Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 17 Mar 2020 05:09:30 -0700
From:   Christoph Hellwig <hch@infradead.org>
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>, Jason Gunthorpe <jgg@ziepe.ca>,
        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: <20200317120930.GA435@infradead.org>
References: <1584333244-10480-3-git-send-email-kernelfans@gmail.com>
 <1584445652-30064-1-git-send-email-kernelfans@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1584445652-30064-1-git-send-email-kernelfans@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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)

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>