
> overall line count and the code's complexity.
> 
> Also, take the opportunity to slightly improve the efficiency of the
> error cases, by doing a mass subtraction of the refcount, surrounded
> by get_page()/put_page().
> 
> Also, further simplify (slightly), by waiting until the the successful
> end of each routine, to increment *nr.
> 
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

Good cleanup.

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

> ---
>  mm/gup.c | 104 ++++++++++++++++++++++++-------------------------------
>  1 file changed, 45 insertions(+), 59 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 85caf76b3012..199da99e8ffc 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1969,6 +1969,34 @@ static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
>  }
>  #endif
>  
> +static int __record_subpages(struct page *page, unsigned long addr,
> +			     unsigned long end, struct page **pages, int nr)
> +{
> +	int nr_recorded_pages = 0;
> +
> +	do {
> +		pages[nr] = page;
> +		nr++;
> +		page++;
> +		nr_recorded_pages++;
> +	} while (addr += PAGE_SIZE, addr != end);
> +	return nr_recorded_pages;
> +}
> +
> +static void put_compound_head(struct page *page, int refs)
> +{
> +	/* Do a get_page() first, in case refs == page->_refcount */
> +	get_page(page);
> +	page_ref_sub(page, refs);
> +	put_page(page);
> +}
> +
> +static void __huge_pt_done(struct page *head, int nr_recorded_pages, int *nr)
> +{
> +	*nr += nr_recorded_pages;
> +	SetPageReferenced(head);
> +}
> +
>  #ifdef CONFIG_ARCH_HAS_HUGEPD
>  static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
>  				      unsigned long sz)
> @@ -1998,33 +2026,20 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
>  	/* hugepages are never "special" */
>  	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  
> -	refs = 0;
>  	head = pte_page(pte);
> -
>  	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
> -	do {
> -		VM_BUG_ON(compound_head(page) != head);
> -		pages[*nr] = page;
> -		(*nr)++;
> -		page++;
> -		refs++;
> -	} while (addr += PAGE_SIZE, addr != end);
> +	refs = __record_subpages(page, addr, end, pages, *nr);
>  
>  	head = try_get_compound_head(head, refs);
> -	if (!head) {
> -		*nr -= refs;
> +	if (!head)
>  		return 0;
> -	}
>  
>  	if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> -		/* Could be optimized better */
> -		*nr -= refs;
> -		while (refs--)
> -			put_page(head);
> +		put_compound_head(head, refs);
>  		return 0;
>  	}
>  
> -	SetPageReferenced(head);
> +	__huge_pt_done(head, refs, nr);
>  	return 1;
>  }
>  
> @@ -2071,29 +2086,19 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>  					     pages, nr);
>  	}
>  
> -	refs = 0;
>  	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> -	do {
> -		pages[*nr] = page;
> -		(*nr)++;
> -		page++;
> -		refs++;
> -	} while (addr += PAGE_SIZE, addr != end);
> +	refs = __record_subpages(page, addr, end, pages, *nr);
>  
>  	head = try_get_compound_head(pmd_page(orig), refs);
> -	if (!head) {
> -		*nr -= refs;
> +	if (!head)
>  		return 0;
> -	}
>  
>  	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> -		*nr -= refs;
> -		while (refs--)
> -			put_page(head);
> +		put_compound_head(head, refs);
>  		return 0;
>  	}
>  
> -	SetPageReferenced(head);
> +	__huge_pt_done(head, refs, nr);
>  	return 1;
>  }
>  
> @@ -2114,29 +2119,19 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>  					     pages, nr);
>  	}
>  
> -	refs = 0;
>  	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> -	do {
> -		pages[*nr] = page;
> -		(*nr)++;
> -		page++;
> -		refs++;
> -	} while (addr += PAGE_SIZE, addr != end);
> +	refs = __record_subpages(page, addr, end, pages, *nr);
>  
>  	head = try_get_compound_head(pud_page(orig), refs);
> -	if (!head) {
> -		*nr -= refs;
> +	if (!head)
>  		return 0;
> -	}
>  
>  	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
> -		*nr -= refs;
> -		while (refs--)
> -			put_page(head);
> +		put_compound_head(head, refs);
>  		return 0;
>  	}
>  
> -	SetPageReferenced(head);
> +	__huge_pt_done(head, refs, nr);
>  	return 1;
>  }
>  
> @@ -2151,29 +2146,20 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
>  		return 0;
>  
>  	BUILD_BUG_ON(pgd_devmap(orig));
> -	refs = 0;
> +
>  	page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
> -	do {
> -		pages[*nr] = page;
> -		(*nr)++;
> -		page++;
> -		refs++;
> -	} while (addr += PAGE_SIZE, addr != end);
> +	refs = __record_subpages(page, addr, end, pages, *nr);
>  
>  	head = try_get_compound_head(pgd_page(orig), refs);
> -	if (!head) {
> -		*nr -= refs;
> +	if (!head)
>  		return 0;
> -	}
>  
>  	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
> -		*nr -= refs;
> -		while (refs--)
> -			put_page(head);
> +		put_compound_head(head, refs);
>  		return 0;
>  	}
>  
> -	SetPageReferenced(head);
> +	__huge_pt_done(head, refs, nr);
>  	return 1;
>  }
>  
> -- 
> 2.23.0
>