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Date:   Wed, 18 Mar 2020 09:15:02 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     linux-mm@kvack.org, Ira Weiny <ira.weiny@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        John Hubbard <jhubbard@nvidia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Christoph Hellwig <hch@infradead.org>,
        Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in
 gup fast path
Message-ID: <20200318121502.GA13926@ziepe.ca>
References: <1584333244-10480-3-git-send-email-kernelfans@gmail.com>
 <1584445652-30064-1-git-send-email-kernelfans@gmail.com>
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On Tue, Mar 17, 2020 at 07:47:32PM +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@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(+)

Much clearer, thank you

Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>

Jason