Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[John Hubbard <jhubbard@nvidia.com>]

On 3/17/20 4:47 AM, 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


s/slow path, slow path/slow path/


> __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@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: Christoph Hellwig <hch@infradead.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> To: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> v6 -> v7: fix coding style issue
>   mm/gup.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 9df77b1..0a536d7 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.
> +		 */

Urggh, this comment is fine in the commit description, but at this location in the
code it is completely incomprehensible! Instead of an extremely far-removed tidbit about
interactions between CMA and huge pages, this comment should be explaining why we bail
out early in the specific case of FOLL_PIN + FOLL_LONGTERM. And we don't bail out for
FOLL_GET + FOLL_LONGTERM...


I'm expect it is something like:

		/*
		 * We can't do FOLL_LONGTERM + FOLL_PIN with CMA in the gup fast
		 * path, so fail and let the caller fall back to the slow path.
		 */


...approximately. Right?


> +		if (unlikely(flags & FOLL_LONGTERM) &&
> +				is_migrate_cma_page(page))
> +			return NULL;
> +
> +		/*
>   		 * When pinning a compound page of order > 1 (which is what
>   		 * hpage_pincount_available() checks for), use an exact count to
>   		 * track it, via hpage_pincount_add/_sub().
> --
> 2.7.5
> 




thanks,
-- 
John Hubbard
NVIDIA