Re: [PATCHv6 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Christoph Hellwig]

On Mon, Mar 16, 2020 at 12:34:03PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
> going to be given to hardware and can't move. It would truncate CMA
> permanently and should be excluded.
> 
> In gup slow path, slow path, where
> __gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
> but in fast path, there lacks such a check, which means a possible leak of
> CMA page to longterm pinned.
> 
> Place a check in try_grab_compound_head() in the fast path to fix the leak,
> and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
> migrate the page.
> 
> Some note about the check:
> Huge page's subpages have the same migrate type due to either
> allocation from a free_list[] or alloc_contig_range() with param
> MIGRATE_MOVABLE. So it is enough to check on a single subpage
> by is_migrate_cma_page(subpage)
> 
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Shuah Khan <shuah@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> To: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  mm/gup.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 9df77b1..78132cf 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -89,6 +89,15 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>  		int orig_refs = refs;
> 
>  		/*
> +		 * Huge page's subpages have the same migrate type due to either
> +		 * allocation from a free_list[] or alloc_contig_range() with
> +		 * param MIGRATE_MOVABLE. So it is enough to check on a subpage.
> +		 */
> +		if (unlikely(flags & FOLL_LONGTERM) &&
> +			is_migrate_cma_page(page))
> +			return NULL;

Wrong indentation.  We either align two tabs for continuation
statements, or after the opening brace of the previous line.  With a
likely or unlikely thrown in the former IMHO looks much better.  E.g.

		if (unlikely(flags & FOLL_LONGTERM) &&
				is_migrate_cma_page(page))
			return NULL;