Re: [PATCHv4 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Ira Weiny <ira.weiny@intel.com>]

On Thu, Jun 13, 2019 at 06:45:01PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM suggests a pin which is going to be given to hardware and
> can't move. It would truncate CMA permanently and should be excluded.
> 
> FOLL_LONGTERM has already been checked in the slow path, but not checked in
> the fast path, which means a possible leak of CMA page to longterm pinned
> requirement through this crack.
> 
> Place a check in gup_pte_range() in the fast path.
> 
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> ---
>  mm/gup.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 766ae54..de1b03f 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1757,6 +1757,14 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  		page = pte_page(pte);
>  
> +		/*
> +		 * FOLL_LONGTERM suggests a pin given to hardware. Prevent it
> +		 * from truncating CMA area
> +		 */
> +		if (unlikely(flags & FOLL_LONGTERM) &&
> +			is_migrate_cma_page(page))
> +			goto pte_unmap;
> +
>  		head = try_get_compound_head(page, 1);
>  		if (!head)
>  			goto pte_unmap;
> @@ -1900,6 +1908,12 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> +	if (unlikely(flags & FOLL_LONGTERM) &&
> +		is_migrate_cma_page(page)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +

Why can't we place this check before the while loop and skip subtracting the
page count?

Can is_migrate_cma_page() operate on any "subpage" of a compound page? 

Here this calls is_magrate_cma_page() on the tail page of the compound page.

I'm not an expert on compound pages nor cma handling so is this ok?

It seems like you need to call is_migrate_cma_page() on each page within the
while loop?

>  	head = try_get_compound_head(pmd_page(orig), refs);
>  	if (!head) {
>  		*nr -= refs;
> @@ -1941,6 +1955,12 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> +	if (unlikely(flags & FOLL_LONGTERM) &&
> +		is_migrate_cma_page(page)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +

Same comment here.

>  	head = try_get_compound_head(pud_page(orig), refs);
>  	if (!head) {
>  		*nr -= refs;
> @@ -1978,6 +1998,12 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> +	if (unlikely(flags & FOLL_LONGTERM) &&
> +		is_migrate_cma_page(page)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +

And here.

Ira

>  	head = try_get_compound_head(pgd_page(orig), refs);
>  	if (!head) {
>  		*nr -= refs;
> -- 
> 2.7.5
>